SMT Process Guideline and Checklist - KAZUS.RUkazus.ru/nuke/modules/Downloads/pub/147/0/IPC-S-816 SMT... · 2004. 7. 18. · IPC-S-816 SMT Process Guideline and Checklist ASSOCIATION

IPC-S-816

SMT Process Guideline

and Checklist

ASSOCIATION CONNECTINGELECTRONICS INDUSTRIES

2215 Sanders Road, Northbrook, IL 60062-6135Tel. 847.509.9700 Fax 847.509.9798

www.ipc.org

IPC-S-816A standard developed by IPC

Original PublicationJuly 1993

The Principles ofStandardization

In May 1995 the IPC’s Technical Activities Executive Committee adopted Principles ofStandardization as a guiding principle of IPC’s standardization efforts.

Standards Should:• Show relationship to Design for Manufacturability

(DFM) and Design for the Environment (DFE)• Minimize time to market• Contain simple (simplified) language• Just include spec information• Focus on end product performance• Include a feedback system on use and

problems for future improvement

Standards Should Not:• Inhibit innovation• Increase time-to-market• Keep people out• Increase cycle time• Tell you how to make something• Contain anything that cannot

be defended with data

Notice IPC Standards and Publications are designed to serve the public interest through eliminatingmisunderstandings between manufacturers and purchasers, facilitating interchangeability andimprovement of products, and assisting the purchaser in selecting and obtaining with minimumdelay the proper product for his particular need. Existence of such Standards and Publicationsshall not in any respect preclude any member or nonmember of IPC from manufacturing or sell-ing products not conforming to such Standards and Publication, nor shall the existence of suchStandards and Publications preclude their voluntary use by those other than IPC members,whether the standard is to be used either domestically or internationally.

Recommended Standards and Publications are adopted by IPC without regard to whether theiradoption may involve patents on articles, materials, or processes. By such action, IPC doesnot assume any liability to any patent owner, nor do they assume any obligation whatever toparties adopting the Recommended Standard or Publication. Users are also wholly responsiblefor protecting themselves against all claims of liabilities for patent infringement.

IPC PositionStatement onSpecificationRevision Change

It is the position of IPC’s Technical Activities Executive Committee (TAEC) that the use andimplementation of IPC publications is voluntary and is part of a relationship entered into bycustomer and supplier. When an IPC standard/guideline is updated and a new revision is pub-lished, it is the opinion of the TAEC that the use of the new revision as part of an existingrelationship is not automatic unless required by the contract. The TAEC recommends the useof the lastest revision. Adopted October 6. 1998

Why is therea charge forthis standard?

Your purchase of this document contributes to the ongoing development of new and updatedindustry standards. Standards allow manufacturers, customers, and suppliers to understand oneanother better. Standards allow manufacturers greater efficiencies when they can set up theirprocesses to meet industry standards, allowing them to offer their customers lower costs.

IPC spends hundreds of thousands of dollars annually to support IPC’s volunteers in thestandards development process. There are many rounds of drafts sent out for review andthe committees spend hundreds of hours in review and development. IPC’s staff attends andparticipates in committee activities, typesets and circulates document drafts, and follows allnecessary procedures to qualify for ANSI approval.

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Thank you for your continued support.

©Copyright 1993. IPC, Northbrook, Illinois. All rights reserved under both international and Pan-American copyright conventions. Anycopying, scanning or other reproduction of these materials without the prior written consent of the copyright holder is strictly prohibited andconstitutes infringement under the Copyright Law of the United States.

IPC-S-816

SMT Process Guideline

and Checklist

Developed by the Soldering Surface Mount Devices Task Groupof the Joining Processes Committee of IPC

Users of this standard are encouraged to participate in thedevelopment of future revisions.

Contact:

IPC2215 Sanders RoadNorthbrook, Illinois60062-6135Tel 847 509.9700Fax 847 509.9798

ASSOCIATION CONNECTINGELECTRONICS INDUSTRIES

AcknowledgmentAny Standard involving a complex technology draws material from a vast number of sources. While the principal membersof the IPC Soldering Surface Mount Devices Task Group of the IPC Joining Processes Committee are shown below, it is notpossible to include all of those who assisted in the evolution of this standard. To each of them, the members of the IPCextend their gratitude.

Joining Processes Committee Soldering Surface Mount DevicesTask Group

Technical Liaison of theIPC Board of Directors

ChairmanDavid SchoenthalerAT&T

ChairmanLes HymesPlexus

Bonnie FenaHibbing Printed Circuits

Soldering Surface Mount Devices Task Group

Abbagnaro, L., Pace Inc.

Abel, C.W., E.I. DuPont de Nemours& Co.

Amick, P.J., McDonnell DouglasElec. Sys. Co.

Aoki, E.M., Hewlett-PackardLaboratories

Aoki, H., Airex, Inc.

Astbury, A., Altron Incorporated

Baker, J., Robotic Process SystemsInc.

Bates, G., Sherwood Medical

Bellia, F., Raytheon Co.

Bernier, D.F., Litton Systems Inc.

Bora, S.T., Smiths Industries

Bradshaw, C., Avex Electronics Inc.

Brill, C., Amp Inc.

Brooks, C.P., Amp Inc.

Burg, J.S., 3M Company

Caci, S.F., Raytheon Co.

Carroll, T.A., Hughes Aircraft Co.

Cash, A.S., Northrop Corporation

Cook, D.A., SynopticsCommunications Inc.

Couble, E.C., Shipley Co.

Currie, D., Teledyne SystemsCompany

Davison, R.C., Jet PropulsionLaboratory

Davy, J.G., Westinghouse ElectricCorp.

Elliott, D.A., Electrovert Ltd.

Engelmaier, W., EngelmaierAssociates Inc.

Evans, G.P., Indium Corp. ofAmerica

Evans, J.W., NASA HQ

Feldmesser, H.S., Johns HopkinsUniversity

Flaten, A.D., AT&T InformationSystems

Gamalski, J., Siemens AG

Gandhi, V., Teradyne ConnectionsSystems

Gechter, J., Delco SystemsOperations

Gonzalez, C.J., SCI Manufacturing

Gray, B.W., Bull HN InformationSystems

Gundotra, V., Motorola Inc.

Hampshire, W.B, Tin InformationCtr.,

Hiett, C.E., Martin MariettaAstronautics

Hillman D.D., Rockwell International

Hinton, P.E., Hinton ‘‘PWB’’Engineering

Holmes, R.R., AT&TMicroelectronics

Hook, M., U.S. Navy

Hymes, L, Plexus Corp.

Inpyn, B., Pitney Bowes Inc.

Jawitz, M.W., Litton Guidance &Control System

Johnson, K.L, Hexacon Electric Co.

Jones, S.A., Wilcox Electric Inc.

Kasilag, M., Aerojet ElectrosystemsCo.

Kenealey, G.W., GWK EnterprisesInc.

Kenyon, Dr. W.G., E.I. DuPont deNemours and Co.

Knowles, L.P., Librascope Corp.

Kraszewski, R., Litton SystemsDivision

Kumar, V., Martin MariettaElectronics

Kuo, S.Y., Boeing Aerospace &Electronics

Lambert, L.P., Digital EquipmentCorp.

Le, Q.N., Boeing Aerospace &Electronics

Leibowitz, J.D., Shireline CompositesInc.

Lichtenberg, L.R., Motorola Inc.

Mackzum, S.C., Ericsson GE

Maguire, J.F., Boeing Aerospace &Electronics

McKenna, G.T., SymbolTechnologies Inc.

Meeks, S., Lexmark International/IBM Corp.

Moffitt, J.H., U.S. Navy

Munie, G.C., AT&T Bell Laboratories

Nickell, R.D., U.S. Navy

Nielsen, R.L., Eastman Kodak Co.KAD

Novick, D.T., Rockwell International

O’Halloran, Dr. H., SiemensStromberg-Carlson Corp.

Officer, R.B., Lockheed Sanders Inc.

IPC-S-816 June 1993

ii

Parkinson, E.H., Digital EquipmentCorp.

Payne, R., Sundstrand Data ControlInc.

Pond, R., Texas Instruments Inc.

Reed, J.R., Texas Instruments Inc.

Rehm, P.M., Intel Corp.

Reithinger, M., Siemens AG

Rietdorf, B.C, Magnavox ElectronicSystems Co.

Robertson, D.E., Pace Inc.

Rosser, J.G., Hughes Aircraft Co.

Rotman, A.B., DCMDN - QTC (Deptof Defense)

Rubin, Dr. W., Multicore Solder Ltd.

Rudy, D., AT&T Bell Laboratories

Rumps, D.W., AT&T TechnologySystems

Rusignuolo, N., Hexacon Electric Co.

