Photec Technical Process Guide

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PHOTEC

Dry Film PhotoresistsTechnical Process Guide

Troubleshooting Guide

Enthone

www.cooksonelectronics.comIssued: 11/02Supercedes: 10/99

2002 Enthone Inc.TPG-Photec: Europe

*Trademark used under license from Hitachi Chemical Co., Ltd.


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PHOTEC*Dry Film Photoresists

Table of Contents

Section Pages

Introduction

Technology

Substrate Preparation

Lamination

Exposure

Development

Etching

Resist Stripping

Questions and Anwers

Appendices

2-3

4-9

10-22

23-28

29-33

34-41

42-50

51-53

54-62

63-73


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Introduction


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INTRODUCTION

This technical process guide will provide detailed instruction in the use andapplication of PHOTEC* dry film photoresists, as well an overview on resist

technology.

Historical Background

Dry film photoresists (resists) were first patented in 1968 by Mr J Celeste ofDUPONT. Since that time they have been used extensively in the primary

image formation on printed wiring boards.

At the time of introduction dry film resists were processed with solvents. It was

not until the mid 1970s that the aqueous (water-based) processing wasintroduced.

Hitachi Chemical creates the resists. Enthone delivers the results.

As noted by the table below, Hitachi Chemical Co., Ltd. has been a leader indry film resists for 25 years.

1974 First PHOTEC products released in Japan1975 First aqueous PHOTEC product released

1985 PHOTEC released to Europe

In 1985, Enthone and Hitachi Chemical Company, Ltd., formed a strongpartnership. For fifteen years, Enthone has been the exclusive distributor andmarketer of PHOTEC dry film resists throughout Europe. Backed by

unmatched technical support, PHOTEC dry film rolls are custom slit at

Enthones ISO 14001 certified facility in The Netherlands.

The dry film resist as supplied consists of a three-layer construction. Theresist is supported on a polyester film and protected on the other side by a

polyethylene-separating layer. This polyethylene-separating layer, which isremoved prior to use, is so that it can be rolled ready for application by the

laminator. Polyester is used as the support film for two reasons:

It has good mechanical and thermal properties so that there is no

distortion or wrinkling when heat and tension is applied at thelamination stage.

Polyester has low permeability to oxygen. If oxygen were to permeatethrough this protective layer to the resist in the unexposed state, free

radicals could be generated which would lead to polymerisation of theresist.


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Technology

Acrylic Polymer

Reactive Monomer

Imaging Agent

Photo sensitisers and Photo initiators

Resist: General Formulation Principles


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TECHNOLOGY

A dry film photoresist is a complex mixture of components, each having a veryimportant role in the function of the resist. A typical dry film resist contains:

Acrylic Polymer Photo-reactive monomer

Photo sensitisers

Photo initiators

Imaging agent

Fillers and adhesion promoters

Plasticisers, etc.

Dry Film Resist Component Properties

Acrylic Oligomer (Monomer) Resolution

Resist Profile

Sensitivity

Stripping (Size of stripped flake)Photoinitiator Resist Profile

Sensitivity

ResolutionImaging Reagent Latent Image (Contrast)

Additives Adhesion Resist stability


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Acrylic Polymer (Binder Polymer)

The polymer is the backbone of the resist and gives physical strength to theresist. The main chemical composition is a copolymer of acrylic or methacrylic

esters with acrylic or methacrylic acid. The molecular weight of the polymer is

a maximum of 250,000.

H H H H I I I I

- CH2 C CH2 C CH2 C CH2 C I I I I

O CO CO CO I I I I OR OH OR OH n

R =Alkyl group

n= Polymer chain length

The ratio COOH groups to the alkyl groups determines the ease of

development and stripping. It will also determine the chemical resistance andin particular the resistance to alkaline etchants.

The carboxylic groups within the polymer chain will react with either SodiumHydroxide (NaOH) or Potassium Hydroxide (KOH) to make the polymer

water-soluble. Hence, the nomenclature aqueous type resist.

Reactive Monomer (Oligomer)

The monomers may be a mixture of mono or polyfunctional acrylates. Themolecular weight of the monomer will affect the odour of the resist, the rate of

polymerisation (i.e. the speed of exposure), and other physical properties ofthe resist.

These monomers are dispersed within the binder polymer mixture such thatwhen the reaction takes place it forms a highly interconnected three

dimensional network of polymers. This then forms a very strong and flexibleresist.

After exposure up to 30-40 % of this monomer is left unpolymerised. In certaincases this may form stickiness of the resist after development and if the

panels are stacked on top of each other they may stick together.


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Imaging Agent

The blue or green colours are the most commonly used colours as theunaided eye judges these to provide the best contrast to the pinkish colour of

the copper surface. Normally a triarylmethane dye such as Crystal Violet or

Malachite green is used. These products are used in the resist formulation intheir Leuco or white form.

The Leuco form oxidises when exposed to ultraviolet radiation to form a

coloured compound and gives the printout image which is essential inassuring correct registration of the phototool to the drilled holes.

Dyes that provide a latent image when exposed are referred to as photo-chromic. These dyes must have a low absorption of light in the spectral range

of sensitivity of the dry film in order not to reduce the radiation energyavailable for resist polymerisation. In some formulation of resists the dyes

used work with a synergistic effect to help initiate the resist polymerisationthrough electron transfer mechanism. Dyes of this form include xanthene andacridinium compounds. The peak spectral absorbency of PHOTEC dry film

resist is 365 nano-metre.

Photo sensitisers and Photo Initiators

In order to polymerise the photoresist must contain some light sensitivemolecules that are able to convert the absorbed energy into a form that

enables the functional monomers to polymerise.

There are two basic types of photoreactive chemicals used in photoresists.

Photo sensitiser: The photo sensitiser absorbs the energy and

transfers it to another molecule, which forms the primary reactivecomponent. The photo sensitiser is not consumed or structurallyaltered and could be considered a photo catalyst.

Photo initiator: A photo initiator is the additive present which

facilitates the initiation reaction. In the case of PHOTEC, the photoinitiator absorbs the radiation emitted by the high pressure mercury

lamp or light in the wavelength of about 365 N.M. and then changes toform the reactive species - "free radicals". The photo initiator isconsumed in the polymerisation reaction.


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Mercury lamps were the first types of lamps to be used in exposure systems.The wavelength of 365 N.M. was originally used because it is the principal

emission of the lamp. Molecules known to respond to radiation of thiswavelength are of many structures and numerous variations. However, themolecules in common commercial use are relatively few. This is because

some of the important considerations for obtaining efficient photo initiation are

wave length sensitivity, relative absorption coefficients, quantum yields, otherreacting species and any side reaction products that do not stoppolymerisation. Typical molecules that are used include:

4,4' Bis (Dimethylamino) bezophenone (Michlers Ketone)

N- aryl - - amino acids

Phenyl imidazolyl dimers

Benzophenone derivatives (no longer used)


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Resist: General Formulation Principles

Increase resolution by

Reducing the molecular weight of polymer.

Increasing the number of carboxylic acid groups on the polymer

chain.

Enhance resist adhesion and provide a wider operating window by

Increasing the glass transition temperature (Tg) of polymer.

Increasing the cross-link density at polymerisation.

Increase conformance by

Decreasing molecular weight of polymer.

Low contamination in the electrolytic solutions and a smaller resist foot, is

achieved by

Decreasing molecular weight of polymer.


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Substrate Preparation

Drilling

Deburring

General

Base Copper Laminate

Electroless Copper

Electro Deposited Copper

Direct Metallisation


Electroless Copper

Electro-deposited Copper


Pretreatment Methods

Abrasive Brush

Brush Pressure, Brush Footprint

Surface Condition of Brushes

Water Placement onto Brushes

Pumice Brush

Spray Pumice

Chemical Clean


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SUBSTRATE PREPARATION

Prior to the final surface preparation and lamination, drilling and deburringmay impact the properties of the dry film resist.

DRILLING

Burrs or holes with jagged edges are caused by drilling and may effect the

tenting property of the resist. Burrs are caused by three major factors:

An incorrect drill speed/feed ratio. (The penetration of the drill into thebase material is too fast for the speed of rotation of the drill bit.)

