Pcba technology -- smthelp.com

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PCBA Technology

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1. Introduction: The electronics assembly process flow Components and Printed Circuit Boards 2. Components used for electronics assembly 3. Printed wiring boards 4. Surface finishes used on printed wiring boardsAssembling Technologies 5. Soldering techniques and solder joint design principles 6. Solder paste stencil printing 7. Component placement methods and process sequences 8. Reflow soldering, reflow ovens and temperature profiling 9. Wave soldering and thermal profiles10. Selective soldering 11. Hand soldering and assembly repairInspection, Test and Quality Management12. In-line inspection and test techniques13. In-circuit and functional test14. Total Quality Management

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Electronics Assembly is the process of attaching component leads or terminations to lands or pads of conductors on a circuit board and then join them by soldering. There are two primary categories of components and assemblies; through hole and surface mount.

Through-Hole Technology (THT)

At through-hole technology (THT), component leads are inserted through holes in the printed wiring board (PWB); and soldered to the lands on its other side. A TH component is the DIL IC (dual-in-line integrated circuit), whose leads usually have a pitch of 2,54 mm (0,1”)

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SMT – Surface Mounting Technology

Process steps: 1. Solder paste application2. Adhesive application for wave soldering3. Components placement4. Adhesive curing by heat or UV light 5. Reflow or Wave Soldering 6. Board cleaning

Terminations of surface mount devices (SMDs) are attached to the board pads by soldering to make both electrical and mechanical connections. In some cases conductive adhesive joints are used.

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Combinations

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SMT assembly line

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Types of discrete components Active components: amplify the electric signal, need energy source, built from semiconductor materialsPassive components: amplify the electric signal, do not need energy source, built on insulating materials Assembling types of components:

Through Hole components

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SMT components

Multilayer ceramic capacitor PLCC:Plastic Leaded Chip Carrier

QFP:Quad Flat Pack

Thick film chip resistor SOIC: Small Outline Integrated Circuit

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SMT ICs with high lead countQFP (Quad Flat Pack)Very high number of leads(up to 1000) are locatedat all the four sides of thepackagePitch minimum is 0,3 mm

FC–PBGA (Flip Chip – Plastic Ball Grid Array

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Types of Printed Wiring BoardsIn an electronic assembly the substrate or circuit board supports the components mechanically, and interconnects their terminations electrically. Printed wiring board is the general term for completely processed printed wiring configurations, including single-sided, double-sided and multilayer boards with rigid, flexible and rigid-flex base materials.For circuit cards and modules the rigid printed wiring boards are used, while flexible and rigid-flex printed wirings are popular for the interconnection of cards placed in different position in equipment or of different units.

Rigid printed wiring board Flexible printed wiring board

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Printed Wiring BoardsPrinted wiring board (PWB) is a substrate, which is made up of an insulating board with copper foil tracks on it. The components are fixed to the PWB and connected to the conductive copper tracks by soldering. In general, the base material of a PWB is a copper clad laminate, and the conductive pattern is prepared from the copper foil by subtractive technology, that is by the combination of photolithography and chemical etching. Multilayer boards are made by the lamination of single boards and adhesive layers. The patterns of the different layers are connected with plated holes (or vias).

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Assembling process steps

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Mixed assembly

applying glue:combinations of reflow and wave soldering

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SolderingSolder joints use some metal alloy material to fasten and electrically contact conductive parts, like soldering pads and component leads. Applying heat and some flux, the surfaces of the parts are wetted by the melted alloy, and the joint is formed when the solder re-solidifies. The soldering alloy has a – preferable eutectic – melting point (MP), lower than to injure the parts. Most popular alloys are:• lead-free SAC alloy, e.g.: 95.5Sn3.8Ag0.7Cu; MP = 217 oC• conventional leaded alloy: 63% Sn 37% Pb; MP = 183 oC The flux helps the wetting process. Flux types are as follows:• Resin (most popular): non-corrosive, soluble in alcohol, needs cleaning• RMA = Rosin Mildly Activated: resin or rosin activated by acid • no-clean flux: transparent, insulating, no need for cleaning• water soluble organics.Two types of soldering are in use in mass production, namely:• reflow soldering when the solder is applied in paste form and then melted (reflowed) to make the joints;• wave soldering, where the solder and heat source is a solder wave.

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Solder paste applicationSolder pastes are formulated to be printable mixtures, and they are applied to the pads of the board using stencil printing.

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Pick & PlaceSurface mount assembly consists of three basic processes:• solder paste application, • component placement, and • reflow soldering.

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Placement to optically recognized pad positions. Polarity checking and simple measurements during placement.Efficiency up to 120K CPH （SF-100)

pick-and-place head

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placement failures - Component falls down from the nozzle- Misplacement and rotational offset (see figures)- Wrong polarity- Billboard (component lays on the edge)- Face down component- Component placed on another component- Injured component

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Reflow solderingReflow soldering is usually carried out in • a convection type or • a combined IR heated hot-air (or N2) furnace.Conveyor belt type ovens are applied in both cases.

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PWB-s are transported through the wave soldering equipment bya conveyor belt, which has a transport speed of: 1,3..1,5 m/min).Both the heat and the solder are provided by the solder wave.

foamed orsprayed flux

Solder wave240…260 ºC

-shaped wave(oxide-free)

pump pump

Pre-heating80…120 ºC

Fluxapplication

Wave Soldering

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Integrated Repair AOI

Visualization of soldering failuresIntegrated shuttleMarking of component positionsPictures of the originally detected failuresStorage of error picturesReporting of failure class

Repair stations are used to correct and repair recognized soldering failures, with functions as follows:

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Fabrication of the LEADOUT In the following slides some details of the fabrication of the LEADOUT demonstrator board is presented.1. Equipment and data of assembly with photos

- stencil printing & stencil design- assembly – component placement- reflow soldering

2. Fabrication problems and their effects- offset of stencil aperture- solder paste misprint- imperfect component placement- paste misprint and smearing

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Stencil Printing & Stencil Design

Thieme TES/S-25 stencil printer features: Printing speed: 20…140 mm/sPressure: 10…60 N, angle: 45…90ºSettings during experiment:Printing speed: 400 mm/sPressure: 55 N, angle: 45 º

Stencil:Laser-cut150 µm thickStainless steelInverse home plate apertures for Surface Mount Components

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Reflow Soldering

Green: 3x3 cm copper area Red: 0603 chip R Blue: air

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after reflow soldering

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Effect of solder paste misprint:wetting corrected misprinting

After stencil printing After reflow soldering

Monitoring the Assembling

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Effect of the offset of stencil aperture:wetting pulled back the solder and the SMD

After stencil printing After reflow soldering


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After component placement After reflow soldering

Imperfect component placement #1:reflow and wetting pulled back the SMD


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Paste misprint and smearing:resulted in solder ball formation


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Paste misprint and smearing:resulted in solder bead formation


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