Russell, J.H., Defense GeneralSupply Center

Scheiner, D., Kester Solder Division

Schneider, A., Alpha Metals Inc.

Schoenthaler, D., AT&T BellLaboratories

Seltzer, M.L., Delco SystemsOperations

Shah, K.K., Racal-Datacom

Siegel, E.S., Pace Inc.

Slezak, E., Litton Systems Inc.

Small, E., Multicore Solders

Smith, W.A., Hughes MissilesSystems Co.

Socolowski, N., Alpha Metals Inc.

Soper, W.A., TRW

Sovinsky, J.R., Indium Corp. ofAmerica

Stanik, E., Loral Defense Systems

Starosta, A., Eldec Corporation

Supp, M., Kester Solder Division

Svensson, J., Ericsson Telecom AB

Theroux, G., Honeywell, Inc.

Turbini, Dr. L.J., Georgia Institute ofTechnology

Underwood, H., U.S. Air Force

Vollmar, E., Methode Electronics Inc.

White, C.E., Indim Corp. of America

White, P.D., Tandem Computers Inc.

Won, Dr. T.S., Allied SignalAerospace

Woodgate, R., Woodcorp. Inc.

June 1993 IPC-S-816

iii

Table of Contents

1.0 SCOPE...................................................................... 1

1.1 Environmental, Safety, and IndustrialHygiene Considerations .................................... 1

1.2 Applicable Documents ...................................... 1

2.0 HANDLING .............................................................. 1

2.1 Electrostatic Discharge (ESD) Concerns.......... 1

2.2 Component Leads.............................................. 1

2.3 Storage ............................................................... 1

2.4 Interim Handling ............................................... 2

3.0 INCOMING INSPECTION OF MATERIALSAND COMPONENTS ............................................... 2

4.0 ADHESIVE APPLICATION ....................................... 24.1 Dispensing ........................................................ 2

5.0 SOLDER PASTE APPLICATION ............................ 4

5.1 Stencil Printing ................................................. 5

5.2 Screen Printing ................................................ 12

5.3 Syringe Dispensing.......................................... 14

6.0 COMPONENT PLACEMENT ................................ 16

6.1 Observed Condition: Wrong ComponentOrientation ...................................................... 16

6.2 Observed Condition: Wrong componentplaced on board ............................................... 16

6.3 Observed Condition: Missing Component ..... 16

6.4 Observed Condition: ComponentMisalignment ................................................... 17

6.5 Observed Condition: DamagedComponent....................................................... 17

7.0 INFRARED/CONVECTION REFLOWSOLDERING .......................................................... 17

7.1 Observed Condition: No Reflow ................... 18

7.2 Observed Condition: Solder Balls .................. 19

7.3 Observed Condition: Charring of Board/Components .................................................... 19

7.4 Observed Condition: DamagedComponents ..................................................... 20

7.5 Observed Condition: Cracked Joints .............. 20

7.6 Observed Condition: Insufficient Solder......... 20

7.7 Observed Condition: Bridging ........................ 20

7.8 Observed Condition: Voids ............................. 20

7.9 Observed Condition: Opens ............................ 21

7.10 Observed Condition: Spatter ........................... 21

7.11 Observed Condition: ComponentAlignment ........................................................ 21

7.12 Observed Condition: Delamination ................ 21

7.13 Observed Condition: Warp.............................. 21

7.14 Observed Condition: Disturbed Joints............ 21

7.15 Observed Condition: Dewetting...................... 21

8.0 VAPOR PHASE REFLOW SOLDERING ............. 21

8.1 General............................................................. 22

8.2 Problems .......................................................... 22

9.0 WAVE SOLDERING .............................................. 24

9.1 Observed Condition: Solder Skips.................. 24

9.2 Observed Condition: Solder Bridges .............. 25

9.3 Observed Condition: Unfilled Via Holes........ 26

9.4 Observed Condition: Solder WaveOverflooding Board......................................... 26

9.5 Observed Condition: Grainy orDisturbed Joints ............................................... 26

9.6 Observed Condition: Cold Joints.................... 27

9.7 Observed Condition: Solder Ballson Assembly .................................................... 27

9.8 Observed Condition: Cracked ChipComponents or Plastic-Bodied LeadedComponents .................................................... 27

10.0 CLEANING .......................................................... 27

10.1 Observed Condition: Visible Residue............. 27

10.2 Observed Condition: Failure of ConformalCoating............................................................. 28

11.0 REPAIR/REWORK ................................................ 28

11.1 Scope................................................................ 28

11.2 General............................................................. 28

11.3 Goals and Guidelines ...................................... 29

11.4 Primary Heating Methods ............................... 29

11.5 Preheating and Auxiliary Heating................... 30

11.6 Vision Systems and Component Placement ... 30

11.7 Selecting Optimum Method of Rework/Repair... 31

11.8 Troubleshooting ............................................... 31

11.9 Observed Condition: Insufficient solderreflow to affect SMD removal ........................ 31

11.10 Observed Condition: Component notremovable after solder reflow or lifted landsupon attempted removal.................................. 31

11.11 Observed Condition: Lands being lifted ordamaged during removal of old solder........... 31

11.12 Observed Condition: Insufficient or pooradhesion of solder on lands duringpretinning operation ........................................ 32

July 1993 IPC-S-816

iv

11.13 Observed Condition: Misalignment ofcomponent to land pattern during SMDinstallation........................................................ 32

11.14 Observed Condition: Dispersion orsolderballing of solder paste during reflow.... 32

11.15 Observed Condition: Blistering or measlingof substrate, or lifted lands during SMDinstallation or removal .................................... 32

IPC-S-816 July 1993

v

June 1993 IPC-S-816

SMT Process Guideline and Checklist

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1.0 SCOPE

This document is intended to provide guidelines and asstance in performing and troubleshooting the steps involvin the process of producing printed wiring assembliincorporating surface mounting attachment of componen

Each section contains a list of problems often observduring a specific part of the surface mount assembly pcess. The list of observed symptoms is matched bydescription of causes often associated with the symptoSuggestions for correction are also included. These sotions may be related to the equipment, materials, or desiAccordingly, some of these corrective measures cannotimplemented on the shop floor.

1.1 Environmental, Safety, and Industrial Hygiene Con-siderations The use of some of the materials referencein this document may be hazardous. Precautions shouldtaken to safeguard personnel and the environment, aslined in the appropriate data supplied with the materiaused. In addition, equipment and procedures should adhto applicable local and federal regulations.

1.2 Applicable Documents The following documents, ofthe issue currently in effect, are applicable to the extespecified herein.

1.2.1 IPC1

IPC-CH-65 Guidelines for Cleaning of Printed Boards anAssemblies

IPC-OI-645 Standard for Visual Optical Inspection Aids

IPC-R-700 Suggested Guidelines for ModificationRework, and Repair of Printed Boards and Assemblies

IPC-SM-782 Surface Mount Land Patterns (configurationand Design Rules)

IPC-SM-786 Recommended Procedures for handlingMoisture Sensitive Plastic IC Packages

IPC-SM-840 Qualification and Performance of PermanePolymer Coating (Solder Mask) For Printed Boards

1.2.2 Joint Industry Standards1

J-STD-002 Solderability Test for Component Leads, Temination, Lugs, Terminals and Wires

J-STD-003 Solderability Tests for Printed Boards

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J-STD-004 Requirements for Soldering Fluxes

J-STD-005 General Requirements and Test Methods fElectronic Grade Solder Paste

2.0 HANDLING

Handling is one topic that is not confined to a specific arof the manufacturing operation. Handling problems are‘‘cradle to grave’’ issue for which no one will claimresponsibility; yet the impact on the surface mount proceyield is substantial. Every major step covered in this guidline, from adhesive application to cleaning of surfacmount assemblies, relies on a comprehensive componand assembly handling strategy. Handling can’t be the ccern of a single group in the manufacturing environmeand must be an integral part of each process step insurface mount assembly line. Although the general formof this document is problem-cause-corrective action, tfollowing handling items need to be considered throughothe entire surface mount assembly process.

2.1 Electrostatic Discharge (ESD) Concerns The fol-lowing are ESD concerns:

1) Has process machinery been properly grounded?

2) Have work stations and storage areas been propgrounded?

3) Are other environmental controls (i.e., ionizers, humidfiers, etc.) in place to reduce the occurrence of ESD?

4) Have operators and all other employees who mhandle components or surface mount assemblies bproperly trained to use ESD precautions such as wstraps or other similar preventative measures?

5) Have the component packaging and assembly contaers been reviewed for potential ESD problems?

6) Have ESD sensitive parts been identified and suciently protected?

