Materials for Electronics1 Electronic card assembly processes

Materials for Electronics 1

Electronic card assembly processes.


Electronic card assembly processes are based on two main technologies:

• Surface Mount Technology (SMT)

• Pin Through Hole Technology (PTH)


Printed Circuit Board with SMT & PTH patterns

SMT: CCGA,CBGA,PBGA,QFP ; PTH: connectors


Types of SMT & PTH components

QFP BGA


Assembled electronic card with SMT and PTH components.


QFP SMT component.


PTH component.


SMT+PTH process flow.First side assembly of Printed Circuit Board (PCB)

• Screening of solder paste on the pads of the first side of the Printed Circuit Board (PCB)

• Component placement on the solder paste deposit.

• Reflow of the solder paste

• Cleaning (not performed for no clean solder pastes)

Second side assembly of PCB.• Duplication of first pass

• Adhesive dispensing on the pads.

• Component placement on adhesive

• Adhesive curing

• Turn PCB over

• Insertion of PTH component on the first side.

• Wave soldering.

• Cleaning (not performed for no clean solder pastes and wave fluxes)


Principal materials for the assembly

processes of electronic cards .

• Protective coating (finish) of PCB copper pad. • Solder paste.• Adhesive• Wave soldering alloy


Copper pad finishes

Organic finishes. • Benzotriazole (2-3 nm) and benzoimidazole (100-300

nm) that avoid oxidation because of the formation of

R-NH-Cu+1 complex.

Metallic finishes.• Ni-P (1-3 /Au (0.2-0.3 coating that prevents oxygen

contact with copper.• Ag (0.1-0.2coating that prevents oxygen contact with

copper.


Solder paste.• Concentrated dispersion of spherical solid particles

(metal powder) in a continuous organic phase (vehicle).

• Viscoelastic (rheological) characteristics of solder paste depend on powder particle size distribution, metal volume fraction and vehicle chemical composition.

• Metal powder is composed of Sn-3.0Ag-0.5Cu alloy, that replaced Pb63Sn37 alloy because of European Union ban legislation


Solder paste types

Type of solder pastes as funtion of particle size distribution


Solder paste alloy.

• RoHS (Restriction of Hazardous Substances) directive of European Union bans since July 1, 2006 the use of lead in electronic devices.

• Eutectic 63Pn37Sn has been replaced by eutectic Sn-3.0Ag-0.5Cu (SAC 305) alloy.


Phase diagram of SAC 305 alloy.

Sn-3Ag-0.5 Cu (weight %)Eutectic melting point at 220°CDensity = 7.6 g/cm3

INTERMETALLICS:Cu3SnCu6Sn5

Ag3Sn


Size dependence melting properties of metal nanoparticles.

The decrease of melting temperature and enthalpy of nanoparticles is inversely proportional to their radius.

The model assumes an equilibrium between the core solid metal and the liquid metal overlayer of thickness t0.

This property can be used to reduce the melting point of the solder paste. Special fluxes shall be developed to avoid the very high oxidation of nanoparticles (very high surface/volume ratio)


Size dependence melting properties of Sn nanoparticles

Tm melting temperatureHm melting enthalpyr nanoparticle radiuss solid-liquid interfacial surface tensiont0 liquid overlayer thickness=1.6 nm

The model is based on classic thermodynamics.For Sn its valid for radius >1.6 nm


Intermetallics structure.

Cu6Sn5

Hexagonal primitive (hP4) Cu3Sn orthorombicside face centered (oC12)

Ag3SnOrthorombic primitive (oP8)


Solder paste vehicle.

• The vehicle is the organic part of solder paste and is compose of flux+solvent.

• The vehicle can be no-clean or water soluble type.• No clean vehicle is composed of no corrosive organic

acids solved in high boiling alcohols and therefore don’t need to be removed by cleaning from the card.

• Water soluble vehicle is composed of potential corrosive organic acid or salts of hydrochloric amines solved in high boiling alcohols and must be cleaned off from the card by water.


Flux functions.

• protect the powder of the solder paste from oxidation

• remove oxide from the surface to be soldered and protect it from further oxidation

• enhance wetting of the melted alloy on the component leads/balls and the PCB pad.

• guarantee enough tackiness to avoid the displacement of components during card handling.

• promote thermal transfer from the oven atmosphere to the solder joint


Examples of solder paste fluxes

Isomers of rosin (colophony)






Examples of solder paste solvents.


Chemical flux reactions.


Poor wetting on SMT pad.

Reduced spread of the alloy on copper oxidized pad.


Solder paste reflow

Types of reflow ovens:• Vapor phase• Infrared• Forced convection

Forced convection in nitrogen atmosphere is the most used oven now.

Convection assures a uniform heat diffusion on any size of component.

Nitrogen prevents the oxidation of PCB pads and the powder of solder paste.


Forced convection oven.

