oWhy did SAC305 become the standard LF alloy? SAC was never considered an ideal replacement for eutectic SnPb, it was simply the best choice at the time. It was readily available, had a reasonable melting temperature and had the least reliability issues compared to other options. oHowever, SAC305 has weaknesses that can be overcome with newer alloys. SAC is a precipitation hardened alloy which means the microstructure and mechanical properties are significantly impacted by reflow temperature and time, cooling rate, and aging (dwell times). It is undesirable for the properties of the solder to be so dependent on the assembly conditions and the customer use environment. oThis presentation addresses the latest research and reliability results for 2nd generation lead free (LF) alloys.

1. © 2004 - 2007
2010
– 2010
2nd Generation Lead Free Alloys: Is SAC the Best We Can Do?
SMTA ICSR Toronto, Canada May 7, 2011
Cheryl Tulkoff, ASQ CRE DfR Solutions Sr. Member of the Technical Staff

2. © 2004 - 2007
2010
2nd Generation Lead Free Alloys Course Abstract
oWhy did SAC305 become the standard LF alloy? SAC was never considered an ideal replacement for eutectic SnPb, it was simply the best choice at the time. It was readily available, had a reasonable melting temperature and had the least reliability issues compared to other options.
oHowever, SAC305 has weaknesses that can be overcome with newer alloys. SAC is a precipitation hardened alloy which means the microstructure and mechanical properties are significantly impacted by reflow temperature and time, cooling rate, and aging (dwell times). It is undesirable for the properties of the solder to be so dependent on the assembly conditions and the customer use environment.
oThis workshop addresses the latest research and reliability results for 2nd generation lead free (LF) alloys.
2

3. © 2004 - 2007
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Instructor Biography
oCheryl Tulkoff has over 17 years of experience in electronics manufacturing with an emphasis on failure analysis and reliability. She has worked throughout the electronics manufacturing life cycle beginning with semiconductor fabrication processes, into printed circuit board fabrication and assembly, through functional and reliability testing, and culminating in the analysis and evaluation of field returns. She has also managed no clean and RoHS-compliant conversion programs and has developed and managed comprehensive reliability programs.
oCheryl earned her Bachelor of Mechanical Engineering degree from Georgia Tech. She is a published author, experienced public speaker and trainer and a Senior member of both ASQ and IEEE. She holds leadership positions in the IEEE Central Texas Chapter, IEEE WIE (Women In Engineering), and IEEE ASTR (Accelerated Stress Testing and Reliability) sections. She chaired the annual IEEE ASTR workshop for four years and is also an ASQ Certified Reliability Engineer.
oShe has a strong passion for pre-college STEM (Science, Technology, Engineering, and Math) outreach and volunteers with several organizations that specialize in encouraging pre-college students to pursue careers in these fields.
3

4. © 2004 - 2007
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Outline
oSAC background & alternative alloys
oShock/Drop Test Results
oSAC vs SnPb
oResults of alternative alloys
oVibration Results
oThermal Cycling
oSAC vs SnPb
oResults of alternative alloys
oWill there be one winner?
oSummary
oSupplementary Material
4

5. © 2004 - 2007
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Why is this important to the Industry?
oSome types of equipment have been RoHS exempt to date.
oThis will change with RoHS 2 which states that many formerly exempt products must be RoHS compliant by 2014 (as defined by Directive 93/42/EEC).
oIn-vitro by 2017
oSome exemptions (including lead solder in portable defibrillators)
oSnPb BGA components are being eliminated from the supply chain (forcing LF transition or reballing).
oGood News. The industry can leverage the improvements made since the initial LF transition.
oSAC305 has weaknesses that can be overcome with newer alloys.
5

6. © 2004 - 2007
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Refresher
oWhy did SAC305 become the standard LF alloy?
oReadily available
oReasonable melting temp
oHad the least reliability issues compared to other options
SAC was never considered an ideal replacement for eutectic SnPb, it was simply the best choice at the time.
6

