mix alloy soldering (sn/pb soldering in a pb free world) alloy soldering (sn/pb soldering in a pb...
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Mix Alloy Soldering
(Sn/Pb Soldering in a Pb Free World)Dr. Neil Poole
January 2007
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Why Backward Why Backward Compatibility?Compatibility?l Some electronics products have exemptions in EU RoHS
directiveLead in solders in high reliability applications such as servers, medicalequipment, telecommunication products until 2010 or beyond.USA has no Pb free legislation (some states looking CA)
l Some SnPb components will be unavailable or command a high premium
Component manufacturers do not want to carry both SnPb andlead-free production lines due to the cost concern
Volume of components for these high reliability products is low
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IntermetallicIntermetallic Bonding (Soldering).Bonding (Soldering).
SUBSTRATE INTERMETALLIC (compound)
Cu Cu6 Sn
Ag Ag3 Sn
Au Au Sn4 (WEAK, BRITTLE)
Ni Ni3 Sn4
- TIN IS THE ‘ACTIVE INGREDIENT’ IN SOLDER -
SOLDERSOLDER
INTERMETALLICINTERMETALLIC LAYERLAYER
SUBSTRATESUBSTRATE
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The The IntermetallicIntermetallic LayerLayerThickness of intermetallic layer depends on: -
l Substrate compositionAu > Ag > Cu > Pd > Ni
l Solder compositionl Temperature!l Time!
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““Mixed AlloyMixed Alloy”” SystemsSystems
l Pb free lead finishes with Pb solderLittle work done so farPossible profile changes
l Pb lead finishes with Pb free solderSome work doneSeen as a transitional problem
l Even with exemptions full Sn/Pb may not be possible
l Mistakes will happen
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““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Component Finishesfree Component Finishes
l Pure SnMost commonWhiskers(?)
l Sn/BiMore popular in JapanReliability Questions
l Ni/Pd/AuVendor SpecificQuestions about wettability
l Sn/Pb + Pure SnWetting issuesReflow Profile
l Sn/Pb + Sn/BiSn/Bi/Pb – low melt temp
l Sn/Pb + Ni/Pd/AuLeast affected
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Alloy Melting PointsAlloy Melting Points
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IntermetallicIntermetallic Thickness Thickness
Cu3SnCu6Sn3Cu3SnCu6Sn3
Sn62 AlloySAC 305 AlloyTreatmentFinish
0.51.01.03.0As Reflowed
Cu
1.53125ºC for 1000 hr
ENIC
12.5As Reflowed
ENIC
0.53.51.04.0125ºC for 1000 hr
Im Ag
0.51.00.03.0As Reflowed
Im Ag
0.53.51.04.0125ºC for 1000 hr
Cu
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Wetting Speed Wetting Speed vsvs Temperature for Temperature for Different AlloysDifferent Alloys
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
200 210 220 230 240 250 260 270 280
Test temperature, °C
tim
e to
2/3
max
wet
tin
g f
orc
e, s
60/40
SnCu base alloys
SnAg & SnAgCu base alloys
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Comparative WettingComparative WettingComparative Wetting Times for Pb Free Terminations
For Sn62 and SAC 387 Alloys
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Comparative WettingComparative Wetting
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Comparative WettingComparative Wetting
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N/AN/AIm Sn
N/AN/AIm Ag
10.3 Kg9.7 KgENIC
10.0Kg9.5 KgCopper
Peak Reflow Temperature 240ºC
Peak Reflow Temperature 215ºCPad Finish
““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Component Finishesfree Component Finishes
Shear testing
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Au 240ºC
Au 215ºC
Cu 215ºC
““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Component Finishesfree Component Finishes
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““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Component Finishesfree Component Finishes
0402
215ºC 240ºC
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““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free free ““LeadedLeaded”” Component FinishesComponent Finishes
215ºC 240ºC
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““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free free ““LeadedLeaded”” Component FinishesComponent Finishes
Au 215ºC Au 240ºC
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Conclusions Leaded and Conclusions Leaded and Leadless ComponentsLeadless Components
l Process temperature can influence joint microstructureHigher peak temperature can increase Ag3Sn dendritesHigher peak temperature can increase Pb phaseHigher peak temperature increase initial intermatallic thicknessHigher temperature reduces voidingHigher peak temperatures better wetting (leaded components)
l Initial joint strengths independent of the profile l Effects on reliability still to be determined!!!!
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““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Bumped Component Finishesfree Bumped Component Finishes
l SAC AlloysSAC 305SAC 405SAC 387SAC 105??
l SACs 305, 387, 405 appear to be equalIPC Solder Value Council
l Many, Many, Many QuestionsUnderfill!!!
All essentially eutectic melt point 217ºC
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l Pb-free BGA bumps / Sn/Pb Pastel Metals in the solder joint:
Sn (bulk)AgCuPbNiAu
l Not evenly distributed not necessarily eutectic structure
““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Bumped Component Finishesfree Bumped Component Finishes
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Bumped Component ChallengeBumped Component Challenge
• What reflow profile??