2.2 Component Leads The following are concernsregarding component leads:

1) Has component lead coplanarity been addressed folloing manual or automatic lead forming operations?

2) Has component lead solderability concerns in relationhandling been reviewed and precautions taken whdeemed necessary?

2.3 Storage The following are concerns regarding stoage:

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IPC-S-816 June 1993

1) Have the effects of the storage environment on comnents, materials or assemblies been reviewed wiyour manufacturing environment?

2) Does your handling system have a method of moniing shelf life issues of stored components or assemmaterials to prevent unacceptable material from beissued into the manufacturing environment?

3) Would the use of date codes on assembly materialscomponents be beneficial in ensuring timely usage?

2.4 Interim Handling The following are concerns regarding interim handling:

1) Would the use of dedicated assembly or componentriers prevent handling defects between process options?

2) Have operators and assembly routers who handle cponents or surface mount assemblies between prooperations been properly trained and made awaretheir potential impact on the assembly process?

3.0 INCOMING INSPECTION OF MATERIALS AND COM-PONENTS

Surface mount assembly complexity can range from jufew components to several hundred on one printed wiboard (PWB). Adhesives, solder, flux, printed circboards, heat sinks, and components are just some ovariety of ‘‘building blocks’’ used in surface mount technology (SMT) assembly. The breakdown of form, fit,function of just one of these building blocks can result inrework operation or even the rejection of a compleassembly. Incoming inspection of the materials and comnents which comprise an assembly is one method of reing, or eliminating, the possibility of this situation occuring. Incoming inspection can accomplish two purposes

1. Conformance of Material to Specification Requirments Surface mount assembly costs rise sharwhen nonconforming materials find their way into tmanufacturing process. Important key material charteristics can be identified and checked prior to the reof the material to the manufacturing floor. Conformantesting can also be utilized for evaluating materiwhich can degrade over time, such as solderabilityadhesive shelf life.

2. Vendor Evaluation When a particular material or component is procured from several different sourcincoming inspection can be used for evaluation coparisons. The comparison may reveal one particuvendor which supplies a material more consistentlywith less variation between different material lots. Thtype of data can be shared with vendors to improvparticular assembly manufacturing process or be inporated into a vendor rating system.

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Incoming inspection of materials and components shohave a defined purpose and be performed in conjuncwith a good statistical process control plan. Emphasis owell-planned incoming inspection system results in lowmanufacturing costs through fewer surface mount assemdefects and a reduction in the number of surface moassembly scrapped.

4.0 ADHESIVE APPLICATION

In surface mount technology the most prevalent useadhesives is for component retention prior to, and duribottom side wave soldering.

The most common method of adhesive application is aumatic syringe dispensing. Hand syringe dispensing is aused for lower volumes and rework/repair operations. Ptransfer is another method often used in high volume, lmix operations.

Control of adhesive location and material propertieseffective dispensing are the most common difficultieencountered.

This section presents a general listing of problems encotered in the adhesive application process, potential cauand suggestions for corrective actions. These solutions mbe related to equipment, materials, design, or prior pcesses. Accordingly some of the solutions cannot be immdiately implemented on the shop floor.

4.1 Dispensing

4.1.1 Dispensing One Part Adhesive

4.1.1.1 Observed Condition: Stringing Most frequentlyrelated to both dispenser and adhesive.

A. Equipment/Process Related

Potential Cause Corrective Action

1. Dispenser pressure toohigh

Reduce pressure

2. Dispensing period toolong

Increase nozzle tip size

3. Nozzle too far from sub-strate

Reduce distance of nozzle tosubstrate

4. Nozzle drools Use vacuum suck back

5. Adhesive on solderingsurface

Change direction of nozzletravel

6. Tip length too long Shorten tip

B. Material/Prior-Process Related


1. Viscosity of adhesive toohigh

Lower viscosityIncrease reservoir tempera-ture

2. Viscosity of adhesive toolow

Reduce reservoir tempera-ture

3. Plastic flow of adhesive(dribble)

Choose more thixotropicmaterial

4. Air in adhesive Remove air by centrifuging

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4.1.1.2 Observed Condition: Nozzle Clogging Most fre-quently related to down time, curing, and foreign matte

4.1.1.3 Observed Condition: Skipping and InsufficientAdhesive Most frequently related to clogged nozzle.

4.1.1.4 Observed Condition: Excessive Adhesive Mostfrequently related to thixotropic properties of adhesitemperature of reservoir, and equipment controls.



1. Down time too long Follow adhesive manufactur-ers’ recommendation

2. Reaction with nozzle ortip

Anaerobic action—do notuse copper or brass; usepassivated stainless steel orplastic.



1. Reaction by absorption ofmoisture (epoxy)

Keep nozzle tip closed dur-ing down time

2. Mixing of two differentadhesives

Thoroughly clean systemwhen changing adhesives



1. Clogged nozzle, bubbles,and foreign material

Remove cured adhesive,lumps, air

2. Worn or bent nozzle tip Replace tip

3. Insufficient height Dispense adhesive on top oforiginal dot (double dis-pense)

4. Variation in dot size Check dispenser output forconsistency



1. PWB not flat Obtain flat boardsCheck hold down

2. Viscosity too high fornozzle tip and pulse rate

Reduce viscosityHeat reservoirSelect another adhesive

3. Insufficient height and/orexcessive spread

Increase viscositySelect a more thixotropicadhesiveCool reservoir



1. Low viscosity Lower reservoir temperature

2. Dispenser pressure toohigh

Lower pressure

3. Tip size too large Reduce tip size

4.1.1.5 Observed Condition: Insufficient Cure Mostfrequently related to insufficient curing energy leaving sogummy, or tacky adhesive.

4.1.1.6 Observed Condition: Lost Components Mostfrequently related to insufficient cure, poor bond or adhsive, or non-wetted component.



1. Low viscosity Increase adhesive viscosity

Select adhesive with higherthixotropic ratio

2. Excessive spread Select another adhesive



1. Insufficient energy Adjust cure time/temperatureCheck output of UV lamps

2. Too much energy whichdamages components

Reduce temperature

Select another adhesive withdifferent cure cycle



1. Local heat sinks Move components on PWB

2. Heat sensitive compo-nents

Move components to bottomside, IR heat top side



1. Insufficient cure Check time/temperature andenergy level of cure cycle

2. Adhesive bond Decrease time betweenplacement and cure

3. Adhesive dot too low withinsufficient componentcontact

Print second dot on top oforiginal dot

Increase tip size

4. Fast speed and stop inX-Y movement

Increase initial tackiness ofadhesive

Reprogram/reduce speed ofplacement

5. Poor placement of com-ponent

Realign equipment

6. Uncured adhesive (UV) Place two dots either side ofcomponent



1. Adhesive bond/strength Check PWB and componentcleanliness/bondability withadhesive

2. Adhesive dot too low withinsufficient componentcontact

Use high dot profile adhesiveand high viscosity for initialtack

3. Adhesive shrinkage dur-ing cure

Select another adhesive

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IPC-S-816 June 1993

4.1.1.7 Observed Condition: Adhesive Voids Most fre-quently related to the presence of moisture in adhesivon surfaces.

4.1.2 Dispensing Two Part Adhesive Similar to 4.1.1with a requirement that if recommended mixture rationot met, the cure will not be satisfactory with a potentloss of components.

4.1.3 Pin Dip Transfer

4.1.3.1 Observed Condition: Stringing Most frequentlyrelated to withdrawal rate and viscosity.

4.1.3.2 Observed Condition: Skipping and InsufficientAdhesive Most frequently related to amount of adhesiin pins.



No information available



1. Moisture in adhesive Check adhesive for excessmoisture

If excessive, cease use anddetermine and eliminatesource of moisture

2. Absorbed moisture onbare PWB

Bake PWB before usingadhesive

Check solder mask forporosity

3. Absorbed moisture onsurface of component

Bake components beforeassembly



1. Rate of pin withdrawal Correct speed of verticaltravel

2. X-Y movement duringwithdrawal

Correct horizontal movement



1. Viscosity too high Reduce viscosity

2. Flow properties do notmatch pin travel

Select adhesive with properproperties

3. Viscosity too low Increase viscosity so adhe-sive does not drip



1. Pin size too small Increase pin size

2. Pin shape incorrect Select different pin shape

3. Pin does not wet Clean pin and/or changematerial

4. Pin immersion incorrect Check pin position and res-ervoir depth

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4.1.3.3 Observed Condition: Excessive Adhesive Mostfrequently related to flow properties of adhesive and vations in temperature of the reservoir.