7. © 2004 - 2007
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Sn
Bi
Ag
Zn
Acceptable wetting And high strength
High Melting Point
217C
Strength
Weakness
Melting point is almost the same as SnPb
Easily oxidizes, corro-
sion cracking, voids,
poor wetting
Mixing with Pb degrades
strength and fatigue
resistance
(silver)
(bismuth)
(zinc)
(tin)
Good wetting and
high strength
In
Inadequate source
of supply & corrosion
(indium)
+ Cu
SnAgCu became the industry accepted Pb- free alloy
Lead-free Alloy Summary
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8. © 2004 - 2007
2010
SAC305 or SAC406?
Eutectic
3.5Ag-0.9Cu
Sn-3.9Ag-0.6Cu
Sn-3.0Ag-0.5Cu
Melting temperature is similar.
SAC406 alloy resulted in higher volume fraction of Ag3Sn precipitates.
SAC406 was higher cost.
SAC305 has lower shear stress (more compliant)
SAC305 eventually won out as the standard
Phase Diagram Source: K-W Moon et al, J. Electronic Materials, 29 (2000) 1122-1236
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Mechanical Properties of SAC Alloys
Ref: Yoshiharu Kariya et al. J. of Elect. Mat, 33, No. 4, 2004.
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10. © 2004 - 2007
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Effect of Cooling Rate on Microstructure in SAC405
Cooling rate of 0.81 °C/s
Cooling rate of 1.86 °C/s
Thilo Sack, Celestica
SAC is a precipitation hardened alloy.
This means the microstructure and mechanical properties are significantly impacted by reflow temp/time, cooling rate, and aging (dwell times).
 It is undesirable for the properties of the solder to be so dependent on the assembly conditions and the customer use environment.
SnPb
Soft Sn and Pb phases in eutectic solder
Hard Ag3Sn phases form in SAC solder.
10

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11
Material Properties - Plasticity
SnPb has lower elastic modulus but the yield strength is more impacted by the temperature.

12. © 2004 - 2007
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oThe creep rate of SnPb does change much after aging at temperatures up to 125C (creep testing performed at RT).
Creep of SnPb
J. Suhling, “Material Behavior of Aging Pb-free Solder Joints”, Center for Advanced Vehicle and Extreme Environmental Electronics, Auburn U.
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13. © 2004 - 2007
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oThe creep rate of SAC305 is much more dependant on aging conditions.
SAC 305 Creep
J. Suhling, “Material Behavior of Aging Pb-free Solder Joints”, Center for Advanced Vehicle and Extreme Environmental Electronics, Auburn U.
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14. © 2004 - 2007
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oImpact of aging on creep of SAC105 is even larger.
SAC 105 Creep
SnPb creep range
14

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Exposed Cu after SAC305 Paste Reflow
oHigh surface tension prevents flow and full wetting of Cu features.
15

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Solders: Copper Dissolution
oPTH knee is the point of greatest plating reduction
oPrimarily a rework/repair issue
oCelestica identified significant risk with >1X rework
o>0.5 mil Cu thickness at knee after rework is a standard requirement.
16
16
S. Zweigart, Solectron

17. © 2004 - 2007
2010
SAC is More Vulnerable to Strain
PCB deflection
Tensile force on
pad and Laminate
PbSn
LF
PbSn limit
LF limit
Laminate Load Bearing Capability
NEMI study showed SAC is more
Sensitive to bend stress.
Sources of strain can be ICT, stuffing through- hole components, shipping/handling, mounting to a chassis, or shock events.
17

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18
Drop Testing Performance of SAC305
Roughly a 2X - 5X
reduction in drops to failure for ENIG
JEDEC (JESD22- B111) standard testing 1500 G’s, 0.5 mS pulse width
Board Level Drop Test Reliability of IC Packages Chai TC, Sharon Quek, Hnin WY, Wong EH, Julian Chia*, Wang YY**, Tan YM***, Lim CT****, Institute of Microelectronics
SnPb better in drop testing
ENIG is much worse than OSP

