1. Polymer Solders: the Cool Lead-Free Alternative
Ken Gilleo, PhD
L-F
SMTA Boston 2000
Ocean State - Rhode Island
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2. Outline Lead-Free Solders: Success or Failure?
Polymers Vs. Metallurgical Solders
Conductive Adhesive Technology
Pros and Cons of CAs for SMT
Conclusions and Prognosis
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3. component Metallurgical Solder Copper - PWB
(shown with Sn only for simplicity) h
phase Cu 6 Sn 5
component
Tin phase Sn
-phase Cu6Sn5 e
-phase Cu3Sn
Copper - PWB
Binary and tertiary alloys form complex structures with many organometallic compounds
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4. L-F Alloy Political Issues
Japan
Using their own proprietary alloys
Alloys; too many and too diverse?
A patent “minefield” from global perspective?
USA
Industry: WHY are we doing this?
Government: electronic Pb, minor envir. issue
ITRI and NEMI are endorsing TAC (Tin-Silver-Copper)
World in General
No global cooperation mechanism?
Low consensus, too many “my alloy” agendas?
Ambiguous deadlines and not in synch!
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5. Sn Alloys Cu Zn? Mg Ba Ce Ag In Bi Some 5-metal alloys
Mostly tin plus, hold the lead Mg Ba Ce Ag Sn In Bi
Some 5-metal alloys
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6. Popular Alloys “TAC”: Sn-Ag-Cu
Promoted by several organizations, appears to be viable
Patent issues, “Doctrine of Equivalence” issues in USA?
Sn-Cu, Sn-Ag; appears to be viable, mostly patent-free
Sn-Ag-Cu-Sb
Much process experience with this alloy
Many have licensed from AIM as Castin®
Sn-Ag-Cu-Bi
Majors in Japan are using this alloy system
Bi supply no issue - low % Bi used
Process at 20oC lower than Sn-Ag-Cu
Licensed from Oatey by Alpha-Fry who will sublicense
Compiled by Dr. Alan Rae
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7. The Real L-F Problem 30 - 60oC Higher - was Sn/Pb already too hot? L-F
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8. Collateral Damage? Equipment
L-F
Can they take the heat and thermal effects
Equipment
oven; N2, faster ramp, different materials
tester & probes; harder flux residue
Substrate; warp, delamination, degradation
Coatings; solder masks; degradation
Board Cleaning; more difficult
Package
substrate; warp, delamination
encapsulant; popcorn
devices like photo, MEMS, low k dielectric?
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9. Flux Issues Lead-free alloys don’t wet as well:
Sn without Pb oxidizes more easily
Nitrogen probably required
High temperature fluxes; more voids in reflow?
Fluxes - halogen-free for rel. and environ
Newer epoxy fluxes may not be viable
Expect new fluxes and processes
They may be harder to clean
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10. Cleaning Baked-on fluxes?
Many users still clean, even no-clean flux, to improve conformal coating adhesion, etc.
Nitrogen atmosphere can reduce residue toughness
“Stronger” cleaning machines
from Speedline Electrovert
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11. Laminates Laminates can degrade at 260oC on multiple passes
Dimensional stability issues
Solder masks; adhesion, color change, degradation
Issue - finding an effective compromise on electrical properties, flammability, mechanical properties
FR4 in jeopardy
Before or after?
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12. Packaging Materials
Additives O
OCH2CH
CH2
Epoxy Resin OH
Hardener
Catalyst A
Silica Fillers
Coupling
Agent
Epoxy Molding Compounds (EMC) may need to be modified or changed to totally different resins.
Formulations are very complex; many formulations have >10 components.
Epoxy Molding Compound (EMC); could be in jeopardy
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13. Long-chained macromolecules of small repeating block monomers
Long-chained macromolecules of small repeating block monomers. Properties like strength, flexibility, melting point and electricals can be engineered into the polymer.
POLYMERS
Put the fire out with COOL
CONDUCTIVE ADHESIVES
Polymers
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14. The inventor of the IC Jack Kilby, uses Conductive Adhesives
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15. Resin, hardener. accelerator
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16. Conductive Adhesives Metal-filled composite
Thermoset (a few thermoplastics)
Filler determines type
Very different than solder
Used commercially for SMT and FC
Two types
Isotropic (best for SMT)
Anisotropic; random or patterned
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17. © ET-Trends LLC