The drill bits are not sharp or have chips in the cutting edges. Choice of the wrong type of support board on the exit side of the drill

stack.

DEBURRING

Deburring is an operation that is necessary when the drilling operation lackscomplete control.

Deburring is a mechanical operation. If not carefully controlled, deburring may

cause deep scratches on the substrates surface and lead to resistconformance issues. The periphery of a drilled hole may be damaged giving adish down on one side of the hole that may lead to conformance or tenting

breakdown.

Substrate preparation is a critical stage prior to the lamination of thephotoresist. The photoresist requires a clean micro roughened surface with ahigh surface energy in order to obtain good adhesion. Unless the copper

surface is correctly prepared, there is a great potential for poor photoresistadhesion.

Mechanical and chemical bonding are two adhesion mechanisms that areimportant in obtaining the optimum properties of the resist. Immediately after

lamination the initial adhesion mechanism is mechanical; after about fiveminutes the resist starts to form a chemical bond.

PHOTEC resists experience a covalent chemical bond that is formed betweena hydrogen atom from the acrylate and the clean copper surface. There are

resists on the market that contain either a sulphur or nitrogen compound toincrease the adhesive forces. In the case of sulphur compound compounds

such as benzotriazole are used.


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To obtain the ideal adhesion values of the dry film resist to the substrate, a

three dimensional, highly micro roughened surface must be generated duringthe pre-cleaning process. This surface allows for the maximum number ofanchorage points required for mechanical interlocking of the photoresist to the

copper substrate. The surface must have a high surface energy for the correct

chemical bond to form between the resist and the substrate.

Checks should be made after pretreatment to ensure that the preparedsurface supports a film of water for at least 30 seconds. This is also known as

the water break test, as failure to hold the water on the surface indicates thatoils or grease remain and should be cleaned. Good water rinsing and drying

after the pretreatment operation is essential. This will ensure completeremoval of any chemical, pumice or copper dust left on the surface and in theholes. If water remains in the holes during lamination this water tends to form

steam which can cause a pressure build-up in the hole resulting insubsequent tent failures.

The pH of the water used for rinsing should be controlled so that it is eitherneutral or very slightly acidic, pH 6-7. Dry film adhesion is affected by the

surface pH of the substrate. An alkaline surface will give rise to less adhesionforces and may result in lifting of the resist. A slightly acidic surface gives

ideal adhesion forces. If the substrate surface is too acidic then resist lock-inmay possibly occur.


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GENERAL

All panels should be free from grease, oil, fingerprints and uncontrolled

oxidation. Panels should be handled using clean, lint-free gloves that arechanged regularly. (See Appendix 3 for specification of panel surface.)

There are basically four types of copper surfaces that are used in circuit

formation.

Base copper laminate

Electroless copper Electro deposited copper

Direct metallisation


A base copper laminate is an electro deposited copper foil that has been

deposited without additives and therefore the crystal structure of the copper iscolumnar. The surface is protected from oxidation by an anti-tarnish, usually a

chromate layer.

Electroless Copper

Electroless copper is normally fine grained but is dependent on the

complexing agent used in the chemical formulation of the deposition process.

Electro Deposited Copper

Electro deposited copper is either a lockin plate of about 5 microns in

thickness or a full deposit of about 2530 microns. These deposits can beeither fine grained equiaxed or lamellar in structure dependent on thechemical composition of the deposition process and the equipment used for

electro deposition. Electro deposition processes are available to deposit fine-grained matte deposits which are ideal for direct lamination of dry film resists.


The surface characteristics are dependent upon the direct metallisationprocess being used. There are several different types of direct metallisation,

including:

Conductive Polymer

Carbon or Graphite

Palladium Chloride

Palladium Sulphide


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BASE COPPER LAMINATE

Base copper laminate is used for innerlayers of multilayer and single-sidedboards or double-sided non-plated through boards. As delivered, the copperfoil is often protected by a chromate or zinc passivation to prevent copper

oxidation. To obtain optimum adhesion this passivation film must be

completely removed.

Accepted pretreatment methods include: Abrasive Brush

Pumice Brush

Spray Pumice

Acid clean Chemical clean followed by a microetch or combined

cleaner/microetch.

Care should be taken since the thickness of the substrate may be as low as

0,1mm. Any bending or creasing of the substrate will give rise to rejects dueto poor conformance of the dry film at the lamination stage.

The pretreatment method should also take into consideration the possible

stretching of the substrate under the influence of mechanical stressesproduced by abrasive brushing etc. Any stretching of the substrate will giveregistration problems at the exposure stage. (See Section on pretreatment

methods for guidance on these processes.)

Electroless Copper

The type of electroless copper process will influence the adhesion of thephotoresist. It will also depend upon whether anti-tarnish is used after theelectroless copper.

In general, surfaces with EDTA-based electroless coppers are somewhatmore difficult to achieve good post-lamination adhesion than Quadrol-based

(Ethylene dinitrilo-tetra-2 propanol) electroless coppers. This is due to thedifferent crystal structures obtained from the two different processes. The

EDTA-based process provides a finer grained, higher crystal densitystructure.

In many electroless copper processing lines an anti-tarnish stage is used toprevent unwanted oxidation of the copper. Alkaline anti-tarnish formulations

have a tendency to provide worse adhesion of the photoresist than thosebased on mineral or organic acids. This is due to the fact that resists adherebetter to a slightly acid surface than an alkaline surface, even after thorough

rinsing the surface pH may follow the pH of the last processing stage.Although dry film resists will adhere to an electroless copper surface it is

normal for these surfaces to be either pumice brushed or chemically treatedprior to resist lamination.


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ELECTRO DEPOSITED COPPER

If either full panel plating or lock in plate is carried out prior to laminationthree problem areas have been identified.

Uneven deposit (i.e. greater deposit thickness in the high currentdensity areas), provides unequal pressure across the board at thelamination stage, resulting in poor adhesion in the centre of the panel.

Unless the boards are pre-treated effectively prior to lamination thesurfaces may be too smooth to obtain good initial, post lamination

adhesion.

Under exceptional circumstances the electrolytic copper deposit maycontain a high concentration of grain refining additives.

Grain refining additives contain sulphur and nitrogen compounds similar tothose used in certain dry film resists imparting adhesion to the substrate. The

electrolytic copper additives may give rise to two side effects.

The high concentration of additives may interfere with the resist-to-

substrate adhesion mechanism.

The additives may chemically bond with the dry film resist and increasethe adhesion to such an extent that stripping of the resist may become

a problem.

The pretreatment used for these panels must take the above points intoconsideration.


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DIRECT METALLISATION

Each type of direct metallisation requires different pretreatment methods.Carbon and graphite processes require that the carbon or graphite be

removed by an aggressive microetch and the surface protected by an organic

anti-tarnish prior to resist lamination. Unless the carbon or graphite iscompletely removed, resist adhesion failures may occur. (Note: Carbon

products are used as lubricants in other industries.) The anti-tarnish normallybased on benzotriazole is used not only to prevent oxidation of the copper

after the microetch but also to assist with adhesion of the photoresist.

As its final chemical stage in the metallisation sequencepalladium sulphide-based processesinclude a microetch which removes the palladium sulphidefrom the copper surfaces. Therefore, it is essential that no additional etching

or mechanical methods are used in the pretreatment process prior tolamination. Additional microetching may cause ring voids at the final copper

electrodeposition stage.

Conductive Polymer

The surfaces that have been treated with direct metallisation using a

conductive polymer, do not normally require more surface preparation otherthan a chemical clean prior to lamination. If, however, the surface is oxidised itmay be treated with a combined cleaner/microetch. The conductive polymer is

only adherent to the dielectric substrate and not the copper surfaces.Therefore, the copper surfaces may be microetched with no problems.


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PRETREATMENT METHODS

The four major methods used for surface preparation require differentequipment. Although each method provides the basic requirement for resist

adhesion, each yields different surface topography that may affect the final

circuit requirements.

Abrasive Brush

The abrasive brush is one of the oldest methods of cleaning copper prior toresist application. It is a simple method that uses a rotating abrasive brush to

mechanically remove oxides from the copper surface. However, successfulpretreatment requires that the following control parameters be taken intoconsideration.