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19
Shock Testing Performance with SAC – surface finish impact
Drop Test Reliability of Wafer Level Chip Scale Packages Mikko Alajoki, Luu Nguyen(* and Jorma Kivilahti Lab. of Electronics Production Technology, Helsinki University of Technology, P.O.Box 3000, 02150 Espoo, Finland*) National Semiconductor Corporation P.O.Box 58090, Mail Stop 19-100, Santa Clara, USA
Roughly a 5X – 10X reduction in drops to failure when switching from OSP to ENIG, failures also occurred on first drop
JEDEC (JESD22-B111) standard testing 1500 G’s, 0.5 mS pulse width
Component Package Qualification for Handheld electronics
CSP metallization – Solder – PWB finish
Ni(P)/Au is ENIG
Cu/ENIG
Cu/OSP
ENIG/ENIG
ENIG/OSP

20. © 2004 - 2007
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What is the Perfect Alloy? Is there one?
Desired Attribute
Comment
Lower Melting Point
Closer to 190C would be desirable
Lower Modulus
Reduction from 51 to 40 GPa (near SnPb)
Good wetting behavior
Wetting time of 0.5 sec or less
Stable behavior
Preferably not precipitation hardened or at least rapidly softens (so properties are consistent after assembly)
Low yield strength combined with low work hardening rate
Similar to SnPb – providing compliance without suffering damage in fatigue.
Low Cu dissolution
To prevent erosion of Cu traces
Low surface tension
For covering of Cu features and wicking up PTHs
There will always be tradeoffs. So there can only be a perfect alloy for a particular application. The following table addresses general consumer applications:
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21
Solder Trends
oSAC305 dominates surface mount reflow (SMT)
oSAC105 increasingly being used in area array components in mobile applications
oSNC pervasive in wave solder and HASL
oIncreasing acceptance in Japan for SMT
oIntensive positioning for “X” alloys (SACX, SNCX)
K-W Moon et al, J.` Electronic Materials, 29 (2000) 1122- 1236

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Likely Elements
oTin will likely be main constituent (forms well understood IMCs with Cu)
oReducing Ag lowers elastic modulus (SAC105 is 11% lower than SAC405)
oSmall amounts of Ni, Co, etc. to arrest IMC formation and reduce Cu dissolution (assuming Tm is above 220C).
oBi to perhaps play a bigger role as Pb is eliminated from supply chain – SnAgCuBi alloys are promising.
oOther elements to be added for improved shock resistance - Mn, Ce show great promise.
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Pb-free Alloys Investigated
SAC105 + Mn or Ce
W. Liu & Ning-Cheng Lee (Indium)
SMTA2006, ECTC 2009
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24
The Current State of Lead-Free
oComponent suppliers
oSAC305 still dominant, but with increasing introduction of low silver alloys (SAC205, SAC105, SAC0507)
oSolder Paste
oSAC305 still dominant
oWave and Rework
oSn07Cu+Ni (SN100C)
oSn07Cu+Co (SN100e)
oSn07Cu+Ni+Bi (K100LD)
oHASL PCB Coating
oSn07Cu+Ni (SN100C)

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LF Rework – Solder Fountain
Time required for rework
There is little to no process window for rework of through hole joints on a thicker board with SAC solder.
Ref: C. Hamilton& M. Kelly, A Study of Copper Dissolution in LF PTH Rework”, SMTA, 2006.
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Hole Fill Challenges with SAC and SnCu
Less than 50% hole fill
To achieve sufficient hole-fill suppliers often increase the preheat temp, solder pot temp and dwell time (this can damage other components).
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Mechanical Shock & Vibration
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Laminate Cracking Leads to Trace Fracture
Bending
Force
Functional failure will occur
Trace routed externally
28

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Drop Test Results – SAC worse than SnPb
29