5.0 SOLDER PASTE APPLICATION

Several solder paste application techniques are usetransfer solder paste to areas being joined. The mostmonly used methods include: stencil printing, screen ping, and syringe dispensing. Less common techniques,as direct pin transfer and wheel transfer, are used for sspecial applications. With the trend toward use of comnents having closer and closer lead spacing, errorapplication of solder paste is more difficult and the dirrelationship between poor quality application and fiyield is more apparent.

Poor application can cause five common defects: inscient solder, excess solder, shorts, opens, and solderEach of these defects can be the result of one or mothe following symptoms: insufficient solder paste depited, excess solder paste deposited, wet bridging, orplaced solder paste. Each of these symptoms can bresult of a variety of causes.



5. Reservoir withdrawal rateincorrect

Check rate of withdrawal

6. Withdrawal from PWBincorrect

Reduce speed of withdrawal



1. Insufficient adhesive onpin

Adjust viscosity andthixotrophy

2. Adhesive drops off duringtransfer

Increase viscosity



1. Pin size too large Reduce pin size

2. Too much adhesive onpin

Reduce depth in reservoir

3. Adhesive reservoir/pinplate not coplanar

Level adhesive reservoir

4. Pins have become rough Replace pins

5. Build up of adhesive onpin

Clean pins

6. Decreased distancebetween pin and PWB

Correct spacing



1. Viscosity increased Adjust viscosity

2. Wrong pins used Replace pins

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June 1993 IPC-S-816

5.1 Stencil Printing

5.1.1 Observed Condition: Insufficient Solder PasteDeposited In order for enough solder paste to be depited in the area to be soldered, the volume of the steopening should be approximately equal to the desamount of wet solder paste.

The width of a stencil opening is usually defined assmallest dimension in the X-Y plane. If the width of topening is too small when compared to the thickness ostencil, it will be difficult for the solder paste to releafrom the stencil hole. Examine the smallest holes instencil after printing. If solder paste is hanging-up in thholes, but not in the larger ones, it is possible that the scil is too thick or the openings are too narrow.

If it is absolutely necessary to use a thick stencil with nrow openings, then one can attempt to acceleraterelease of the solder paste from the stencil by increathe snap-off distance to 50 to 90 mils. With this methodsolder paste has a better chance of pulling away fromstencil, but there will also be a greater chance of print malignment.

In addition to design defects in stencils, various stemanufacturing defects are also possible, such as incometch, improper image placement, or rough surfaces inthe stencil holes. If too little solder paste is consisteobserved on one part of the board, then examine the csponding section of the stencil for possible defects.


Potential Cause Corrective Active

1. Stencil too thin Increase stencil thickness

General Guideline of Stencil Thickness for Various Applications

Stencil Components

8-20 mils chip components only

8 mils leaded with >31 mil pitch

6 mils leaded with 20–25 mil pitch

IPC-S-816 June 1993


7. Improper squeegeepressure

If squeegee pressure is toolight, solder paste may notbe effectively forced to thebottom of the stencil holeand will not transfer to thepad. If the pressure is toostrong it is possible that thesqueegee blade will deformand scoop solder paste outof the larger stencil open-ings. To properly adjust thesqueegee, first decreasepressure until a thin layer ofsolder paste is left on thestencil after making a pass.Pressure should then beincreased slowly until all ofthe paste is cleaned off thestencil after each pass.



1. Too little tackiness The solder paste may not betacky enough for the givenprint parameters. In order forsolder paste to roll in front ofthe squeegee blade, insteadof slide, it must have suffi-cient tackiness to adhere tothe stencil material. A solderpaste which does not rollhas difficulty filling all of theholes. Solder paste tacki-ness is most easily con-trolled by the vendor whendeveloping the formulation,but may also be enhancedby increasing the flux per-cent or by raising the solderpaste temperature.

2. Too much tackiness It is also possible for a sol-der paste to be too tacky fora given application. If thesolder paste is so tacky thatit hangs up in the stencilholes without fully transfer-ring to the pads, then it istoo tacky.

3. Large or irregular solderparticles

Particles which are too largeor irregular may have diffi-culty being forced into smallstencil openings. If there isany doubt about the solderpowder particle size orshape, it can be examinedby spreading out a smallquantity of solder pasteunder a 10X to 100X micro-scope.

6



4. Viscosity too high The solder paste viscositymay be too high for the printparameter being used. It isdifficult to force very viscoussolder paste into narrowstencil holes unless the otherpaste characteristics areproperly balanced. In orderto maintain consistency, it isbest for the solder pastevendor to make any perma-nent viscosity changes; how-ever, as a temporary step, itmay be lowered by increas-ing the solder paste tem-perature, adding thinner toincrease the flux percent,stirring vigorously or increas-ing the squeegee speed.

5. Viscosity too low Low viscosity solder pastescan be scooped out of largestencil holes, leaving a thin-ner deposit than on otherpads. This normally happenswhen low viscosity solderpaste is used in combinationwith soft squeegee bladesand high squeegee pressure.Although the solder pastevendor can increase the sol-der paste viscosity, the bestway to handle this problemis by using a harder squee-gee blade.

6. Low percent metal The percentage of metal inthe solder paste may belower than required to give aproper sized solder joint.Even though the stencilopening may be properlydesigned for printing, solderpastes having too low a per-centage of metal may notgive a sufficiently large jointafter reflow. Solder pastevendors report the amount ofmetal in solder paste as aweight percent. Because ofthe large difference betweenthe solder and flux, approxi-mately 7:1, a small changein metal weight percent hasa larger effect on the metalvolume percent. Consult thesolder paste vendor if itappears that too little solderis in the joint area, eventhough the recommendedamount of solder paste isbeing deposited.

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June 1993 IPC-S-816

5.1.2 Observed Condition: Excess Solder Paste Depos-ited Excess solder after reflow or solder shorts causeexcess solder paste deposits and wet bridging.

Although it is very difficult to create excessively heavy filets by any means other than depositing too much sopaste, there are many reasons for the creation of soshorts or bridges: improper reflow profile, excessive coponent placement pressure, component movemimproper solder paste, or incorrect solder paste applica

There are many signs of incorrectly applied solder paincluding: large deposits, wet bridges, smeared printmisplaced print.



7. Bubbles in solder paste Bubbles may surface in thesolder paste as it rolls infront of the squeegee, eitheras the result of air entrap-ment or as an inherent char-acteristic of the fluxthickening system. Bubblescan prevent the correctquantity of solder paste frombeing deposited in the stencilholes by displacing solderpaste. The solder paste ven-dor should be consulted ifbubbles continue to beobserved after severalsqueeqee passes.

8. Low solder pastetemperature

Solder paste used at lowerthan normal operating tem-peratures will have a higherthan published viscosity. Thelower viscosity may causethe solder paste to slide onthe stencil and not bepushed through the holes.Consult the solder pastevendor for alternatives if thesolder paste or environmentcannot be warmed up.

9. High solder pastetemperature

As temperature increases,the viscosity of solder pastewill decrease. Lower viscos-ity solder paste has a ten-dency to be scavenged fromthe pads by soft squeegeeblades.

10. Not enough solder pasteon stencil

If too little solder paste isused, it is difficult to getproper roll in front of thesqueegee and some holesmay be skipped. A roundbead of solder paste,approximately 1/2 inch to 1inch in diameter, is normallyrecommended.

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1. Damaged stencil One very frequent cause ofwet bridging is damage tothe stencil. In order for sol-der paste to print cleanly,intimate contact between thestencil and circuit boardmust be maintained as thesqueegee passes the open-ing. Frequently, the fragilestencil area between adja-cent holes can be bentslightly. The bent area allowssolder paste to pass under-neath the stencil and form awet bridge. If one consis-tently observes poor qualityprint in one area of theboard, then one should lookfor damage in the corre-sponding area of the stencil.

2. Stencil defects If the stencil is not properlyimaged or etched, inconsis-tency will exist from openingto opening. Improper imag-ing or etching can causesome holes to be larger thanothers and stencil areasbetween holes may becomevery narrow. If this happens,one can expect a higheroccurrence of wet bridges.

3. Inadequate stencil framesize

Because stencils areanchored within finiteframes, the center of thestencil is capable of greaterdisplacement from the frameplane than the stencil edges.When using small snap-offdistances, the ability of thesqueegee to press down onthe stencil will not beaffected. Inconsistent printthickness and wet bridging ispossible when using largesnap-off distances. If consis-tent wet bridges or smearedprints are found at the edgesof the stencil near the frame,but not near the center, thenconsider decreasing thesnap-off distance or using alarger frame size.