Grade of abrasive used for the brush

Brush pressure and the resulting brush footprint Surface condition of the brush

Placement of water spray onto the brush

Abrasive brushes are by definition brushes that remove surface imperfections

and the base copper by mechanical means. The result is that the surface ofthe copper will be degraded to an extent that depends on the grade of

abrasive used. Abrasives used in the brush manufacture are normally basedon silicon carbide that has excellent abrading power and long life. To obtainthe surface finish required for resist application, the particle size of the silicon

carbide must be chosen carefully.

To achieve good resist conformance, a controlled surface topography isrequired. This enables excellent adhesion without problems of non-conformance.

The grades of brush recommended are a combination of brushes with two

different particle sizes. For the first stage it is recommended that a 320 gritsize is used to remove all oxides, surface imperfections etc., followed by a600 or 800 grit brush to smooth the surface to the final roughness

recommendations.

By removing surface imperfections, a surface finish with controlledtopography is provided with a surface energy that enables the resist to haveinitially good mechanical adhesion. The resist may then form a chemical bond

to the surface.


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Brush Pressure, Brush Footprint

The actual pressure applied to the brushes is dependent on the thickness ofthe panel and also the width of this panel. The pressure is normally controlled

by a torque metre fitted to the motor that turns the brush. Actual pressure

applied is normally controlled by the brush footprint, which is set at between10 and 15 mm. This brush footprint is the area of the brush that is in contactwith the surface of the panel at any time.

Surface Condition of Brushes

The brushes will wear with the number of panels processed and must beinspected for evenness of wear. It is recommended to position the panelsalternatively across the total length of the brush to ensure even wear over the

brush. If this is not carried out then uneven wear will result in a possiblepretreatment difference across the panel being processed.

Although copper is considered a relatively soft metal it becomes much harderas it is subjected to mechanical forces. Any copper particles that remain in the

surface of the brush will result in a deep scratch in the panel being processed.

Water Placement onto Brushes

During use the brushes are sprayed with water to both keep the temperature

constant and to remove copper particles from the surface. It is essential thatthe water be sprayed onto the surface of the panel at the point where the

actual brushing action takes place.

Abrasive Brush Pretreatment


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Pumice Brush

Pumice brush is a means of cleaning a copper surface by using a suspensionof pumice in water which is brushed onto the surface by means of a nylon

brush. If properly controlled, it does give a three-dimensional structure on the

copper surface that is suitable for resist application.

The pumice brush stage may be one of three different combinations.Specifically, there are three abrasive media that are used in combination with

a nylon brush: normal pumice, white pumice, and an aluminium oxide that isused as a replacement to pumice. Normal pumice is a grey/cream colour and

comes in various particle sizes. Typically 3 ON grade are used but this canvary between ON and 6N.

Pumice is a natural ore product that is ground and sieved with different meshsizes to produce the different products. Due to the fact that it is a natural

product, the particles are of random shape and with different angularconfiguration. It must be noted that the grade of pumice is normally controlledby passing through different sieve sizes. Checks should be made to

determine the minimum particle size and concentration contained in theavailable product. A high concentration of small particles does not give the

required surface topography.

Pumice is relatively soft and during use the angular shapes are converted to a

rounded structure. Changes in angular structure impacts its effectiveness asan abrasive medium. The action of the brushes with the abrasive particles

remove any inorganic contamination from the copper surface and leaves the

copper in an active state with a surface that is ideal for resist lamination.

In practice pumice is used in 1218 % weight/volume pumice-to-water ratio.This pumice concentration must be checked at least every working shift and

adjusted if necessary to the correct concentration. Since copper is removedduring the brush/pumice operation, the copper concentration in the pumiceincreases with use. This copper concentration must be controlled and the

pumice changed regularly to ensure constant quality. Commercial equipmentis available to reduce the copper particles in the pumice mixture.

The white pumice or B grade is a harder material with a more consistent

particle size.

Pumice in a particle form may be trapped in through-holes. This entrapped

pumice must be removed by either high pressure water rinsing or by the useof ultrasonics. It must be remembered that high-pressure water is onlyeffective on the surface of the panel, actual pressure of the water in any

through-hole is much lower than the force on the surface.

If pumice brush is used it is essential to remove all particulate matter from thepanel surface prior to lamination otherwise conformance of the resist willresult. Poor resist conformance will give rise to either open or short circuits

dependent on which part of the circuit the pumice particle is trapped or the


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As with all mechanical methods, only inorganic contaminants will be removed

from the copper surface. The method will not remove oils or grease from thesurface.

Aluminium oxideis an ideal replacement for pumice. It is harder with a more

angular particle. Being harder it has a longer working life. The majordrawback, however, is that aluminium oxide is more expensive and does givemore wear on the equipment. Spray nozzles must be changed to allow thismaterial to be used. The same guidelines presented for pumice should be

noted for aluminium oxide.

Pumice Brush Pretreatment


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Spray Pumice

Spray pumice a common method used in the PWB industry. The methodemploys pumice that is sprayed under pressure onto the copper surface. The

mechanical action of the pumice under pressure removes all inorganic

impurities and also copper from the surface. Nozzles must be checked toensure even application of the pumice over the entire surface otherwise resistadhesion may become a problem. All guidelines presented for thepumice/brush operation should be followed for the spray pumice.

Chemical Clean

Provided that the chemistry is well controlled, the chemical cleaning method is

the preferred method of cleaning the copper surface prior to resist application.This method eliminates the possibility of particulate matter on the surface

which may become entrapped under the laminated resist.

There are two different chemical cleaning options:

A two-stage process (cleaner, followed by microetch)

A combined cleaner/microetch

Either option will achieve a clean micro-roughened surface.

The thickness of copper removed during the etching process must be

controlled for consistent performance. Although the actual amount of copper

removed during processing is dependent on the process chemistry, it isnormal to remove about 0,61,0 micron of copper. Technical data suppliedfrom the process supplier must be observed for optimum surface topography.

The temperature of the process and the dissolved copper content determinesthe actual crystal structure of the copper surface. As the copper concentration

increases in the microetch, the crystal structure changes from an angularstructure to a more polished rounded crystal structure. It is the angular form ofthe crystal that is required for optimum resist adhesion. Good water rinsing is

required to remove all chemical products from the surface before drying andresist application.

In practice there are three different oxidising chemicals used for the microetch

Hydrogen peroxide

Sodium or potassium persulphate

Sodium or potassium monopersulphate

Hydrogen peroxide is the least expensive material and has the highest weightper volume of oxidising power, however unless carefully controlled it does notproduce the ideal crystal structure. Various additives are available to control

the stability of the hydrogen peroxide and each may have different effects on

the crystal structure produced.


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Normal persulphate is the most commonly used material for the microetchformulation. However, monopersulphate is preferred for the ideal surface

topography.

Chemical Clean Pretreatment

Both spray pumice and cleaner/microetch preparation prior to resist

application produces a homogeneous three-dimensional microroughendsurface. For high density fine line circuitry a properly controlled chemical

cleaning method is the preferred method of pretreatment since it does

eliminate the possibility of particulate matter from the copper surface.


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Lamination

Preheat Temperature

Rollers

LaminatingTemperature

Pressure

Speed

Hold Time after Lamination


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PHOTEC*

Dry Film Photoresists

Lamination


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LAMINATION

Lamination is the application of the dry film resist to a properly prepared

substrate.

The lamination process must be carefully controlled to ensure that therequired mechanical adhesion of the resist to the substrate is obtained byflowing the resist into the surface irregularities. The resist should not flow too

much into any drilled holes or slots. Over flowing of the resist will causethinning along the periphery of holes and result in tent breakage. Therefore, acorrect balance of all the lamination parameters is crucial to ensure optimum

performance of the resist.

There are different types of proprietary laminators on the market ranging frommanual to fully automatic and from hot rollers being heated by resistanceheaters within the roller to indirect heating of the rollers by infrared. In all

cases the recommendations of the manufacturer must be followed. The basicprinciple of the operation is to preheat the resist to a temperature of 110 +/-

100C to lower the viscosity of the resist just prior to application, underpressure, to the substrate.