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Drop Test Results
Ref: B. Roggeman, “Comparison of Drop Reliability of SAC105 and SAC305 on OSP and ENIG Pads”, Unovis, 2007.
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Shock Testing with SAC105
Aging causes precipitate coursening and softening of the alloy
M.Ding and A. Porras, “AGING EFFECTS ON DYNAMIC BEND TEST PERFORMANCE OF Pb-FREE SOLDER JOINTS ON Ni/Au FINISH », SMTA Proceedings, Chicago, 2006
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Drop Results for SACx Solders
W. Liu, N. Lee, “NOVEL SACX SOLDERS WITH SUPERIOR DROP TEST PERFORMANCE”, SMTA Proceedings, Chicago, 2006.
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33. © 2004 - 2007
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Drop Testing
o244 I/O BGA, 0.5 mm
oElectrolytic NiAu on package substrate.
oOSP on PCB.
oJEDEC Drop Test method used.
o250 thermal cycles precondition
Ref: W. Liu et al., “Achieving High Reliability Low Cost LF SAC Solder Joints via Mn or Ce Doping”, ECTC, 2009.
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Mechanical Properties – SN100C
oTensile stress-strain curves compared to SnPb
34

35. © 2004 - 2007
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Mech Properties of SN100C
SN100C overlay (-40 to 125C range)
35

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Vibration
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Vibration Test Results
oSolder joint fatigue life for a 2512 Resistor
Vibration Strain = 2400μϵ
SnPb better than SAC
Vibration Strain = 1200μϵ
SAC better than SnPb
37

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38
Vibration Results: Effect of Solder Material
Resistor
TSOP
CSP
Resistors:
Generally, SAC < SN100C < SnPb
TSOP:
At 50% failure, SN100C < SnPb < SAC
CSP
Generally, SAC << SN100C ~ SnPb
SAC: b ~1
ENIG Surface Finish, 30G vibration

39. © 2004 - 2007
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Vibration Results for SACM and SACC
244 I/O BGA, 0.5 mm
Electrolytic NiAu on package substrate.
OSP on PCB.
Cyclic bend test
Preconditioning at 150C for 250 hours
Cyclic Bend Testing (Fatigue)
In high cycle fatigue SACM and SACC perform better than SnPb but worse than SAC305
Ref: W. Liu et al., “Achieving High Reliability Low Cost LF SAC Solder Joints via Mn or Ce Doping”, ECTC, 2009.
39

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40
Alloy Comparison - Vibration
oTimes to failure for all three solders at extreme test conditions varied based upon the solder joint geometry
oWhy? Stiffness (SAC > SN100C > SnPb)
oFor a given force / load, a stiffer solder will respond with a lower displacement / strain (elastic and plastic)
oLow-cycle fatigue (plasticity driven)
oUnder displacement-driven mechanical cycling, lower stiffness solder will tend to out-perform higher stiffness (e.g., chip scale packages [CSP])
oUnder load-driven mechanical cycling, higher stiffness solder will tend to out- perform lower stiffness (e.g., leads of thin scale outline packages [TSOP])
oHigh-cycle fatigue (elasticity driven)
oStiffer solder (i.e., SAC and SN100C), lower strain range

41. © 2004 - 2007
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41
Shock & Vibration Summary
oSAC (Ag ≥ 3%) should not be used with ENIG in high G environments, vibration testing at 30G’s yielded “random failures” on a small PCB (organic solder protection) with chip scale packages
oPerformance of Pb-free solders in high cycle fatigue is the same or better than SnPb
oSAC105 and SN100C have almost identical creep behaviors and likely have very similar modulus and plastic properties, should have
oDrop/shock performance
oHigh cycle fatigue
oSAC105
oPasty range of SAC alloys increases as the silver content drops, 217 - 226C, reflow greater than 226C necessary
oShrinkage cracks, and effect on life under vibration
oSN100C
oMelting point 227C (liquidus and solidus are at 227)

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Thermal Cycle Performance
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Non underfilled flip chip – SnPb better
Ref: E. Al-Momanl and M. Mellunas, “Lead-free Thermal Cycle Progress, Unovis, June, 2008.
Stiff Component – SnPb is better
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TBGA – SAC better
Ref: E. Al-Momanl and M. Mellunas, “Lead-free Thermal Cycle Progress, Unovis, June, 2008.
Compliant Component – SAC is better
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45
45
Thermal Cycling: SnPb vs. SAC
Where does SnPb outperform Pb-free?
Leadless, ceramic components
Leadless ceramic chip carriers (crystals, oscillators, resistor networks, etc.)
SMT resistors
Ceramic BGAs
Severe temperature cycles
-40 to 125ºC
-55 to 125ºC
Syed, Amkor