7

IPC-S-816 June 1993



4. Stencil not centered inframe

When using fixed rail in-lineassembly systems, the sten-cil surface is not centered inthe frame; instead, it is ori-ented relative to the fixedrail. Unequal stencil deflec-tion is a common cause ofinconsistent print becausethe edge of the stencil closeto the frame will not come incontact with the board aseasily as the center of thestencil. If an unbalancedstencil must be used, thenone should use on-contactprint with the correspondingstencil thicknesses and printparameters.

5. Improper tension ofscreen border

If the screen border aroundthe stencil is loose or hasbeen stretched in one direc-tion, there will be more sten-cil movement in the X-Yplane. As the stencil movesin the X-Y plane, the likeli-hood of a smeared printincreases. X-Y stencil move-ment increases as squeegeepressure is increased,because the amount of fric-tion between the squeegeeblade and stencil surface isincreased. It is common tofind X-Y movement whenusing thick stencils with highsnap-off distances andgreater than normal squee-gee pressures.

Poor circuit board design orquality can affect solderpaste printing. It is very diffi-cult to get a consistent printon an inconsistent surface.

6. Squeegee speed too fast At high squeegee speeds,solder paste will shear thinand heat up. As this hap-pens, the solder paste vis-cosity drops. If the drop islarge enough, there will be adeterioration of print defini-tion. This can be seen by theincreased formation of wetbridges and poorly definedmounds of solder paste. Ifthe print quality is good, butthere is a gradual deteriora-tion, the squeegee speedshould be decreased. If thisis not possible, then the sol-der paste manufacturershould be consulted for fur-ther recommendations.

8



7. Environment too warm Not all solder paste usershave the ability to controltheir working environment.Because solder paste gener-ally thins as temperatureincreases, room environmentplays an important role inprinting quality. If the roomtemperature cannot be con-trolled, the solder paste ven-dor should be consulted.

8. Heat generated by screenprinter

After prolonged periods ofuse, some screen printersgenerate heat which affectsthe solder paste and stencil.This heat generation reducesthe solder paste viscosityand changes print quality.Consult the equipmentmanufacturer for recommen-dations or the solder pastevendor for a solder pastedesigned for higher operat-ing environments.

9. Squeegee pressure toolow

Unless sufficient squeegeepressure is applied, the sten-cil will not come into contactwith the circuit board andsolder paste will not becleaned thoroughly from thestencil surface. With thestencil not in contact with theboard, excess solder pasteis extruded into an areabetween the stencil and theboard. In addition, anyadded thickness of solderpaste left directly above thestencil holes is deposited.

10. Squeegee blade angletoo small

With flat blade squeegees,as the angle of the blade tostencil plane decreases, theamount of deformation of theblade increases. Excess sol-der paste will be deposited ifthe blade is not able to pushthe stencil into contact withthe board or wipe the stencilclean.

June 1993 IPC-S-816



1. Incorrect solder particlesize

As the size of the stencilopenings decrease, the sizeof the solder particles in thepaste should also bedecreased. Solder particleshaving a diameter in therange of 45–75 microns arerecommended for mostapplications where stencilopenings are greater than 14mils wide. Although it is pos-sible to print through smalleropenings with the large par-ticle size, it is easier usingsmaller particles. Most solderpaste vendors can provideseveral different particle sizeranges depending on theneeds of the application.

If good solder paste depositdefinition is seen on thelarge pads, but not on thesmall pads, check to makesure that the proper particlesize solder paste is beingused.

2. Viscosity too low Most higher viscosity solderpastes leave more sharplydefined solder paste depos-its than low viscosity solderpastes. If the solder pasteviscosity is too low, it may bedifficult to control the quan-tity of solder paste depositedand wet bridging. Solderpaste viscosity decreases astemperature and shear onthe solder paste increases. Itmay not be necessary tochange solder pastes, onlyreduce the operating tem-perature or squeegee speed.

3. Solder paste too tacky Although a solder pastemust have sufficient tacki-ness to roll in from thesqueegee blade, the solderpaste should have a greatertendency to stick to itselfrather than to the stencil.This helps solder pasterelease cleanly from insidethe stencil walls as the sten-cil lifts away from the circuitboard. If the solder paste istoo tacky, it will be very diffi-cult to control print qualityand the occurrence of wetbridging may increase. Tacki-ness is most easily con-trolled by the solder pastevendor.

It is best to consult the sol-der paste vendor for materialrecommendations. They maynot always be aware of thespecific requirements ofone’s process.



4. Board warpage Non-planar circuit boardsmake printing very difficult,because the stencil hastrouble making intimate con-tact with the board. Thegaps between the stenciland circuit board may fill upwith solder paste resulting ina smear, wet bridge, orexcess deposit. Most screenprinters are equipped with avacuum plate which can testand compensate for minorboard warpage. If a tightseal is achieved between thevacuum plate and the board,then the circuit board is gen-erally planar enough to get agood print. Off-contact print-ing with increased squeegeepressure also helps compen-sate for board warpage.

5. Inconsistent circuit boardsurface

Even though a circuit boardmay be planar, it may still bedifficult for the stencil tocome into contact with allsegments of the board. It israre that one will find a cir-cuit board which containssoldermask and has a per-fectly smooth surface. Sometypes of mask, such as dryfilm, tend to have smoothersurfaces than others, butthere is still no guarantee. Ifthe stencil cannot come intocontact with the circuitboard, then the incidence ofexcess solder, wet bridges,and smeared printsincreases.

6. Circuit board patternmisalignment

It is very common to see thepads on a circuit board varylocation by several mils. Thisdoes not normally cause dif-ficulties when working with50 mil center devices or chipcomponents, but can causeproblems when doing finepitch assembly. Very littlecan be done short of using avision system to realign thestencil for every print. If avision system is not avail-able, the other effects thatcontribute to misalignmentshould be reduced bydecreasing snap-off distanceand squeegee pressure.

9

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IPC-S-816 June 1993

5.1.3 Observed Condition: Solder Paste Will Not Roll inFront of the Squeegee If the solder paste does not roll ifront of the squeegee, then it is more difficult to fill thholes in the stencil. The solder paste may skip overholes.



7. Lack of soldermaskbetween pads

Most circuit boards havesoldermask separating everypad. Apart from the electricalconsiderations, soldermaskperforms several otherimportant functions. It helpsmake a seal between thestencil and circuit board andforms a well into which thesolder paste can be depos-ited. It is common to observeinconsistent soldermaskheight between adjacent finepitch pads. Some circuitboards are designed with nosoldermask between closepads, making it virtuallyimpossible to guarantee aclean, consistent print.

8. Inconsistent solder padheight

It is common to find hot airleveled boards with solderdeposit heights varying from1–3 mils. The variation canoccur from pad to pad orbetween different areas ofthe same pad. Inconsistentheight is a problem, becausestencil openings are not nor-mally the same size as thepad and some part of thestencil usually comes in con-tact with the solder deposit,leaving other areas of thestencil away from the board.Solder paste is thensqueezed under the stencilin the areas that do not meetthe board.

Pattern misalignment, stencilmisalignment, and stencilmovement all increase thisproblem by moving the edgeof a stencil opening closer tothe pad center where thedeposit is generally thicker.

Circuit boards with bare cop-per or thin solderable coat-ings are the easiest to printsolder paste onto, becausethey have the most level sur-face.




10

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5.1.4 Observed Condition: Solder Paste Adheres to theSqueegee at the End of the Stroke In order for eitherdiamond or flat blade squeegees to perform inbi-directional print mode they must be able to lift cleanaway from the solder paste at the end of the stroke. Sopaste must remain on the stencil for the return pass.



1. Too little solder paste onthe stencil

In general, solder paste willadhere to itself rather thanadhere to anything else. Aminimum amount is requiredto allow the solder paste tocome in contact with boththe squeegee and boardwithout adhering to one orthe other.

2. Insufficient solder pastetackiness

If tackiness is too low, thenthe solder paste will not beable to adhere to the stencil.If the solder paste cannotadhere to the stencil, then itwill slide over the stencilrather than roll.

3. Viscosity too high Viscosity should be balancedwith tackiness to yield solderpaste with the best printingcharacteristics possible. Ifsolder paste viscosity is toohigh in combination with lowtackiness, the solder pastewill slide over the stencilrather than roll.






1. Insufficient solder pasteon the stencil

Most solder pastes willadhere better to themselvesrather than a squeegeeblade. If too little solderpaste is used, there will beinsufficient mass to breakthe adherence to the blade.

2. Solder paste viscosity toohigh

If solder paste slides overthe stencil and adheres tothe blade, then there is agood chance that the viscos-ity is too high for the applica-tion. High viscosity may be aresult of the formulation,inadequate shear from thesqueegee, or low workingtemperature.