A normal sequence of operation is: - Preheat substrate (40-500C)

Heat photoresist (110 +/- 100C)

Apply resist by roller pressure to the substrate. Typically a

pressure of 2-4 Kgf/cm2 is applied.

Lamination speed 1,0-3,0 meters per minute

Although preheating the substrate is not essential, it does ensure that the coldsubstrate does not act as a heat sink and thus reduce the actual temperature

of resist at the lamination stage. If it were to do so it would affect the adhesionand conformance of the resist to the substrate.

There must be a correct balance of lamination temperature, pressure speedand resist tension to ensure that maximum resist adhesion and tenting ability

from the photoresist is obtained.

During lamination the resist is heated on one side and is laminated onto the

cooler substrate surface, thus a temperature gradient exists through theresist. The lower molecular weight and lower boiling point fraction of the

chemicals within the resist will migrate to the cooler surfaces. A hold time afterlamination prior to further processing is to ensure that mobile chemicalsequilibrate with the higher molecular weight fraction.


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Preheat Temperature

The actual preheat temperature required is dependent upon both thethickness of the dielectric and the copper on the surface. As the thickness of

either or both increase, the greater the thermal heat sink. In order to obtain a

surface temperature of 40500

C, the preheat temperature must be adjustedaccordingly.

If the preheat temperature is too over 550C, wrinkles may occur during

lamination. When using an automatic cut sheet laminator, wrinkling isparticularly predominant at the edges of the panel. Thinning of the resist at the

periphery of holes or slots can occur if lamination speed is too low or thelamination pressure too high.

If the preheat temperature is too low poor resist adhesion immediately afterlamination will result. Resist conformity to the substrate, especially in deep

and narrow areas, will be imperfect at best.

Rollers

The condition of the rubber on the rollers is important to ensure a constantpressure over the entire panel. Any imperfections in or on the surface willappear as defects on the laminated resist. A cut or a piece of rubber removed

from the roller will give a lower pressure at that spot during lamination and willin severe cases show up as a blister on the surface of the laminated resist.

The hardnessof the rubber should be about 65 Shore hardness. If it is harderthan this then the resist will be pushed into the holes or slots on the panel. If

the hardness is too soft poor conformance will result. The thicknessof rubberon the rollers should be as recommended by the equipment manufacturer.

Care should be taken when re-coating any rollers since the removal of therubber is normally mechanical and at this stage the steel shaft is reduced in

diameter. To obtain the required outside diameter of the roller, a thickerrubber will be applied. Rubber is a poor conductor of heat and therefore, the

actual transfer of heat from the heating elements inside the roller will be lessthan normal.

During lamination of a batch of boards the roller may not be able to maintainthe correct lamination temperature. On most laminators the heating is

accomplished by a resistance heater located within the steel core of the roller.To ensure that the heat is transferred from this element to the actual roller aheat transfer gel is used. Unless this gel is evenly coated around the heating

element, irregular heat transfer will occur along the length of the core andaround the diameter. All rollers should be checked whenever they are

changed and also on a regular basis. If there are major temperaturedifferences on the roller defects will occur. The two heated rollers should bechecked periodically to ensure that they are parallel with each other.


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If pressure is applied by air at both ends of the rollers and narrow width

panels are laminated frequently, a bow may form on the rollers. If thiscondition exists, pressure applied to the centre of the panel will be less thanthat applied to the centre. In severe cases there is a possibility that poor

adhesion of the resist to the substrate will result.

Laminating Temperature

During lamination the resist must be heated above its glass transition

temperature to make it semi-liquid and hence be in a state to be pressed intosubstrate defects by roller pressure. Although the glass transition temperature

is about 350C, the resist must be heated to a higher temperature to accountfor cool air and the heat sink effect of the substrate. Heat must pass throughthe polyester support film prior to heating the resist. It has been shown that

the roller temperature, measured by a contact temperature-measuring probe,should be 105-110 0C to provide optimum flow characteristics of the resist. On

many laminators the hot rollers are heated by resistance elements in the coreof the roller. If the contact gel used to transmit the heat from these elements(firstly to the steel core and the rubber coating) is not operative, the heating

will be much slower and the rollers may not return to the set point betweenlamination of subsequent panels.

The temperature indicated on the read out on the laminator is via a contactprobe situated at one end of the roller. If this contact point is dirty or loose it

will lead to an incorrect read out temperature.

Pressure

The dry film resist is heated to about 1100C so that it becomes semi-liquid

and will flow under the influence of pressure. The pressure that is applied tothe rollers is to ensure that the dry film resist is forced into the micro

roughness and surface defects that are present on the copper surface.Unless the pressure is sufficient to enable this action to take place, the resistwill not have the necessary physical properties to withstand subsequent

processing.

If insufficient pressure is applied, poor conformance of the resist to thesubstrate irregularities will result. Development, etching or electroplating

chemicals will penetrate under the resist resulting in rejects being produced. Ifthe appliedpressure is too high, the resist will be forced into any holes thatrequire tenting and the tent strength will not be sufficient to withstand

subsequent processing.

Pressure must be sufficiently high to enable the resist to flow into the macro

roughness of the panel formed by the different thickness of glass fibres usedin the construction of the dielectric substrate. A normal pressure range of 3-5

bars is used depending on the type of laminator.


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Speed

The resist requires a finite time to flow into the irregularities of the substrateunder the influence of temperature and pressure. The actual speed of

lamination determines the time of applied pressure.

The lamination speed is adjusted to give optimum conformance and at thesame time the productivity that is required. Lamination speed is 1,0 to 3,0metres per minute.

The overall lamination parameters are a balance of temperature, pressure,

speed and substrate thickness. Substrate thickness is important since this willimpact the amount of heat required to obtain the correct temperature on thepanel.

If the speed is too highthen the resist will not have sufficient time to flow into

surface irregularities and hence poor conformance. If the speed is too lowandall other parameters are at optimum, the resist will flow into tented holescausing tenting failures.

Hold Time After Lamination

After lamination the temperature of the laminated substrate must cool to roomtemperature as quickly as possible.

The resist is made semi-liquid at the laminating stage and as such will

flow into surface irregularities. Once laminated the resist must be cooled

quickly to prevent continual flow into holes that require tenting. If theresist continues to flow into the hole the resist thickness around the

periphery of the hole will thin and may not have sufficient mechanicalstrength to withstand subsequent processing. A tent failure will result.

The resist must stabilise prior to subsequent processing. The photo

sensitisers and photo initiators move during the time that the resist isexposed to ultraviolet light and continue to move for a period after thelight is switched off. The polymerisation of the resist will stop once the

molecules are at rest. The higher the temperature of the resist, thefurther the molecules will travel. This is why the resist should be cooled

as quickly as possible.

The minimum hold time after laminationis the time taken for the resist to

cool to room temperature.

The maximum hold time after laminationis normally four days. However,to hold the boards for an extended time the panels should be covered inblack opaque plastic. Yellow light does contain an element of visual light

that dry film resists are sensitive. In the worse case, polymerisation willoccur and prevent development of the resist.


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Exposure

Equipment

Lamp Type

Illumination Intensity

Exposure Energy

Phototool Quality

Degree of Collimation

Vacuum Delay

Exposure Unit Temperature


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EXPOSURE

The exposure process is the polymerisation of the oligomers/monomers in theresist chemistry. Polymerisation is accomplished by exposing the resist to a

known amount of ultra violet radiation.

The following sections detail eight critical areas that should be taken intoconsideration to ensure successful exposure.

Equipment

There are several different types of exposure units used in the industry whichrange from single-sided exposure manual printers to double-sided exposure

fully automated types. Different types of light reflectors are used with varyingefficiency of light transfer. The height of these reflectors above or below thepanel to be exposed will determine the light energy reaching the panel. This

will directly impact the rate of exposure. The quality of the reflectors willdetermine the ENERGY DISTRIBUTION across the exposure frame.

Light will not be reflected evenly across the frame leading to resist exposuredifference across the imaged panel. On some older manual exposure units

the difference in light energy across the exposure frame can be as high as40%. This does mean that we can have a difference of up to one step on a 21Step tablet across the frame.

