1. Electronic card assembly processes.
Materials for Electronics

2. Materials for Electronics
Electronic card assembly processes are based on two main technologies:
Surface Mount Technology (SMT)
Pin Through Hole Technology (PTH)
Materials for Electronics

3. Printed Circuit Board with SMT & PTH patterns
SMT: CCGA,CBGA,PBGA,QFP ; PTH: connectors
Materials for Electronics

4. Types of SMT & PTH components
BGA
QFP
Materials for Electronics

5. Materials for Electronics
Assembled electronic card with SMT and PTH components.
Materials for Electronics

6. Materials for Electronics
QFP SMT component.
Materials for Electronics

7. Materials for Electronics
PTH component.
Materials for Electronics

8. 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)
Materials for Electronics

9. Principal materials for the assembly processes of electronic cards .
Protective coating (finish) of PCB copper pad.
Solder paste.
Adhesive
Wave soldering alloy
Materials for Electronics

10. Materials for Electronics
Copper pad finishes
Organic finishes.
Benzotriazole (2-3 nm) and benzoimidazole ( nm) that avoid oxidation because of the formation of R-NH-Cu+1 complex.
Metallic finishes.
Ni-P (1-3m) /Au ( m) coating that prevents oxygen contact with copper.
Ag ( m) coating that prevents oxygen contact with copper.
Materials for Electronics

11. Materials for Electronics
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
Materials for Electronics

12. Solder paste types
Type of solder pastes as funtion of particle size distribution
Materials for Electronics

13. Materials for Electronics
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.
Materials for Electronics

14. Phase diagram of SAC 305 alloy.
Sn-3Ag-0.5 Cu (weight %)
Eutectic melting point at 220°C
Density = 7.6 g/cm3
INTERMETALLICS:
e Cu3Sn
h Cu6Sn5
g Ag3Sn
Materials for Electronics

15. 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)
Materials for Electronics

16. Size dependence melting properties of Sn nanoparticles
Tm melting temperature
DHm melting enthalpy
r nanoparticle radius
ss solid-liquid interfacial surface tension
t0 liquid overlayer thickness=1.6 nm
The model is based on classic thermodynamics.
For Sn its valid for radius >1.6 nm
Materials for Electronics

17. Intermetallics structure.
h Cu6Sn5
Hexagonal primitive (hP4)
e Cu3Sn
orthorombic
side face centered (oC12)
g Ag3Sn
Orthorombic primitive (oP8)
Materials for Electronics

18. Materials for Electronics
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.
Materials for Electronics

19. Materials for Electronics
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
Materials for Electronics

20. Examples of solder paste fluxes
Isomers of rosin (colophony)
Materials for Electronics

21. Examples of solder paste fluxes
Materials for Electronics

22. Examples of solder paste fluxes
Materials for Electronics

23. Examples of solder paste solvents.
Materials for Electronics

24. Chemical flux reactions.
Materials for Electronics

25. Materials for Electronics
Poor wetting on SMT pad.
Reduced spread of the alloy on copper oxidized pad.
Materials for Electronics

26. Materials for Electronics
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.
Materials for Electronics

27. Forced convection oven.
Q=Hc As (Tg-Ts)
Q heat rate transfer
Hc thermal convection conductance
As assembly surface area
Tg gas temperature near surface
Ts assembly surface temperature
Materials for Electronics

28. Materials for Electronics
Thermal
Profile of
Pb-free
Solder paste
with SAC 305
alloy
Materials for Electronics

29. 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.
Materials for Electronics

30. Functions of thermal profile zones.
Dwell time
the time above the melting temperature lasts about 30 sec to produce good solder joints without too much thick intermetallics.
Cooling time
a 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.
Materials for Electronics

31. 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.
Materials for Electronics

32. Wave soldering process
Wave soldering of Pin Through Components
Materials for Electronics

33. Wave soldering profile
Materials for Electronics

34. 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
Materials for Electronics

35. 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.
Materials for Electronics

36. Young-Laplace capillary rise equation.
g liquid surface tension
r liquid density
h liquid height
r capillary radius
g gravity constant
q contact angle
Low g decreases contact angle
Low r and r increases h
Alloy has high r a
Good meniscus requires low g and r
Materials for Electronics

37. Good wetting in PTH solder joint
Positive meniscus and complete
rising in PCB hole
Materials for Electronics

38. 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.
Materials for Electronics

39. 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 solvent
because of incorrect preheat step during solder paste reflow.
Materials for Electronics

40. Solder joint metallurgy. SAC 305 Pb-free solder paste & alloy
PTH solder joint with voids
because of wrong preheat
during wave soldering
Regular capacitor solder joint fillet
obtained with SAC 305 Pb-free
no clean paste.
Materials for Electronics

41. Solder joint metallurgy
BGA balls made by SAC 305 Pb-free alloy after soldering with 305 Pb-free
no clean paste.Surface roughness is due to alloy shrinking and formation
of Ag3Sn and Cu6Sn5 intermetallics (right magnified optical picture).
Pad copper is protected with Entek 106A finish (200 nm).
Materials for Electronics

42. Soldering interface Cu
Finish
Solder
alloy
High T Au
Diffusion

43. Solder alloy
Junction
Cu/Ni-P/Au/Sn-Ag-Cu
Junction
Cu/Ni-P/Au/Sn-Pb

44. Solder joint metallurgy
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.
Materials for Electronics

45. Solder joint metallurgy
SEM backscattering image of solder joint formed by SAC 305 ball and 305 solder paste on pad with
Cu/Ag (200 nm).
Intermetallics are due to reaction between Cu,Sn and Ag.
Materials for Electronics

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