46. © 2004 - 2007
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46
46
Thermal Cycling: Effect of Dwell Time
Normalized time to failure as a function of dwell time at maximum temperature for SAC solder
•40% to 60% drop in the number of cycles to failure as dwell is increased past 8 hours
•As the CTE mismatch decreases and the part becomes more compliant, the effect of dwell decreases

47. © 2004 - 2007
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47
47
Thermal Cycling: SnPb-SAC Transition
I. Kim, ECTC 2007
But not this simple – slope of curves and transition will depend greatly on component type and dwell time.

48. © 2004 - 2007
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48
Test Spec
48
Thermal Cycling: When is a Failure Not a Failure?
D Temperature
Time to Failure
Field Condition
Test
SnPb
Pb-Free
Life Requirement
Understanding the acceleration factor is very important

49. © 2004 - 2007
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SnCuNi Data at Different ΔT
The same was done for resistor and TSOP components
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SnCuNi – Acceleration Factor
y = 1E+07x-2.153
y = 2E+07x-2.011
y = 2E+07x-2.175
0
500
1000
1500
2000
2500
3000
3500
40
60
80
100
120
140
160
180
Cycles to 1% Failure
Delta Temperature C
SN100C Thermal Cycle Results
Resistor (2512)
TSOP 44IO Alloy 42
CSP 96IO, 7mm, 0.5mm
Power (Resistor (2512))
Power (TSOP 44IO Alloy 42)
Power (CSP 96IO, 7mm, 0.5mm)
n ~ 2.1 for
SN100C
AF =
ΔTt
ΔTf
(
)
n
50

51. © 2004 - 200107
Typical Pb-Free Thermal Cycle Results
Stiff Component More Compliant Component
Raw data from Qi et al., U. Toronto, Microelectronics
Reliability, 2005 Raw data from Ahmer Syed, Amkor
Thermal Cycle Analysis
27mm PBGA
1000
2000
3000
4000
5000
6000
7000
8000
50 70 90 110 130 150 170 190
Delta Temp
Cycles to 1% Failure
SnPb
SAC
SnPb (n=1.55)
SAC (n=1.75)
Large/stiff components typically perform worse with Pb-free solder.
Acceleration factor is different – typically higher for Pb-free solder
Theoretical ► N1%App = N1%test(ΔTtest/ ΔTApp)n
y = 4E+06x-1.731
y = 8E+06x-1.895
300
500
700
900
1100
1300
1500
1700
1900
60 80 100 120 140 160 180
Cycles to 1% Failure
Delta T
Resistor 2512 - SnPb
Resistor 2512 - SAC
Power (Resistor 2512 -
SnPb)
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52. © 2004 - 2007
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Testing of SAC, SnPb, and SN100C
Similar results when testing from -40/125C (large delta T)
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Expected Field Failure Time (1%)
0
2000
4000
6000
8000
10000
12000
14000
0
20
40
60
80
100
120
140
160
180
Cycles to 1% Failure Rate
Temperature Change in Application
Extrapolation for a CSP (compliant component)
Sn100C
SnPb
SAC
n = 2.18
n = 1.75
n = 1.55
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Estimation for 1% Field Failure Rate
0
2000
4000
6000
8000
10000
12000
14000
0
50
100
150
200
Cycles to 1% Failure
Temperature Change in Application
Extrapolation for a Resistor (stiff component)
Sn100C: n=2.15
SnPb: n=1.73
SAC: n=1.90
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Thermal Cycle Requirements of New Alloys
oTest to failure with two or more delta T values.
oCreate Weibull plots and calculate N1%.
oUsing best fit, calculate n value in acceleration factor.
oUse n value to perform extrapolation to your desired field use conditions.
Results in temp cycling (example -40/125C) are not meaningful without the acceleration factor for that solder.
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56. © 2004 - 2007
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Thermal Cycle Results for SACM and SACC
Thermal Cycle (- 40/125C)
244 I/O BGA, 0.5 mm
Electrolytic NiAu on package substrate.
OSP on PCB.
JEDEC Drop Test method used.
Preconditioning at 150C.
Characteristic Life in Thermal Cycle Testing
SACM and SACC perform similar to SAC305 in ATC but are much better in shock.
Ref: W. Liu et al., “Achieving High Reliability Low Cost LF SAC Solder Joints via Mn or Ce Doping”, ECTC, 2009.
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SAC 105 vs SAC 305 Thermal Cycle
oTesting of a BGA memory device with SAC305 passed thermal cycle requirements of 0/100C for 1000 cycles with no cracks shown after dye and pry.
oThe samples with SAC105 failed this testing with 30% of the samples having cracks over 50% of the solder joint area.
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SAC105 +Ni +Cr
oAddition of both elements improved shock performance.
Ranjit S Pandher, Robert Healey, “Reliability of Pb-Free Solder Alloys in Demanding BGA and CSP Applications,” Proceedings 58th Electronic Components and Packaging Technology (ECTC), Orlando, May 27-30, 2008.
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SAC0307+Bi
oSACX (Cookson) is a version of this alloy.
oThermal cycle results are said to approach that of SAC305.
Ranjit S Pandher, Robert Healey, “Reliability of Pb-Free Solder Alloys in Demanding BGA and CSP Applications,” Proceedings 58th Electronic Components and Packaging Technology (ECTC), Orlando, May 27-30, 2008.
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Creep Results after Aging at 100C
Ref: J. Suhing, “Material Behavior of Aging LF Solder Joints”, CAVE, Auburn U, 2009
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61
Divergence in Solder Selection
oConsiderations include
oPRICE!
oInsufficient performance
oNewly identified failure mechanisms
oMarket still unsteady; proliferation and evolution of material sets
oSolder seeing the fastest increase in market share?
oSnCu+Ni (SNC)
SAC405
SAC305
SAC105
SACX
SNC
SnAg
SNCX
SnCu
SnAgCu
??