Lamp Type

PHOTEC photoresists have a peak spectral absorption of about 360manometers. Therefore, the lamp used must have a peak spectral output at

the same wavelength. If there is a mismatch between output of the lamp andthe absorption characteristics of the resist there may be incorrect exposure,

even if the required number of millijoules is applied. In general, the type oflamp used is a high pressure mercury lamp. To obtain the requiredwavelength output the mercury is normally doped with small quantities of

iron.

Lamps should be changed regularly as the efficiency of the lamp changeswith age. Regulating the current that is applied to the lamp controls the light

output. In addition, spectral output may also change. It is common practice tochange the lamps after 1000 hours of use. A meter is normally incorporatedin the machine to record the time that the lamp is used.

Some lamps are cooled with a water jacket around the lamp. This coolingdoes prevent heat build up within the exposure machine but it also reduces

the amount of energy transmitted. In severe cases this may increase thelength of exposure time to reach the desired number of millijoules.


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Illumination Intensity

A short, high illumination intensity is preferred to a longer, low intensityillumination. The optical density of the resist changes during exposure. Since

the resists are of the surface polymerisable type, the surface layer of the

resist will consume energy, resulting in reduced energy passing to the lowerlevels within the resist. Therefore, unless there is sufficient energy intensity,most of the energy will be consumed as it passes through the resist and theresist at the copper interface will not be exposed sufficiently.

During normal exposure we will see an exposure difference between the

resist on the surface and that at the base of the resist. The difference inexposure can be two steps on a 21 Step Density tablet. Two steps are areduction of 50% of the light energy.

Exposure intensity is measured in milliwatts.

The actual time of exposure in seconds is millijoules. milliwatts

Millijoules = milliwatts x time

Exposure Energy

Each resist requires a certain amount of energy to reach the optimum

exposure state. The amount of exposure will determine the actual chemicalproperties of the resist. In all cases the technical data sheet of the resist

should be followed.

If the exposure energy is too low, then the resist will be attacked by both the

developing solution and the subsequent processing chemicals. The adhesionof the resist can be affected leading to open circuits after innerlayer etching

and short circuits on electroplated circuits.

If the resist is exposed with too high an exposure energy, the image on the

phototool will not be transferred in a 1:1 ratio. This means that the exposedtraces will be wider than on the phototool.

Phototool Quality

Phototool quality is extremely important for high circuit density products.

Image density ( Dmax) should be in excess of Dmax3,5. Edge definition of theopaque areas should be sharp and well defined. Diffused image edges canlead to variation in track widths after development or incorrectly exposed

edges which may cause problems in the development, electroplating oretching processes.

Background clarity of the clear areas of the phototools is essential to ensureshort exposure times. A high background density may increase exposure

times by up to 100%. If not corrected to compensate for high background


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The use of phototool emulsion protection systems extends the life of the

phototool. When setting exposure times the thickness of the protective coating(normally 3-5 microns) must be considered to ensure accurate line widthreproduction. This thickness increases the off-contact distance from the resist

and thus makes the beam collimation of the exposure unit more important. If

the exposure unit has poor beam collimation, the use of a phototool protectivesystem will be dependent upon the permitted deviation from the normal 1:1reproduction of the phototool.

Degree of Collimation

To obtain accurate phototool reproduction, the exposing energy (light) whichis hitting the panel at right angels to the resist is required. This light should beevenly distributed across the exposure frame.

Collimated light is defined as light that is very close (90 degrees) to the panel

surface. Normally the angle of declination is about 0,5 degrees. It must benoted that the higher the degree of collimation, the more dirt and scratcheswill have on the phototool. These defects will be reproduced on the exposed

panel.

There are several factors that can influence the angle at which the light hitsthe panel being exposed. The light emitted by the exposure lamp passesthrough several different layers before it actually reaches the photoresist

These include the glass or polyester vacuum frame, phototool, phototoolprotective layer and the polyester cover sheet on the resist. All of these layers

can give diffraction of the light.

Any air trapped between any of the layers will also cause light scatter. This air

gap can lead to off contact printing. The actual print out image will bediffused and lead to rejects.

Vacuum Delay

A vacuum delay is necessary to allow time for all the air between the variouslayers to be removed. We need the closest contact possible between the

phototool and the photo-resist, the vacuum delay allows the time for the air tobe removed and for the phototool to pull down onto the resist surface. A

typical vacuum delay is about 10 seconds but for high circuit density boards orhigh-resolution boards then the vacuum delay is increased up to 30 secondsin severe cases.


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Exposure Unit Temperature

Heat is a key factor during exposure. Polymerisation reactions started by ultraviolet light can be continued by heat energy and continue after the light

energy has been stopped. This may result in over exposure of the resist even

if the number of millijoules applied is correct. The actual degree of overexposure caused by the heat in the exposure unit will depend on the actualtemperature and the inhibitors used in the formulation of the resist.

The exposure unit should have sufficient airflow within the unit to remove theheat emitted by the lamp and to maintain the temperature within the unit at or

near room temperature.


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Development

pH concentration

Resist Loading

Temperature

Development Time (breakpoint)

Spray Nozzles

Antifoam

Filtration

Water Rinsing


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PHOTEC*

Dry Film Photoresists

Development


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DEVELOPMENT

Development is the removal of unexposed portions of the negative workingresist. The development stage is critical as it determines the quality of the

resist remaining on the surface in terms of track profile, adhesion, etc.

As circuit density increases, the track width becomes smaller and moreclosely packed. As such, the development process becomes more important.On exposure the resist is polymerised and this alters the dissolution kinetics

between the exposed and unexposed resist.

The development mechanism is a diffusion controlled process, that is thedeveloping solution penetrates the unexposed resist and partially removes theresist in the form of a colloid of binder polymer carboxylate salts. This layer

must then be removed by mass transfer of developing solution on the resistsurface and mechanical action by spray pressure before the next layer can be

attacked.

Eight key factors must be taken into consideration to achieve the correct

development action, cleanliness of the developed surface and optimum trackprofile.

pH concentration of the developing solution

Resist loading within the developing solution

Temperature of the developing solution

Development Time (breakpoint): removal of the resist Spray nozzles (type, volume of solution, spray impact)

Antifoam type and quantity

Filtration

Water rinsing (time and temperature)

The above factors demonstrate how the mechanical aspects of thedevelopment machine and the chemical control of the process are extremely

important. Equipment design is very important and often the dry film resistsupplier has to use equipment that is already installed. However, the resistsupplier must ensure the following points so that resist performance is not

jeopardised.

The spray from the nozzles is effective and even over the entire boardsurface. Development is even on the upper and lower board surfaces.

The spay impact pressure is sufficiently high to remove unexposed resistfrom within fine lines and spaces and yet is not too high to break tents,

particularly those over oblong holes.

There is no uddlin on the u er surfaces of the anel.


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If more than one chamber is incorporated in the development part of the

machine, the space between the chambers should be clean and theconveyor mechanism should remain wet. This can be achieved by usingmisting nozzles.

Rinsing is an integral part of the development mechanism. Ideally, therinsing time should be 50 % of the development time.

Drying reduces swelling of the resist that occurs in the development stage.If wet resist passes directly to a cupric chloride etching solution, increasedorganic contamination may occur in the etchant.

The development mechanism is a diffusion controlled reaction, The sodiumcarbonate reacts with the carboxylic (acid) radicals in the unexposed resistand solubilises the resist. This carbonate resist mixture must be removed with

fresh sodium carbonate before the reaction may proceed.

Dry film resist is not dissolved in the developing solution, but is held insuspension in a colloid form. Any mechanism by which this colloid is brokendown can lead to scum formation on the panel surface. It may also appear as

an oily substance on the solution surface. These mechanisms includemechanical stress created by sheer forces in the pumps, spray nozzles, anti-

foams, dry film resist load in the development solution, etc.

If the colloid is destroyed then a scum will deposit in the developing solution

and an oily substance may be seen floating on the surface of the developing

solution.

COOH COONa

COOH + Na2CO3 COONa

COOH COONa

Carboxylated Polymer Sodium Salt

(Colloid)


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pH Concentration

The concentration of sodium carbonate used for development must be withinthe range specified for each dry film resist. If the concentration is too lowthe

unexposed resist will not be completely removed and a scum remaining on

the surface will lead to etching or electroplating problems. If the concentrationis too highthe resist will be attacked at the resist substrate interface leading topoor track profile, and in severe cases, lifting of the resist. This in turn will leadto electroplating deposits under the resist or track width reduction during

etching.