Q=Hc As (Tg-Ts)

Q heat rate transferHc thermal convection conductanceAs assembly surface areaTg gas temperature near surfaceTs assembly surface temperature


ThermalProfile ofPb-freeSolder pastewith SAC 305alloy


Functions of solder paste thermal profile zones

Preheat • Uniform the temperature of components with different thermal

capacity• evaporate the moisture inside the components avoiding cracking• reduce the rate of thermal expansion.

Soak• evaporate the solder paste solvent and activate the flux• assure the maximum coalescence of liquid alloy and reduce the

formation of solder balls.

Reflow• the peak temperature is 20°C higher than alloy melting point to

make fluid the alloy and to wet properly pad and lead component.


Functions of thermal profile zones.

• Dwell timethe time above the melting temperature lasts about 30 sec to produce

good solder joints without too much thick intermetallics.

• Cooling timea proper rate allows the formation of a suitable solid alloy (no too much

large inclusions of Cu-Sn and Ag-Sn intermetallics in Sn matrix) and no thick intermetallics between alloy and pad.


Adhesive dispensing and curing

•Stick on the first side of PCB SMT small conponents to be soldered with wave soldering machine

•Epoxy resin with viscoelastic properties dispensed by syringe (viscosity depends on dispensing velocity and syringe opening)

•Quick curing operation: 90 sec at 150°C

•Uncured product is fluorescent in ultraviolet light for failure control.

•High glass transition higher than melting point of wave soldering alloy.


Wave soldering process

Wave soldering of Pin Through Components


Wave soldering profile


Functions of wave soldering process steps

1) Flux dispensing.• No clean or water soluble flux (similar chemistry of solder paste flux)• Remove oxides from hole walls and pin component• Reduce surface tension to improve the rising of liquid alloy through

the hole.

2) Preheat• Uniform the temperature on all card components• Evaporate the solvent and activate the flux


Functions of wave soldering process steps

3) Contact with liquid wave solder • Liquid SAC 305 alloy is pumped to form a wave wich goes in

contact

wirh the pins of the component.• Few seconds contact time due to high speed of alloy rising through

the hole• Peak temperature 20°C above alloy melting point (220°C)

4) Cooling.

• A proper rate allows the formation of a suitable structure of solid alloy (no too much large inclusions of Cu-Sn and Ag-Sn intermetallics in Sn matrix) and no thick intermetallics between alloy and pad.

5) Cleaning.Residues of water soluble flux are removed by water washing.


Young-Laplace capillary rise equation.

liquidsurface tension liquid densityh liquid height r capillary radius g gravity constantcontact angle

gr

cos2h

Low decreases contact angleLow r and increases hAlloy has high

Good meniscus requires low and r


Good wetting in PTH solder joint

Positive meniscus and completerising in PCB hole


Poor wetting and voids in PTH solder joint

Negative meniscus and incomplete rising in PCB hole due to poor wetting of pin component . Voids are formed because of insufficient solvent evaporation during preheat step of wave soldering process.


Solder joint metallurgy. SAC 305 Pb-free no clean solder paste

Regular QFP front & back fillet QFP fillet with voids

Voids are due to incomplete evaporation of no clean solder paste solventbecause of incorrect preheat step during solder paste reflow.


Solder joint metallurgy. SAC 305 Pb-free solder paste & alloy

Regular capacitor solder joint filletobtained with SAC 305 Pb-freeno clean paste.

PTH solder joint with voidsbecause of wrong preheatduring wave soldering


Solder joint metallurgy

BGA balls made by SAC 305 Pb-free alloy after soldering with 305 Pb-freeno clean paste.Surface roughness is due to alloy shrinking and formationof Ag3Sn and Cu6Sn5 intermetallics (right magnified optical picture).Pad copper is protected with Entek 106A finish (200 nm).

Soldering interfaceSoldering interface

CuCu

FinishFinishSolderSolderalloyalloy

High T

AuAuDiffusionDiffusion

Solder alloySolder alloy

JunctionCu/Ni-P/Au/Sn-Pb

JunctionCu/Ni-P/Au/Sn-Ag-Cu



Scanning Electron Microscope (SEM) backscattering image of solder joint formed by SAC 305 ball and 305 solder paste on pad withCu/Entek 106A (150 nm).

Intermetallics are due to reaction between Cu,Sn and Ag.



SEM backscattering image of solder joint formed by SAC 305 ball and 305 solder paste on pad withCu/Ag (200 nm).

Intermetallics are due to reaction between Cu,Sn and Ag.



SEM backscattering image of solder joint formed by SAC 305 ball and 305 solder paste on pad coated with Cu/Ni-P(2.5)/Au(150nm)Layers: 1) Ni-P-Sn-Cu (reaction alloy with Ni-P); 2) Ni-P finish; 3) Cu padAu is not detectable because dissolved in the alloy at very low concentration .

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Materials for Electronics1 Electronic card assembly processes