62. © 2004 - 2007
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oIt was easiest to widely adopt when the LF transition was required.
oIts high strength provides better thermal cycle behavior for compliant packages (BGAs, CSPs, QFPs).
oIts high yield strength enables better high cycle fatigue performance (low amplitude vibration).
oThe wetting properties are sufficient for surface mount components (although head & pillow defects are more common and lack of flow results in exposed Cu).
oIts higher creep resistance enables higher operating conditions.
Advantages of SAC305
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63. © 2004 - 2007
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oIts high liquidus temp requires up to 260C processing (higher energy usage – not really “green”, stresses the PCB and components).
oIt is a precipitation hardened alloy so the mechanical properties change dramatically depending on processing and aging conditions.
oIts marginal wetting behavior (high surface tension), Cu dissolution, and cost are not ideal.
oIts high modulus results in pad cratering as a common failure mode (under dynamic strain).
oThe thermal cycle reliability is worse for “stiff” components such as resistors and capacitors.
oShock performance is much lower than SnPb.
Weaknesses of SAC305
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SAC105
oImproved drop and shock performance over SAC305
oThermal cycle life is less than SAC305
oCreep rate is very high
oLower copper dissolution rates in SMT joints
oReduced intermetallic compounds and occurrences of silver tin platelets
oIs greatly improved with additions of Mn or Ce (more data may prove these to be winners as a ball alloy)
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Wave Solder Alloys
oSnAgCu – moderate wetting, much dross, excessive Cu dissoluton, expensive.
oSnCu – poor wetting, lower cost, higher pot temp required.
oSnCuNi (or SnCuX) – good wetting, moderate cost, bath control required (to keep Cu & Ni ratio in spec).
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SN100C
oProving promising for wave solder and HASL coating.
oCareful control of Cu & Ni levels are required.
oPromising data for surface mount (ATC, vibration, shock).
oSmooth surface reduces crack initiation sites.
oHigher reflow temp required.
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Supplementary Material
67