The dry film resist has acid radicals within its chemical structure. As the resistreacts with the sodium carbonate, the acid radical is neutralised and the pH ofthe developing solution will fall. If the pH of the developing solution falls below

a specified pH, the developing mechanism will cease. Therefore, it is essentialto replenish the developing solution by either pH control, conductivity, or area

of resists developed etc.

Resist Loading

The quantity of dry film resist within the developing solution is defined as

resist loading. As resist loading increases,the dissolution kinetics is changedand therefore the speed of development changes. There is also a greatertendency for the colloid resist particles to become unstable and precipitate

back onto the panel. This leads to open circuits in the case of pattern platingand short circuits in the case of innerlayer production or circuits produced by

the tent and etch technique.

The specific recommendation for resist loading will depend on the type of

circuit being produced. Circuits with lines and spaces greater than 150microns a loading of 0,4 square metres of 40 micron thick resist per litre of

developing solution is recommended. As the lines and spaces becomesmaller, the resist loading becomes correspondingly lower. For lines andspaces of 100 microns and lower, the resist loading should be less than 0,1

square metres per litre of developing solution. (See Appendix 1 for method ofanalysis for resist loading in developing solutions.)

The dry film resist has acid radicals within the chemical structure. As the resist

is dissolved in the sodium carbonate solution this acid radical is neutralisedand the pH of the developing solution will fall. A 10-gpl Sodium Carbonatesolution will have a pH of about 11,2.


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Spray Nozzles

Development is a diffusion-controlled mechanism. This means that thedeveloping solution must diffuse through the unexposed resist and through

the interaction of the sodium ion in the sodium carbonate solution react with

the carboxylic acid groups within the resist to form a colloidal particle. Thesecolloidal particles are then washed from the resist surface by fresh developingsolution. As the colloids are removed it leaves the surface in a condition formore sodium carbonate to diffuse into the resist surface. This mechanism is

repeated until all the unexposed resist is removed.

Because resist removal is a diffusion controlled mechanism, a large volume oflow pressure sodium carbonate to reach the resist surface is required duringthe first stage of development. To achieve, spray nozzles are used in the first

part of the development machine that spray a cone shaped pattern onto thesurface (i.e. cone nozzles). These cone nozzles give a low pressure on the

resist surface. It is recommended that each nozzle deliver in excess of 4 litresper minute of developing solution. (A typical cone shaped nozzle will have aspray impact of 0,4-0,5 Bar on the panel surface.)

To ensure the complete removal of resist at the base of fine lines and spaces

a higher pressure is required so that the developing solution will reach thebase of resist at the resist to copper interface. In fluid dynamic terms this is adeep recess. A nozzle that delivers a fan shaped spray pattern is used for

this purpose.

The nozzle with a fan shaped spray is referred to as a high impact nozzle and

will have a typical spray impact of 8-9 Bar on the panel surface. Equipmentdesign is important to achieve the correct development of the resist across the

entire panel. Nozzle height and the angle of the nozzle jet must be such thatall the entire panel is sprayed at equal volume and pressure and that no

overlapping of the sprayed solution occurs. Any overlapping of the sprays willreduce the effective pressure that impinges onto the surface. Prevention ofdevelopment solution puddling on the top of the panel must be prevented,

otherwise this will affect the replenishment of fresh solution on the resistsurface.

To even out the differences in solution replenishment on the top and lower

panel surfaces different spray pressures are used between top and bottom.Normally a 0,2 - 0,3 Bar higher pressure on the top spray bar is used. Typicalspray pressures for development are Top 1,5 Bar and Lower 1,3 Bar.

However, the exact spray pressure is equipment related and should bearrived at experimentally.


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Antifoam

The type and quantity of any antifoam used in the development solution will

determine the stability of the colloid formed. If the incorrect antifoam is usedprecipitation within the development solution will occur. This may lead to scumformation on the panel surface that will interfere with the subsequent plating

or etching operations. To eliminate these problems, an antifoam that is a

polyethylene oxide or polypropylene oxide block copolymer based should beused.

Siloxane based antifoamsare known to cause instability of the colloids in the

development solution and give rise to scum or sludge. Silicone basedantifoams should not be used as they may provide problems in subsequent

processing operations such as electroplating. Trials should be made with allnew antifoams to check for colloid stability prior to use on production.

The concentration of antifoam used should be minimised to limit the foambuild-up in the development solution. Actual concentration used should be

within the range recommended by the supplier.

Filtration

To prevent the possibility of resists particles or large colloidal particles being

re-deposited back onto the panel it is essential to filter the solution. The filtershould be in line between the pump and the spray bar manifold and equippedwith filter to remove particles greater than 30 microns. The filters should be

changed regularly to prevent loss of developer solution volume being suppliedto the spray nozzles. Pressure indicators should be fitted into the spray

manifold after the filters; this will give a more accurate indication of the spray

pressure.

Water Rinsing

Water rinsing after development is an integral part of the process. The type,volume and temperature of the water are important to ensure dry film resistperformance. Water with a hardness of 8 - 12 on the DIN scale should be

used. (See Appendix 4 for water hardness conversion.)

The reason for using slightly hard water for rinsing is that the divalentcautions. Calcium and magnesium converts the soluble sodium form of the

polymers present on the resist sidewalls after development into less solublecarboxylate salts. This effectively stops further development while improvingboth resolution and resist sidewall profile.

Soft water or softened water is not an effective rinsing medium. The pH of softwater is easily affected by drag-in of developer solution and in severe

circumstances may continue the development action.

The pH of the rinse water should be less than pH 9,0 and the temperature ofthis rinse water should be less than 30 0C to stop development action andprevent attack on the resist surface.


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Etching


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Photoresists are employed as an etch resist in the print and etch technology

for the production of innerlayers or in the "tent and etch" method of final circuitproduction. The etchant used in these technologies is primarily cupric chloride

or ferric chloride. Following the etching process defects can remain on thesubstrate in the form of copper spots or fine shorts.

This section details the chemistry involved with the etching process and some

of the reasons for defects. In addition, means of preventing these defects willbe provided. It is essential that care be taken to identify the cause of theproblem to enable the corrective action to be taken so that the problem maybe eliminated.

Factors Affecting Copper Etching

Substrate and Substrate Preparation

Dry film photoresist exposure and development

Cupric chloride etching solution

Substrate and Substrate Preparation

The innerlayer copper foil or the electroplated copper used in the tent andetch process must be checked for the defects listed below.

Epoxy resin through pinholes in the copper foil during lamination of thesubstrate

Copper nodules on the plated via hole boards or tent and etch panels Excessive shiny spots on the surface

Heavy oxidation of the copper surface

Handling defects such as scratches, dents or fingerprints

Incomplete removal of any anti-tarnish coating

The standard practice of using 18 micron copper foil for innerlayers and the

increasing use of 9 micron copper has highlighted the problems with pores inthese thin copper foils.

Epoxy resin

During the substrate manufacture the epoxy resin of the dielectric flowsthrough pin holes forming circular spots of epoxide resin on the surface.

These epoxide resin spots are not removed during normal surface preparation

of the copper prior to resist application. These spots may change from circularin appearance to an elongated form after any brushing operation. In general

resin spots should not exceed one spot per square metre.

Corrective Action: Incoming inspection should be able to detect this problemwithout difficulty.


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Copper Nodules

The increasing requirement for buried via holes and panel plated copper fortent and etch technology has necessitated drilling and metallisation (oftendirect metallisation) followed by electrolytic copper plating. This three step

sequence may result in both nodules and gas pits on the surface prior to

lamination. Copper nodules may result in copper remaining on the etchedsurface. Both copper nodules and gas pits may result in poor conformance ofthe resist at the lamination stage and allow the development or etchingchemistry to penetrate beneath the resist and lead to open circuits.

Corrective Action: Inspect the panels after electroplating prior to imaging. The

most probable causes of nodules after electroplating include:

Thick electroless deposits in localised areas are caused by insufficient

control of the pre-activation and activation stages prior to electrolesscopper deposition. These nodules appear after electroless copper and

produce localised high current density areas in the electrolytic copperstage producing higher than average copper deposit thickness.