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June 1993 IPC-S-816

5.1.5 Observed Condition: Solder Paste Hanging Up inthe Stencil Holes A balance of properties: tackiness, vicosity, stencil design, and print parameters contribute tosolder paste’s ability to release cleanly from the stenholes. Rarely is there only one cause.



3. Solder paste is too stringyfor application

Although some applicationsfavor stringy solder paste, itis generally undesirable forstencil printing. The solderpaste vendor should be con-sulted.



1. Stencil too thick Stencils which have a highstencil thickness to openingwidth ratio are difficult toprint with. Most solderpastes will adhere to thewalls of the stencil hole. Ifthe force holding the solderpaste to the walls of the holeis greater than the forcebetween the paste and thecircuit board, there will notbe a clean and consistentprint. Stencil thickness mustbe decreased as the dimen-sions of the stencil hole aredecreased.

2. Snap-off distance toosmall

Cone-shaped deposits arecaused by the solder pasteadhering to the stencil holewalls as the stencil isremoved from the circuitboard. A larger snap-off dis-tance can cure the problemby pulling the solder pasteout of the hole before it hasa chance to set up. Higherviscosity and lower tackinesssolder pastes also print morecleanly.




Solder paste with a viscositytoo high for the applicationmay have difficulty beingforced to the bottom of thestencil hole or may set upand have difficulty transfer-ring.

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5.1.6 Observed Condition: Lateral Deposit Skips It isvery common to have poor print quality in only one sectioof the circuit board while the rest of the circuit board iprinted cleanly. A number of factors can contribute to thi



2. Improper solder pastetackiness

Solder pastes can havetrouble printing if they areeither too tacky or not tackyenough. A solder pastewhich is not tacky enoughwill not roll. A solder pastewhich is too tacky willadhere too strongly to thewalls of the stencil hole andnot transfer cleanly.

3. Solder paste particle sizetoo large

The smaller the size of thesolder powder used to makethe solder paste, the easierthe solder paste will print,because there is less frictionbetween particles. Disadvan-tages to using very smallsolder particles includegreater solder paste slumpand increased occurrence ofsolderballs. It is best to usethe largest particle size avail-able which will still printcleanly.



1. Improperly made stencil Make sure that the stencil isnot too thick for the width ofthe opening being printed. Ifsolder paste is continuallyhanging up in the hole thenthis is a strong possibility.Also check the area which isprinting poorly for signs ofinconsistent etch. If the sten-cil is off center in the frame,then adjustments should bemade to print technique.

2. Damaged stencil Examine the area of thestencil which is having con-sistent problems. If the sten-cil is bent, buckled, or dented(even slightly), then this maybe the cause of the problem. Adecision must be made,whether to accept the poorquality print or use a newstencil.

11

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IPC-S-816 June 1993

5.1.7 Observed Condition: Inconsistent Print When theSqueegee Travels in Opposite Directions Althoughbi-directional printing should yield the same results whtraveling in either direction, this is not always the casemost cases, the inconsistency can be traced to print paeters.



3. Inconsistent vacuum Most manufacturers will usea vacuum plate to hold thecircuit board in place andcompensate for minor non-planarity. If there is inad-equate vacuum, inconsistentvacuum, or a vacuum leak,then part of the board maynot be level. Make sure thatno vacuum holes are in thesame location as via holes inthe circuit board.

4. Squeegee blade not level Some squeegees have fixedpositions, while others rockor float in order to remainparallel with the stencil. Ifone side of the stencil is notwiping clean, then thesqueegee may be level. Iflarge streaks of solder pasteremain on the stencil, thenthe squeegee blade may bedamaged or else there maybe something peculiar aboutthe way the squeegee bladeis fixed in the squeegeeblade holder.

Some squeegee blade hold-ers use screws or hex boltsto hold the blade securely.When tightened, the boltsmay put pressure on the softrubber squeegee bladecausing it to deform. As thishappens, the cross-sectionof the blade elongates in thearea of the bolts and thesqueegee blade edgebecomes non-linear. Sincesome areas of the blade pro-trude farther than others,greater pressure is put on thestencil in these areas.



1. Circuit board not planar If the circuit board is warped,then it is very likely that cer-tain areas of the board willbe printed differently thanothers.

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12

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5.2 Screen Printing Screen printing is different fromstencil printing. A mesh screen, blocked off in the areasto be printed by a polymer emulsion, is used in place ometal mask stencil. Screen printing always requires theof a snap-off and has different limitations in the typecircuitry which can be printed. It is difficult to print soldepaste onto very large land areas with a stencil and, cversely, difficult to print small areas with a screen. Sofsqueegees are used when screen printing, because thder pastes are less viscous, more conformance is requand screens are easily damaged. Most of the mechanicscreen printing are identical with stencil printing with thexceptions mentioned below.



1. Inconsistent squeegeespeed

Although the squeegeetravel speed is supposed tobe identical when traveling ineither direction of bi-directional print, this is notalways the case. Time thespeed of the squeegee tomake sure that the speedsare identical. With machinesthat have independent speedsettings for both directions, itwill be easy to adjust thespeed so that it is consistent.If independent settings can-not be made and the speedsare not identical, then theequipment manufacturershould be consulted.

2. Inconsistent travel dis-tance

In order to get a consistentprint, particularly when usingoff-contact print, the distancethe squeegee travels pastthe stencil pattern should beidentical in both directions.This will reproduce theamount of stencil peel andsolder paste set-up time inboth directions.

3. Inconsistent squeegeeangle

Independent adjustments tosqueegee angle can bemade on screen printershaving a double flat bladesystem. Independent bladesare used to print in eitherdirection. In a screen printer,which has a single-diamondblade squeegee, problemsmay arise when the squee-gee is not tight or some ofthe fixturing parts havebegun to wear. It is notuncommon to find the angleof the squeegee bladechanging slightly when print-ing in opposite directions asthe print head rocks in aworn or loose fixture.

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June 1993 IPC-S-816

5.2.1 Observed Condition: Insufficient Solder PasteDeposited To eliminate insufficient solder or open cicuits caused by too little solder paste deposited in the ping operation.



1. Insufficient emulsionthickness

In screen printing, the thick-ness of print should beapproximately equal to thethickness of the emulsion,plus the thickness of thescreen weave. Smallincreases or decreases inthis value can be achievedby either changing snap-offdistance or squeegee pres-sure. Large changes in printthickness must be done byincreasing the amount ofemulsion.

2. Insufficient snap-off dis-tance

In order for solder paste totransfer cleanly from thescreen to the circuit board,the solder paste must bepulled out of the screen asquickly as possible with theleast amount of resistance.By using a large snap-offdistance, the solder pastestarts to pull out of thescreen as soon as thesqueegee passes.

3. Wire blocking opening ina very small hole

When printing fine linesusing a screen, it is difficultto get around the problem ofwires in the way. If the areato be printed is less than twoscreen openings wide, thereis a very good chance thatthe average open area willbe constricted to less thanone opening wide. This mayresult in poor solder pastedeposits, because the par-ticles have difficulty fittingthrough the opening.

4. Emulsion too thick forapplication

Just as with stencil printing,if the thickness of the screenis too great for narrow open-ings, the solder paste willhang up in the openings andnot transfer completely.Increasing the snap-off dis-tance will help somewhat,but only to a point. The widthof the opening may have tobe increased in relation tothe thickness of the emul-sion.

5. Improper screenmanufacture

Occasionally openings in thescreen are not complete dueto improperly placed emul-sion. If one consistently getsinsufficient solder in one par-ticular area of the pattern,the screen should beinspected for defects.

-

5.2.2 Observed Condition: Excess Solder Paste Depos-ited Excess solder after reflow or solder shorts causedexcess solder paste deposits and wet bridging.

Excess solder and solder shorts occur when too muchder paste is deposited. As with stencil printing, thereseveral mechanisms for excess solder deposits.



6. Damage to screen Screens are more fragilethan stencils and wear morerapidly with use. Damage tothe screen which restrictssolder paste depositsincludes: bent wires, swollenemulsion from solvents inthe solder paste or cleaningmaterial, and large particlesof solder, squeegee, or othermaterial permanentlywedged in the screen open-ings.



1. Solder paste too viscous Solder pastes used in screenprinting have a lower viscos-ity than those used withstencils. This is becausethere is greater frictionbetween the solder pasteand screen, due to the largersurface area the solderpaste is being squeezedpast. Thicker solder pasteshave a tendency to set up inthe screen weave and nottransfer consistent amountsof material.

2. Solder particles too largefor the screen used

There is a physical limitationto the size particles that willfit through a screen.Although in theory, solderparticles will go through anyopening larger than they are,this is not a reasonableexpectation in a productionenvironment. Solder particlesmust pass smoothly andquickly through the screen inorder to achieve a consistentprint. Solder particles shouldbe no larger than one halfthe average diameter of theopenings.