68. © 2004 - 200107 68 68
Thermal Cycling: Stress Relaxation
 Pb-free alloys demonstrate higher creep resistance
 Results in greater durability under accelerated testing (fast ramps, short dwells)
 Exception: Very high temperatures (>125oC), high stress loadings (leadless, ceramic)
 When will Pb-free be less reliable in the field?
Time
Stress
SAC
SnPb
Temperature

69. © 2004 - 200107 69 69
Thermal Cycling: Effect of Dwell Time
0
2
4
6
8
10
0 100 200 300 400 500
SAC life / SnPb life
Dwell Time (min)
Ceramic BGA on FR4
0 to 100C (experimental data, Bartello, 2001)
2512 Resistor on FR4
25 to 80C (modeling, Blattau, 2005)
Based on creep laws developed by Schubert
and damage model developed by Syed

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Copper Dissolution by Alloy
Initial Thickness = 1.7 mil
Ref: C. Hamilton& M. Kelly, A Study of Copper Dissolution in LF PTH Rework”, SMTA, 2006.
SnCuNi is similar to SnPb with respect to Cu dissolution rate.
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Intermetallic Growth
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72. © 2004 - 2007
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Intermetallic Growth
72

73. © 2004 - 2007
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Intermetallic Growth
73

74. © 2004 - 2007
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Reflow of SAC105
The narrow thermal margin between the liquidus temperature of the low silver spheres and the peak temperature of the assembly raises concerns about incomplete ball collapse and incomplete mixing of the solder alloy with the sphere material, resulting in non-homogenous solder joints. This head-in-pillow solder joint was formed under temperatures high enough to melt the SAC 305 solder, but too low to melt the SAC105 sphere.
REFERENCE:
LOW-SILVER BGA ASSEMBLY PHASE I – REFLOW CONSIDERATIONS AND JOINT HOMOGENEITY
SECOND REPORT: SAC105 SPHERES WITH TIN-LEAD PASTE
Chrys Shea
Ranjit Pandher
Cookson Electronics
South Plainfield, NJ, USA
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Dwell Time Impact on SAC351
Ref: E. Al-Momanl and M. Mellunas, “Lead-free Thermal Cycle Progress, Unovis, June, 2008.
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Testing of SAC, SnPb, and SN100C
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77. © 2004 - 2007
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SAC Mechanical Properties
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Auburn Drop Results
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79. © 2004 - 2007
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80. © 2004 - 2007
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Failure Analysis
80

81. © 2004 - 2007
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Evaluation of Lead-less Resistor Reliability
FEA results and calibration of model predictions with experimental results. SnPb performs significantly better under these conditions.
Bend cycling ENIG
81

82. © 2004 - 2007
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SnPb Aging Effects
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SAC 405 Aging Effects
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oTin and copper bond to form intermetallics of Cu3Sn and Cu6Sn5
oIrreversible
oOccurs rapidly in the liquid state, but rate still appreciable in solid state (even at room temperature)
oTotal intermetallic thickness after all assembly and rework should be between 1 to 4 um
oElements
oBi is in solid solution in the tin-rich phase or precipitates out (>1%)
oIn will form binary intermetallic species with Ag and Cu and ternary intermetallic species SnAgIn and SnCuIn
oCo seems to display similar behavior to Ni
Intermetallic Basics
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Zn additions can cause SCC Mechanism
Suganuma, et.al, JIEP project paper, Soldertec/IPC conference, Brussels, 2003
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Pb Contamination Results
Sn-Ag-Bi alloys have attractive mechanical properties but if mixed with a small amount of Pb severe degradation occurs.
These hold promise as Pb is eliminated from the supply chain.
T. Woodrow, Boeing
Company, IPC Pb-free
conference proceedings,
San Jose, 2003.
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Indium Containing Pb-Free Alloys
Indium has attractive low melting temperature properties.
The primary issue is source of supply.
oYearly amount of Pb solder used in electronics = 60,000 tons.
oYearly world wide production of In = 100 tons.
Max wt% Indium allowable in a complete Pb-free replacement alloy = 0.20% (to use up world wide supply and drive cost up).
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