Particulate material in the electrolytic copper electrolyte is co-depositedwith the copper. These particles may arise from edges of the panels

particularly if an excessive desmear operation has been carried out onan in-line processing sequence.

Gas pits may be caused by insufficient control of the additive system in theelectrolytic copper electrolyte. Localised over concentration of these additives

may result in passivation which reduces the cathode efficiency of the

deposition process and result in hydrogen evolution and a localised reductionin copper thickness.

Excessive "Shiny" Spots

The excessive shiny spots on the copper foil, as received from the laminate

supplier, are generally caused by the glass cloth used for the construction ofthe dielectric. A high localised thickness where the weft and the warp of theglass fibres cross may lead to shiny spots (depending on the diameter of the

glass fibre used in the construction) being formed after pressing the copperfoil onto the dielectric. The excessive pressure on these high spots during the

pressing operation to produce the laminate changes the structure andhardness of the copper in these localised areas. Any differential hardness orstructural differences in the copper cause changes in etch rates, often slowing

the etch rate such that reduced copper thickness spots are left on the surfaceafter etching.


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Corrective Action

After laminate preparation, check to see if pumice, pumice brush,abrasive brush or chemical pretreatment have removed these spots.

Check sprays or brush pressure. Increase pressure if necessary.

Ensure that the pumice concentration is correct; increase pumiceconcentration to 20 volume percent.

Ensure pumice suspension is acidic in nature.

Change pumice more frequently.

Check pumice grade being used.

If equipment is suitable, consider use of aluminium oxide as

replacement of pumice.

If chemical pretreatment is used, increase the thickness of copperetched to 1,0 micron.

Heavy surface oxidation

Heavy surface oxidation should be confined to those panels that have beenwet processed prior to resist lamination. If the copper oxide is not removed it

may cause a "lock in" of some resists.

Corrective Action

Check to ensure that the rinse water after electro-plating is clean and

that the panels are rinsed sufficiently.

Increase the temperature and airflow volume during the dryingoperation.

Ensure that any through holes are dry before stacking the panels


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Anti-tarnish

The anti-tarnish coating on base laminate as received from the laminate

supplier is normally of the chromate type. Excessive chromate conversioncoating may not be removed during cleaning and could either lead to a "lockin" of some resists or may etch slower than surrounding copper.

Chromate layers are normally removed using an acidic medium. Alkalinepumice spray or pumice brush operations may lead to incomplete removal ofthe chromate layer.

Corrective Action

Ensure that the pumice suspension is acidic in nature.

Change pumice more frequently as a build-up of chromate may occur

in the pumice suspension. If electrolytic methods are used to removethe chromate layer, ensure that the correct anodic voltage is applied forthe correct time

Dry Film Photoresist Exposure and Development

It is not generally recognised that the combination of exposure anddevelopment may provide problems during the etching operation.

As part of the resist matrix, a dry film resist comprises oligomers, sensitisers

and photoinitiators that during exposure to ultra violet radiation combinetogether to form polymers and stable reaction bi-products. Insufficientexposure will lead to an excessive quantity of these chemicals remaining on

and in the resist matrix. These unused chemicals react in the acidic media of

the etchant to produce an oily product which, if not removed, will float on thesurface of the etchant.

Underdevelopment of the resist leaves a scum residue on the copper surfaces

that will not be completely removed by rinsing and will prevent etching of thecopper surfaces. Underdevelopment also leaves a larger than normal "foot" at

the base of the resist track profile. This "foot" will be undercut during etchingand fall into the etchant. This again may lead to organic contamination in theetchant.

Over development provides an unstable sidewall to the resist that is attacked

by the cupric chloride etchant. This will leach out photoinitiators, oligomersand imaging agents from the resist, which unless removed by carbontreatment, will provide an oily residue in the etchant.

It is recommended that the dry film resist content of the developing solution ismaintained below 0,2 m2/ litre. If the resist content is higher there is a danger

that the resist is re-deposited in a particulate form which is very adherent tothe copper surfaces and difficult to remove by water rinsing and prevents

etching. Any solid particulate material, especially sodium carbonate, willpenetrate into the unexposed resist and prevent development of theunderlying resist.


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Rinsing is an integral part of development. Rinse stations should be at least

50% of the development stations.

Development should be maintained at 1,3 - 1,7 times that of the time forminimum development breakpoint.

Corrective Action

Check that the lamination temperature is not too high and that the hotrollers have a good thermal profile. Hot spots on the hot roller may leadto heat polymerisation of some resists. Panels should be cooled as

quickly as possible to room temperature and not stacked until thiscooling process has taken place.

Check for correct exposure level. In the case of PHOTEC resists,

ensure that the Step Tablet exposure is maintained between Step 7-9.

Ensure that the phototools are of good quality with no scratches or

pinholes in the opaque areas. The phototool should have opaque areasof sufficient density to prevent partial exposure of the resist in areas

that should not be exposed. Scratches or "touch-up" marks in the clearareas of the phototool will lead to partially polymerised resist that maybe removed during development and re-deposit back on to the panel at

a later stage of development.

The acuity (the change between the clear areas and the opaque areasof the phototool) should be a sharp boundary. If there is a gradualchange between clear and opaque areas this will lead to partial

exposure along the edges of these tracks and development problems.

In severe cases this may increase organic levels in the etchant.

Check for correct developer concentration. Follow the resist

manufacturer recommendations for concentration and developersolution temperature.

Ensure that there is no particulate matter in the development solutionby using a filter of 10-30 micron filter size

Check for the resist loading in the developing solution. (See Appendix1 for the analytical method.)

Ensure that the breakpoint of the development is correct in order to

obtain a stable sidewall of the resist

Rinse water temperature should be between 12-27 0C and rinse watertime should be at least 50% of the development time

If necessary, increase the hardness of the rinse water to 8 - 12 dH.

Reduce scum formation by using water containing less than 10mgm/litre of divalent cautions for make up and replenishment.


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Reduce the amount of antifoam and ensure that it is a compatible type

with the resist being developed. The higher the concentration ofantifoam, the higher the formation of scum and the chance of residues

remaining on the panel after development.

Clean all transport rollers to prevent any particulate matter pressinginto the undeveloped resist.

Ensure that all spray nozzles in the development section are notblocked or worn. Worn, blocked or partially blocked nozzles may cause

poor or underdevelopment.

Ensure that as the polyester foil is removed no flakes of this material

are formed that will be attracted to the resist surface by electrostaticelectricity.

Check the resist after development and drying to see if the resist is

correctly developed and rinsed. Over development or rinsing with too

soft water can leave a soft resist that can be attacked by the etchant.After development and drying, place the panel in a tank of water for

one minute. Take out the panel and hold it vertically. The water shouldimmediately run from the resist (i.e., it should immediately de- wet).

The resist should have a gloss appearance without any matte areas

Ensure that the resist is thoroughly dried after development and

rinsing. The resist swells slightly during development and will shrink assoon as it comes in contact with an acidic medium. Heating the resist

at this stage stabilises the sidewall and shrinks back the resist to itscorrect size and prevents leaching into the etching solution.


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ETCHING

The primary etchant that is used to produce innerlayers and boards by thetent and etch process is a non-proprietary solution of cupric chloride. Thisetching solution contains cupric chloride and hydrochloric acid. The etch rate

and the etch factor is determined by the concentrations of these two

chemicals in solution.

The basic chemistry of the etching mechanism is:

CuCl2 + Cu0 Cu2Cl2

Cu2Cl2 + 4 HCl 2CuCl32- + 4H+

During this reaction there is a build-up of the insoluble Cu2Cl2 (cuprous

chloride). Insoluble Cu2Cl2precipitates onto the surfaces preventing etching atthese points. Further reactions take place with the oxidant to oxidise the

cuprous to cupric chloride.

With hydrogen peroxide each gram of copper etched requires 75 ml of 100

volume hydrogen peroxide and 1 ml of 35% hydrochloric acid.