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l Outer Appearance – BGA bumps
New BGA
““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Bumped Component Finishesfree Bumped Component Finishes
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l Outer Appearance – BGA bumps
Lead paste Reflowed@ 210 C
Ball did not melt
““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Bumped Component Finishesfree Bumped Component Finishes
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l Outer Appearance – BGA bumps
Lead paste Reflowed@ 210 C
Ball did not melt
““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Bumped Component Finishesfree Bumped Component Finishes
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l Pb-free BGA bumps / Sn/Pb Paste & ProfileBump doesn’t melt/collapse
Component at a higher standoffStress distribution differentComponent tiltImpact to reliability
Segregated joint
““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Bumped Component Finishesfree Bumped Component Finishes
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““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Bumped Component Finishesfree Bumped Component Finishes
l Pb-free BGA bumps & Profile / Sn/Pb PasteFlux exhaustion of the Sn/Pb Paste
Poor coalesanceIncreased voiding/blowholes
Insufficient activityPoorer wetting
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““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Bumped Component Finishesfree Bumped Component Finishes
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Mixed Alloy Reflowed BGAMixed Alloy Reflowed BGA
Rockwell
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Mixed Alloy Reflowed BGAMixed Alloy Reflowed BGA
Thermal Cycling-55 / +125
10C/min, 11 min dwell“Normal” Sn/Pb= 1000+
Mixed Metal = 267
Rockwell
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““Mixed AlloyMixed Alloy”” Reflowed BGAReflowed BGA
Ag3Sn Platelet inside the void indicates that the void formed during the soldering process or during cooling.
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““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Bumped Component Finishesfree Bumped Component Finishes
Bump collapsed and part self centeredFull mixing of alloys.
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Micro Structure of Joints with Micro Structure of Joints with Low Levels of PbLow Levels of Pb
0.2% Pb
4.0% Pb
Forsten, Steen, & Wilding, Soldering & Surface Mount Technology
Pb-rich phase, 179°C melting temperature
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Final Joint Compositions & Final Joint Compositions & LiquidusLiquidus for for Sn3Ag0.5CuSn3Ag0.5Cu Ball and Sn37Pb PasteBall and Sn37Pb Paste
2080.32.111.91090411 (Sq)0.5
203(200*)
0.31.617.01080514 (Sq)0.65
2090.42.211.11485516 (Sq)0.8
2150.42.64.92290518 (Sq)1.0
2160.52.82.9281005
2160.52.73.428100621 (Ro)1.27
Liquidus temp. (C)
% of Cu
% of Ag
% of Pb Ball Dia. (mil)
Transfer Ratio (%)
Stencil Thickness (mil)
Aperture Size (mil)
Pitch (mm)
J. Pan, et al., IPC/JEDEC Pb-free Conference, San Jose, 2006.* Calculated by Dr. Ursula R. Kattner of NIST from NIST thermodynamic database
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Final Joint Compositions & Final Joint Compositions & LiquidusLiquidus for for Sn4Ag0.5Cu Ball and Sn37Pb PasteSn4Ag0.5Cu Ball and Sn37Pb Paste
2060.32.711.91090411 (Sq)0.5
201(197*)
0.32.217.01080514 (Sq)0.65
2070.42.911.11485516 (Sq)0.8
2170.43.54.92290518 (Sq)1.0
218*0.53.72.9281005
2180.53.63.428100621 (Ro)1.27
Liquidustemp. (C)
% of Cu% of Ag% of Pb Ball Dia. (mil)
Transfer Ratio (%)
Stencil Thickness
(mil)
Aperture Size (mil)
Pitch (mm)
J. Pan, et al., IPC/JEDEC Pb-free Conference, San Jose, 2006.* Calculated by Dr. Ursula R. Kattner of NIST from NIST thermodynamic database
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““Mixed AlloyMixed Alloy”” ReliabilityReliability-- Thermal CycleThermal Cycle
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Effect of Effect of UnderfillUnderfill on BGS/on BGS/CSPCSP’’ss
1.00 1000.0010.00 100.001.00
5.00
10.00
50.00
90.00
100.00 ReliaSoft's Weibull++ 6.0 Drop Testing .5mm CSP
Drop Count
% Failure
12/19/2006 16:28HENKEL
Tom White
Underfill B 0.5 SAC Drop
Underfill B0.5 SnPb Drop
Underfill A 0.5 SAC Drop
Underfill A 0.5 SnPb Drop
(2900 Gs, 0.3 msec Pulse)
?
?
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ConclusionsConclusionsl There are successes with mixed metal systemsl Reliability window is reduced (underfill?)
Much work still to be donel iNEMI committee looking into mixed metal BGA/CSP
assemblyl Other companies also investigatingl Avoid it if possible, understand the risks if required
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SourcesSources
l Y. Kariya et al: J. Electronic Materials, 1998, 27, p1229-1235l M. Harrison et al: Soldering & Surface Mount Technology, vol 13 no 3, p21-
38l NCMS Report no 0096RE01, 2001l M. Bozack: Auburn University, 2002l G. Swan et al: Proceedings APEX, 2001, paper LF2-6l A. Syed: Proceedings APEX, 2001, paper LF2-7l D. R. Banks et al: Proc SMI conf, 1996, p121-126
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AcknowledgementsAcknowledgementsl Dr. Neil Poole – Sr. Applications Chemistl Dr. Hector Steen – Sr. Development Scientistl David Hillman – Rockwell Collinsl Paul Wood – OK Industriesl Ramon Mendez – Elcoteql Tom White – Henkell Pauline Chan – Henkell Dr. Jainbiao (John) Pan - Assistant Professor Cal Poly State University,
San Luis Obispo.
Any Questions
Dr. Neil Poole