3. Solder paste metal con-tent too low

The most common methodof decreasing solder pasteviscosity to facilitate screenprinting is to decrease themetal content. In doing thisthe volume of solder afterreflow is also decreased.This may result in insufficientfillet size.

13

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IPC-S-816 June 1993

5.2.3 Observed Condition: Misaligned Solder PasteDeposits



1. Emulsion too thick Emulsions that are too thickmay cause excess solderpaste to be transferred to thecircuit board. The nominalthickness of solder pastedeposited is equal to thescreen weave thickness plusthe emulsion thickness.

2. Excess snap-off orinsufficient squeegeepressure

Excessive snap-off dis-tance will lead to distor-tion in the printed patternand misalignment.

It is possible to control theamount of solder pastetransferred by altering thesnap-off distance and squee-gee pressure. By using ahigh snap-off in combinationwith a low squeegee pres-sure, it is possible to extrudethe low viscosity solderpaste used in screen printingto a greater thickness thanwould normally be expected.Conversely, if one is gettinga greater thickness than isdesired, then one shoulddecrease the snap-off dis-tance and increase thesqueegee pressure.

3. Improper screen designor manufacture

If bridging or excess solderis noticed in only one par-ticular print area, then themost common cause wouldbe a defect in stencil designmanufacture. Look for areaswhich should have emulsion,but do not.

4. Damaged screens It is not uncommon for ascreen to become damagedthrough normal use.Scratches, holes, bentscreen, etc., can all contrib-ute to excess solder depos-its. Very little can be donewith the exception of install-ing a new screen.



1. Screen stretching Screens are constantly beingstretched, because they arealmost exclusively used inan off-contact mode. Overtime, the screen material willfatigue and it becomes moredifficult to control the printalignment, particularly whenprinting bi-directionally. Verylittle can be done, with theexception of decreasingsnap-off distance and squee-gee pressure. This reducesthe amount of frictionbetween the squeegee andscreen.

14

5.3 Syringe Dispensing Syringes are used both in repaioperations and programmable multipoint dispensinSyringes used in repair are generally hand-held and empair or manual pressure to dispense the solder paste. Mupoint dispensers use air or other forms of pressure to dpense solder paste to points programmed into an X-Y tab

In syringe dispensing applications, the most importathings are continuous dispensing and consistent sizdeposits.

5.3.1 Observed Condition: Solder Paste Flo Stops



2. Excess snap-off or insuffi-cient squeegee pressure

Excessive snap-off distancewill lead to distortion in theprinted pattern and misalign-ment.






1. Dispensing pressure toohigh

Dispensing systems whichuse pressure on the back ofthe syringe to push the pasteout of the needle requirespecial consideration. Solderpaste responds best to lowpressure and longer shottimes, as opposed to highpressure and short shottimes. The high pressure ismore likely to separate theflux from the solder paste.

2. Solder paste separation Some applications appear toseparate the solder pasteregardless of what is done.This is common when verysmall diameter needles mustbe used. Some manufactur-ers have found that it helpsto shorten the length of theneedle in order to reduce theamount of friction. Otherforms of dispensing equip-ment have been developedwhich do not stress the sol-der paste as much as directpressure dispensing. Thesemethods use variations ofpinch valves or pistons toapply high pressure to verysmall amounts of solderpaste.

ersitlyen

June 1993 IPC-S-816

5.3.2 Observed Condition: Inconsistent Solder PasteSize Solder paste deposit size is not generally a concin repair operations, because the amount of paste depocan be determined by the operator. But how consistentsolder paste deposits during an automated syringe disping application will have a bearing on yield quality.



1. Solder paste stops com-ing out of the tube beforeit is empty

Once solder paste stopscoming out of the tube, it isvery difficult to continueusing that tube. Separationof the flux from the powderhas already started to occur.It may be possible toincrease the dispensingpressure to get some solderpaste out, but this will onlywork to a point. It is far bet-ter to use a preventativemethod to stop initial separa-tion.

2. Incorrect solder paste for-mulation for dispensing

Many solder paste manufac-turers have different solderpastes for stencil printingand syringe dispensing. Therheological properties maybe very different. A solderpaste designed for syringedispensing will have a con-sistent viscosity under differ-ent shear rates. In addition,the percent metal by weightis generally lower to give thesolder paste added lubricity.

3. Solder powder particlestoo large for needle sizeused

If the solder powder particlesare too large for the needleused, they will bind in theneedle and prevent thepaste from coming out.Smaller solder powder par-ticles or a larger needle canbe used.




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5.3.3 Observed Condition: Solder Paste Deposit NotRounded Enough

5.3.4 Observed Condition: Solder Paste Deposit NotPeaked Enough



1. Incorrect solder paste for-mulation

Consult the solder pastevendor if the dispenser hasdifficulty delivering consistentamounts with each shot. Thesolder paste may not bedesigned for the dispensingapplication or there may beair entrapped in the solderpaste.

A sure sign of inconsistentsolder paste deposits is ifthe solder paste stringswhen the dispensing headmoves from one pad toanother. This is generallycaused by too much solderpaste being pulled out of theneedle and will leave thenext deposit short of solderpaste.



1. Improper needle tipdesign

Use a beveled needle, whichdispenses solder paste toone side of the needle, notdirectly under the needle.



1. Solder formulation nottacky enough and doesn’tslump

Use a solder paste whichself-rounds or slumpsslightly.



1. Improper needle tipdesign.

Use a flat-tipped needle.



1. Solder formulation tootacky and slumps

Use a different formulationwhich is less tacky anddoesn’t slump.

15

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IPC-S-816 June 1993

5.3.5 Observed Condition: Solder Paste Will Not Adhereto the Pad

6.0 COMPONENT PLACEMENT

Surveys have indicated that a major portion of soldeassembly defects result from manufacturing operatioprior to soldering. Component placement has been idefied as the most prevalent source of these defects. Thisbecome increasingly evident with the increased use ofpitch technology.

This section presents a general listing of problems encotered in the component placement process, potential caand suggestions for corrective actions. These solutionsbe related to equipment, materials, design or prior pcesses. Accordingly some of the solutions cannot be imdiately implemented on the shop floor.

6.1 Observed Condition: Wrong Component Orientation



1. Too high a pressure Use a lower pressure for dis-pensing.



1. Incorrect solderformulation

Use a different formulationthat is more tacky.



1. Placement programproblems

Verify correct revisionsAdd rotational data toprogramChange rotational datain program

2. Different board/programversion

Verify correct version



1. Components in feederhave wrong orientation

Correct orientation of compo-nents

2. Components have mixedorientations at pickuplocation

Orientate components infeeder

3. Board marking incorrect Verify orientation with bothassembly drawing andschematic

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6.2 Observed Condition: Wrong component placed onboard

6.3 Observed Condition: Missing Component



1. Wrong pickup callout Verify/change pickup location

2. Incorrect sequence steporder

Verify sequence program


Verify correct board/programversion

4. Incorrect placement loca-tion

Verify/change placementlocation

5. Component from previouspickup stuck to chuck

Clean chuck; vacuum pickuptip

6. Incorrect data file used Verify correct program



1. Wrong component putinto feeder

Inspect/verify components infeeders

2. Wrong components mixedin with correct parts

Inspect/verify componentsnumbers

3. Parts list is incorrect Update program with latestparts list



1. Vacuum pickup not on Verify there is vacuum atpickup

2. No components at pickuplocation

Restock feeder

3. Component not picked up Reprogram for correctpick-up

4. Feeder malfunctiona) Low/off air pressure

b) No electricity

c) Waste tape is clogging

d) Excessive vibation orlow vibration

e) Excessive vibration orlow vibration

Verify correct pressure/vacuum at feederVerify power is turned on atfeeder and verify feeder iscorrectly connectedRemove excess tape andre-threadRemove jammed component

Adjust vibratory feeder vibra-tion control

5. Incorrect chuck used Modify program to use cor-rect chuck

6. No placement data Add placement data to pro-gram

7. Incorrect placementlocation

Reteach correct placementdata

8. Component did notrelease from chuck

Clean chuck and/or checkfor vacuum release


Verify correct board/programversion

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6.4 Observed Condition: Component Misalignment






1. Placement teach is notaccurate

Reteach placement

2. Board is misregistered Verify board loading equip-ment and tooling

3. Incorrect pickup chuckselected

Modify program to use cor-rect chuck for pickup or mayrequire vision alignment

4. Component is rotated Verify correct rotation data inplacement program

5. Board revision differentthan program

Verify program versions

6. Equipment needs calibra-tion

Calibrate to manufacturersmaintenance instructions

Check pulleys/ gears for slip-page on shafts

7. Incorrect data file used Load correct data file

8. Machine placement toler-ance is not adequate foraccurate/acceptableplacement

Equipment cannot be usedfor the required accuracy ofplacement.