Cu2Cl2+ 2HCl + H2O2 2 CuCl2 + H2O

With chlorine:

2 CuCl32- + Cl2 2CuCl2 + 4 Cl

-

It is normal for these reactions to be controlled by the use of an Oxidation-Reduction Potential control system (ORP). However, the oxidant added to

control the copper chloride etching solution is not only consumed by oxidisingthe copper from the cuprous to the cupric state.

Organic material dissolved from the dry film resist is also oxidised and thetotal reaction consumes a higher amount of oxidant. It has been shown by x-

ray mass analysis that the oily residues present on the surface of the cupricchloride etchant is a mixture of oligomers, photoinitiators and imaging agent. It

has been determined that the oxidising agent in the etchant affects allaqueous developable photoresists. It has also been determined by

experiment that it is essential to control the Oxidation Reduction Potentialclosely as the range of eliminating or obtaining copper spots on an etchedpanel is within a range of 20mV.


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Control of Etching Solution

Maintain the oxidation-reduction potential at 540 +/- 40 mV. Below500mV the incidence of copper spots increases considerably.

Continuously carbon treat the cupric chloride etchant to remove

organic material.

Remove any sludge formed on the surface of the etching solution at

least once per week.

Clean once per week the conveyor and squeegee rollers, especially

rollers at the entrance of the etching machine.

The "oily" layer on the surface of the etching solution may be reducedby heating the developed boards to a temperature of 40-60 0C prior to

etching.

The oil-adsorbing mat (polypropylene-type) in the etching machine

should be maintained regularly.

If pumped through the spray nozzles, the oily layer forms an emulsionthat is strongly adherent to copper surfaces.

Increase the addition rate of the oxidant (hydrogen peroxide or chlorinegas) for organic material.

Ensure that the filters on the etching machine are maintained regularly.

Filters capable of removing 10-30 micron particles are recommended.

Ensure that the total etch time is at least 1,3 times the minimumetching time.

Cuprous chloride concentration increases as the oxidation reductionpotential decreases. At 540 mV the concentration is about 1 gm /l and

at 450 mV it rises to about 20 gm /l.


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Resist Stripping


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RESIST STRIPPING

The objective of resist stripping is to remove the resist from the copper panel

(including fine lines and spaces), while ensuring a non-oxidised surface.

Most alkaline type dry film resists do not dissolve in the stripping solution but

are detached from the copper surface in small flakes. This extends the

working life of the stripping solution.

Stripping solutions are normally alkaline metal hydroxides, such as sodium orpotassium hydroxide, or based on amines such as mono or tri ethanolamine

and tetra methyl ammonium hydroxide.

The stripping solution should break the polymer chain at the cross-linkingpoint of the three dimensional structure, which is formed during thepolymerisation of the resist and before the bond between the resist and the

copper surface is broken. The stripping mechanism depends not only on thecross-link density of the resist but also the number of carboxylic acid groups

on the polymer chain. Therefore, the type and concentration of strippingsolution should be optimised for each resist and these must be set to allowthe stripping solution to have time to penetrate the resist and break the

polymer chain before the resist-to-copper bond is broken.

If stripping trials are conducted in the laboratory prior to production it must benoted that the resist characteristics do change during the electrolyticdeposition of copper and tin-lead or tin. This means that any stripping trials

should be conducted on panels that have been through the deposition cycleused on production.

The trials are performed not only to optimise the stripping time and flake size,but also to set the concentration of stripping solution that minimises the

swelling of the resist in the solution. To obtain complete stripping of the resistfrom within fine lines and spaces the resist must be removed before it swells

and is trapped by mechanical forces within these fine traces.

The stripped flake size for any resist depends on four major factors:

The type of stripping solution

The concentration of the stripping solution The temperature of the stripping solution

Design of the stripping equipment

When using any stripping solution the stripping time and stripped flake size isa balance of concentration and temperature. The higher the concentrationresults in a faster stripping time but with a larger flake size. Conversely, a

lower concentration will give smaller flake size but the stripping time is muchlonger. Increasing the temperature will reduce both the stripped flake size and

the stripping time.


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Stripping is a diffusion-controlled mechanism. A high volume and spraypressure through each spray nozzle is required. Typically, a minimum flow of

4 litres per minute at a pressure in excess of 1,5 Bar is recommended. Whenalkaline metal hydroxide solutions are used for stripping electroplated boards,a pressure as high as 6-10 Bar is often used.

An ideal equipment configuration is for the spray jets in each stripping moduleto be angled at 30 degrees in the four axes of the board. This will ensure thatthe resist is stripped between fine tracks. Where there is a possibility ofoverhang of plated metals onto the resist surface, particularly in high current

density areas, it is beneficial to have a flooded stripping cell as the finalstripping section. With PHOTEC resists it is helpful if the plating overhang is

less than 7-8 microns onto the surface of the resist.

Antifoam may be necessary to prevent foaming of the solution. Any antifoam

used must be in a minimum concentration and compatible with the strippingsolution being used. The antifoam used in the development solution will

probably not be suitable for the stripping solution. The total alkalinity of thestripping solution determines which type of antifoam is suitable.

To prevent the dry film from being dissolved in the stripping solution, it isnecessary to filter the solution to remove the stripped flake of resist. This

filtration will extend the working lifetime of the stripping solution. There aremany different types of filtration methods, including external or internal beltfilters or angled screen types. PHOTEC resists have been demonstrated to

work with all of these. If an angled screen is used as the filtration method, theactual angle of this mesh may have to be adjusted to obtain the best results.

For optimum stripping it is necessary to replenish the stripping solution. Thismay be achieved by either analytical or by bleed and feed methods. In all

cases, the area of PHOTEC resist stripped should not exceed 0,5 squaremetres per litre of stripping solution. The following example demonstrates the

effect of stripping solution concentration on stripping time and stripped flakesize.

Dry film resists thickness: 50 micronStripping solution: Sodium Hydroxide

Temperature: 550CAgitation: Zero

Concentration (%) Complete removal of resist

Time (sec.)

Stripped

Flake Size

0,5 360 ~2 x 10 mm*

1,0 150 ~10 x 20 mm*

1,5 100 Large sheet**



*Self releasing from copper **Requires agitation to release from copper


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Questions and Answers


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COMMONLY ASKED QUESTIONS

Where is PHOTEC manufactured? Wh ere is i t s l i t?

PHOTEC is currently manufactured in Japan and Malaysia. It is produced

with Japanese products and slit on machines that are specified and run byHitachi Chemical engineers.

Enthone-OMI has been the exclusive distributor and marketer of PHOTEC

dry film resists throughout Europe. Backed by unmatched technicalsupport, PHOTEC dry film rolls are custom slit at Enthones ISO 14001

certified facility in The Netherlands.

How is cons is tent qual ity and su pply assured?

All manufacturing is carried out under the same exacting conditions. The

products used during manufacture are identical and quality control is thesame.

There is always 6-8 weeks of Jumbo rolls in stock at Enthone Benelux.This stock is based on our forecast for sales over a period of 5-6 weeks

and the delivery time from the manufacturing site.

What is a Jum bo Rol l?

A jumbo roll is a master roll that is produced during manufacture. This roll

is typically 1,5 - 1,6 metres wide and with a length of up to 2 kilometres.

How do w e know wh ich type of PHOTEC to stock?

The type of PHOTEC inventoried is based on customer current and

forecasted buying patterns.

What cutt ing width s are avai lable?

Enthone slitting facility is capable of cutting widths according to customer

requirements within the range 145mm to 610mm with an accuracy of +/-1mm.

What are the standard co re sizes?

The core size relates to the internal diameter of the core. 3 inch and 5 inchcore sizes are available.

What is m eant b y a 6-inch core size?

The 6-inch core was originally quoted when large diameter paper coreswere first introduced. It was a standard core for electrical coils that utilisedthe outside diameter of the roll. The paper cores had a wall thickness of

0,5 inches; the inner diameter was the normal 5-inch.


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What class of clean room is recommended?

Class 10,000 clean room is required for todays PWB lines and spaces(i.e. 75 micron). The air quality should be checked regularly and all cleanroom procedures must observed.

How sho uld ro l l s be stored?

To prevent deterioration of the dry film resist, reduction in photosensitivity,edge fusion, etc. the rolls should be stored between 5

Documents

Photec Technical Process Guide