18. Classes of Joining Materials
Solders - metallurgical
POLYMER-BASED
Isotropic Cond. Adh. (ICA)
Anisotropic Cond. Adh. (ACA)
Hybrids
Non-conductive
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19. Iso- vs. Anisotropic
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20. + Conductive Polymer Coating
Conductive Particles
still the standard
silver powder
silver flake
use blend
custom penetrating
copper
particles
+ Metal coatings: Ag, solder
+ Anti-corrosion films
+ Conductive Polymer Coating
No real success yet!
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21. Typical CA Composition
Pre-polymers or monomers (epoxy is common)
Hardener and accelerators
Silver filler; major component (>79% wt.)
Additives for wetting, flow, adhesion
Mix under vacuum
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22. Silver is Dominant Moderate cost (some impact)
(but why silver?)
Moderate cost (some impact)
Oxide is electrically conductive
Malleable; can be shaped
Only moderately abundant
Chemically reactive
Safe for humans (biocide)
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23. ICA Adhesive Model Electrical pathways Polymer binder oxide &
The many
interfaces
add electrical
resistance
surfactant
Electrical pathways
Mechanical & Electrical properties are mostly independent
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24. Adhesive Assembly Processes
Stencil
Adhesive
Place
Component
Cure
Adhesive C
Adhesives run cool
Remove or deactivate flux
Stencil
Solder
Place
Component
Reflow
Solder C
Runs on today’s
SMT lines
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Cookson - Foxboro, MA

25. Solder Adhesive Differences <1% reduction 50% reduction Oxide
Half as much volume
<1% reduction
50% reduction
Oxide
Adhesive
Oxide-Tolerant
Much more compatible
Solder Dewetting
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26. SMT Line for Adhesives Digitizer made by SOLDERLESS SMT
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Poly-Flex Circuits - Parlex

27. Test the System MECHANICAL Bond Strength After: ELECTRICAL
Die Shear
Shock; drop
Bond Strength After:
Heat Age
Chemical Exposure
Temp. & Humidity
ELECTRICAL
Volume Resistivity
Electrical Stability
Thermal Cycle
REWORKABILITY
SOLDER: 4-6 mW Per junction
CA: 8-12 mW Per junction
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28. Junction Instability Many PWB finishes oxidize
Most metal oxides are insulators
Oxidation can occur under CAs
Most CAs can form unstable junctions
METAL
INITIAL
OXIDIZED METAL
AFTER AGING
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29. One Proven Solution: Junction-Stable Penetrating Particles Metal oxide
Oxide-Penetrating particles
Junction-Stable
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Poly-Solder used by Poly-Flex a Parlex Co.

30. Penetrating Particle Type Adhesive:
1000 hrs
+18% incr.
+15% incr.
-8%
-3%
-10%
-22%
500 hrs
PLCC-44
1206 resistor
PWB Finish
Bare Cu
Silver Ink
Solder
Gold
85%rh/85oC
Note resistance drop
Junction Resistance - 85%RH/85oC
Sn/Pb solder, the reference, shows no change
Components are solder-coated
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31. - Silver Migration? + Ag Silver is encapsulated by resin.
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32. Flip Chips with CAs Work well, in production
Underfill provides high strength
Underfill eliminates migration concerns
Used for memory, RFIDs
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33. Advantages Low Thermal Processing Stress Eliminate Solder Mask
Excellent Fine Pitch
Wide Process Window
Less Used: (1/4 to 1/6 as much by wt.)
No Flux, No Residues, No Cleaning
No Lead
No a-Particle Emission
Laptop mouse pad,
polyester’ max 150oC
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34. Limitations Lower mechanical shock No self-alignment
Some require non-oxidizing surfaces
Higher electrical resistance
Higher thermal resistance
Much more difficult to rework
Higher cost (than Sn/Pb; L-F?)
Silver: limited supply, aquatic toxicity
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35. Comparison .000015 ohm.cm .00006 ohm.cm Typical Junction R 8 - 15 mW
Characteristic
SOLDER
Best Adhesive
Volume Resistivity
ohm.cm
ohm.cm
Typical Junction R mW
<25 mW
Thermal Conductivity
W/m-deg.K
3 - 8 W/m-deg.K
Shear Strength (1206)
>2000 PSI
> 1600 PSI
T & H (85%/85C)
no change
no change
Mechanical Shock
Pass 6 ft. x 6
Fail 6 ft., once
Thermal Fatigue
Yes
Lower, gradual
Proc. Temp. oC oC
Fine Pitch
Good
Excellent
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36. Conclusions Adhesives run on std. SMT lines
Much lower process temperatures
Low mechanical shock issue
Need to move away from silver
A niche product without a breakthrough
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37. Prognosis Copper adhesives will succeed
metal-coated
conductive polymer coated - boost strength?
Mixtures with solder; now used in inks
Mechanical properties will be the key
Cost probably lower than L-Fs
Metallurgical joints likely to dominate
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