Acquire equipment with anacceptable placement accu-racy.

9. Vision system is notworking

Check electrical connectionsand fuses:

a) Check target sizes/ com-position; verify compliance.

b) Camera is not looking atthe component heads; soft-ware problem. Verify compo-nent within field.

10. Placement teach isabove board surface

Reteach placement



1. Board is misregistered Verify fiducials

2. Incorrect footprint onboard

Verify with design standardsand/or latest parts list andschematic

3. Incorrect componentpicked up

Verify with parts list andassembly drawing

4. Component is rotated Review component loadingprocess

5. Board revision differentthan program

Verify board version

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6.5 Observed Condition: Damaged Component

7.0 INFRARED/CONVECTION REFLOW SOLDERING

This process is generally carried out with inline convorized equipment incorporating multiple heating zowith individual temperature controls. Heat sources uinclude short wavelength tubes, medium-to-longer walength panel secondary emitters, and/or heated air/nitrin the convection reflow systems.

In many cases convection reflow systems have overcsome of the perceived drawbacks of this reflow procwhich included: (1) the need for individual assembly pcessing profiler as a result of differences in mass and cponent characteristics, (2) shadowing of some areasexposure to Infrared Reflow (IR) energy as a result of cponent size or spacing, and (3) the lack of fast responsset temperature changes.

As with any mass soldering process, defects detected isoldered assemblies are often related to prior processmaterial variables. The defects that relate to prior proor material are much larger in number and can be veryficult to classify after the fact. For example, onceassembly is soldered, it is hard to determine whethcomponent was out of position before reflow, or whetthe solder paste deposit was properly registered.



1. Pickup/placement headdamaged component

a) Verify pickup and compo-nent placement. Z-axis loca-tions are not impactingcomponents; adjust height ofpickup/placement.

b) Pickup head is not correctfor particular component; usepickup head designed forthat component type/style.

2. Component leads arebent, or missing

a) Adjust pressure of dam-aged, or missing ‘‘square off’’fingers.

b) Verify that pickup head iscorrect for that component.

c) Fragile leads requirevision alignment, notmechanical alignment.



1. Component was dam-aged prior to placement

Inspect components prior toplacement. Discard damagedcomponents.

2. Component leads arebent, damaged or missing

a) Verify at incoming ormissing inspection

b) Improve handling proce-dures

c) Review excising die

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There are some defects which may not make themselknown immediately, such as cracks in chip capacitorsflux residue under parts. Tight process control is a necsity to prevent these defects, as it is far too late to stcorrective action when the defects don’t appear until tproduct built is in service.

This section presents a general listing of problems encotered in the infrared/convection reflow soldering procespotential causes and suggestions for corrective actioThese solutions may be related to equipment, materiadesign or prior processes. Accordingly some of the sotions cannot be immediately implemented on the shfloor.

7.1 Observed Condition: No Reflow

7.1.1 Observed Condition: No Reflow of Solder Paste/Cold Solder Joints

7.1.2 Observed Condition: No Solders No solders maybe the result of a non-reflow condition or they may be dto problems associated with placement, solder paste apcation, solderability of the parts, or errors in the designthe assembly that make reflow of some of the joints vedifficult. Thus a ‘‘no solder’’ may be defined as a condition

1. In which solder is completely absent from the intendarea.

2. Where the solder paste is present, but reflow hasoccurred.

3. Where solder is present and has reflowed, but haswet either the lead, the board, or both.

4. Where the solder is present, has reflowed, has wetboard/lead, and pulled back away from the solder joini.e., dewetting.

7.1.2.1 Observed Condition: No Solder—Solder NotPresent



1. Reflow profile incorrect Increase/change reflow tem-perature

2. Thermal load too great Decrease product loading formachine capacity rate



1. Incorrect solder pastealloy

Use the solder alloy speci-fied




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7.1.2.2 Observed Condition: No Solder: Solder notReflowed



1. Clogged stencil/dispenser Clean stencil/dispenser(see section 5)

2. Print process out of con-trol (See Section 5.0 formore detail)

Adjust squeegee pressure ordurometerChange print speedCheck stencil alignmentCheck dispenser accuracy(see section 5)

3. Paste drying out (SeeSection 5.0 for moredetail)

Change process to reducetime paste remains onstencil

Increase humidity andreduce drafts in the screenprinting area

Do not reuse old paste(see section 5)


Obtain lower viscosity paste(see section 5)

5. Paste not ‘‘wetting’’ thestencil

Change paste(see section 5)

6. Solder clogging stencil/dispenser

Use different particle sizepaste

Check paste for unevennessof texture

Use different viscosity paste

Use paste with slower dryingtime(see section 5)



1. Reflow temperature nothigh enough

Adjust profile

Increase time above liquidus

Verify that all heaters areworking

2. Machine being thermallyoverloaded

Reduce rate of product inputto match equipment thermalcapacity and recovery capa-bility

3. Board layout causingshadowing

Reorientation of board onconveyor

Change reflow profile

Select alternate reflow pro-cess

4. Heat sinking from board/parts



Increase preheating stage

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7.1.2.3 Observed Condition: No Solders: No Wetting

7.1.2.4 Observed Condition: No Solders: Dewetting

7.1.3 Observed Condition: Uneven Reflow Sometimesan assembly will reflow in some sections and not in otheReasons for this may be similar to the ones for cold andsolders discussed above, i.e., thermal or solderability.



1. Incorrect solder alloy Use solder alloy specified


Change component selectionand placement


Reorientation of board onconveyor








1. Lead not wetting Check part solderability (seeJ-STD-002)

More active paste may behelpful

2. Board not wetting Check board solderability(see J-STD-003)

More active paste may behelpful

3. Part may not be in con-tact with paste

Check print accuracy

Check part placement (seesection 6.0)

Check lead planarity/skew

Check board warp



1. Excess time above reflowtemperature

Change profile to reducetime above reflow



1. Board has excessintermetallics

Increase solder thickness onboard

Reduce storage time

Improve storage condition

2. Chip termination has poorleach resistance

Use parts that have barrierlayers (see J-STD-002)

7.2 Observed Condition: Solder Balls Solder balls areoften due to excessive outgassing of the paste or boaduring reflow or an excess of oxide on the paste particcomponents, or boards.

7.3 Observed Condition: Charring of Board/Components Thermal damage to the boards or compnents during reflow can be due to oven temperatureuneven thermal transfer during reflow.



1. Incorrect profile forassembly

Check profile, customize foreach assembly

2. Poor heat transfer Check positioning of boardto insure even heat transfer

3. Uneven heat transferacross oven

Increase convective airacross oven flow in oven

4. Other effects Check items under 7.1.2above

5. Thermal capacity andrecovery capability

Repair or replace oven




Design change required


Design change required

3. Other effects Check items under 7.1.2



1. Incomplete paste cure Pre-cure paste or changepreheat portion of reflow pro-file for cure

2. Conveyor speed too high Reduce conveyor speed



1. Incomplete paste cure Change paste

2. Moisture in boards Pre-bake boards

3. Entrapped solvents inboards

Pre-bake boards

4. Oxidized paste Refer to J-STD-005 forincoming paste controls

5. Solder mask/pasteinteraction

Change in paste or maskneeded

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7.4 Observed Condition: Damaged Components Dam-aged components are a subset of thermal damage ineral. Thermal damage to the components may be eicharring (see 7.3), melting, or cracking. As with most masoldering techniques, it has been found that preheat isessential part of the process. In order to avoid crackcomponents, an acceptable ramp rate should be establifor sensitive compoents.

7.5 Observed Condition: Cracked Joints Physical dam-age to the solder joints can occur if the assembliesmoved while the solder joints are still liquid or if thassemblies are handled in such a way as to damagecooled solder joints, e.g., dropped. Stress after reflow,during, is the most common cause of cracked joints.



1. Oven temperature toohigh

Reduce temperature set-points

Shield affected areas fromdirect radiation

Increase conveyor speed

2. Loading rate too low foroven settings

Check/change rate

3. Uneven/incorrect position-ing of assemblies inreflow

Check positioning to insureheat transfer






1. Temperatures too high Change profile

Keep temperatures belowlimit for components

2. Components being dam-aged in pick and placeoperation (see Section6.0)

Adjust pick and place equip-ment



1. Components incompatiblewith IR reflow

Ch