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EFFECT OF Ag COMPOSITION, DWELL TIME AND COOLING RATE ON THE RELIABILITY OF Sn-Ag-Cu SOLDER JOINTS g
Mulugeta Abtewg
Typical PCB Assembly Process
Component Placement
Solder Paste ApplicationPCB Loading
Automatic Optical
Inspection(AOI)
SolderPaste
Inspection(SPI)
Wave Soldering
Component Hand Load
Reflow Soldering
Visual Inspection
Automatic Optical
Inspection(AOI)
Research DomainFunctional
TestIn-circuit
TestFinal
Assembly
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Ch i i t t k i d R li bilit i t
State of PCB Assembly and Electronics Manufacturing
Change in interconnect packaging and Reliability requirements
o Solder alloys being deployed into structural applications under demanding temperatures (room temperature is 0.5-0.8 Tm) and strain ranges.
Mi i t i ti th t i t t hi th h i l li it f ld t id d d o Miniaturization that is stretching the physical limits of solder to provide sound and reliable solder joints.
o Reduction in cross section area and stand-off height of solder joints from the PCB resulting in a considerable increase in stress and strainresulting in a considerable increase in stress and strain.
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State of PCB Assembly and Electronics Manufacturing
Continuous and dramatic increase in metal costo Sn is up 30% in the past 2 yearso Ag is up 55% in the past yearso SAC305 solder is up 25% in the past
yearso SAC105 17% in the past years
S Al h C k
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Source: Alpha‐Cookson
Research Methodology
1. Full Factorial Design of Experiments
Three factorsThree factors
Two Levels
α = 0.05
FACTORSLEVELS
High (1) Low (2)
Ag Composition 3.0 wt.% (SAC305) 1.0 wt.% (SAC105)
Dwell Time 10 minutes 60 minutes
Cooling Rate 3.0oC/s 1.0oC/s
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Test Vehicle Configuration
T V hi lTest VehiclePCB assembly
1.6 mm thick
Epoxy-glass (FR-4) substrate
18 CBGA components per PCBA
Single sided SMT
Reflow Soldering
In circuit TestIn-circuit Test
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CBGA Package with 360 I/ODaisy Chain Configurations
Time Temperature ProfilesSAC105
SAC305
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Accelerated Thermal Cycling (ATC)
Testing Method
1. Accelerated Thermal Cycling (ATC)
2. Minimum Temperature: 0oC
3. Maximum Temperature: 100oC
4. 10 and 60 minutes dwell at Minimum and
Maximum Temperature
5. Ramp Rate of 10oC / minute
6. Vertical orientation of the assemblies in
the Chamber during testing
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g g
Results & AnalysisResults & Analysis
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RESULTS
At the 95% confidence level, the experimental results showed that:
Dwell time at extreme temperatures during ATC testing to be the most critical factor that determines the number of cycles to fail which corresponds to the fatigue that determines the number of cycles to fail, which corresponds to the fatigue resistance of the solder joints.
Cooling rate and Ag composition were found to have no significant effect on the fatigue life of the Pb-free solder joints. j
Source DF Seq SS Adj SS Adj MS F P
Cooling Rate 1 5202 5202 5202 5.37 0.259g
Dwell Time 1 228488 288488 288488 236.04 0.041
Alloy Type (Ag Composition) 1 3528 3528 3528 3.64 0.307
Cooling Rate*Dwell Time 1 3120 3120 3120 3.22 0.324
Cooling Rate*Alloy 1 364 365 365 0 38 0 650
ANOVA
Cooling Rate Alloy 1 364 365 365 0.38 0.650
Dwell Time*Alloy 1 613 613 613 0.63 0.572
Error 1 968 968 968
Total 7 242283
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Total 7 242283
Initial Microstructure before ATC
Fast Cooled SAC105Slow Cooled SAC105Slow Cooled SAC105
Fast Cooled SAC305
Slow Cooled SAC305
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Initial Microstructure before ATCFAST COOLED SLOW COOLED
The microstructure of the ld h b d
SAC305
SAC305 solder joints exhibited Sn cells with regions of binary eutectic decomposition at their boundaries. The Sn cell size is larger with the slow cooling g grate.
SAC105The microstructure of the SAC105 solder joints exhibitedSAC105 solder joints exhibited Sn cells with regions of binary eutectic decomposition at their boundaries. The Sn cell size is also larger with the slow
li
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cooling rate.
Initial Microstructure of SAC105 before ATC
FAST COOLED SLOW COOLED
In addition, the microstructure of the slow cooled SAC105 exhibited:• Relatively more Cu6Sn5
IMC particles• Wider area of binary
SAC105
• Wider area of binary eutectic at the Sn boundaries
• Intermetallic spalling*
*S lli i th lt f lidifi ti diti th t t i it ti ti i S b d
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*Spalling is the result of solidification conditions that promote a quasi‐peritectic reaction in Sn based solders in the presence of Cu and Ni [L. Snugovsky, Materials Science and Technology, vol. 25, no. 10, 1296‐1300, (2009)].
Initial Microstructure of SAC305 before ATC
FAST COOLED SLOW COOLED
The microstructure of the slow cooled SAC305 exhibited:• Relatively more Cu6Sn5
IMC particles • Wider area of binary
SAC305
• Wider area of binary eutectic at the Sn boundaries
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Summary of Weibull Data for ATC Testingy g
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Weibull plot of ATC Testing Results
Slow cooled SAC305 solder joints showed slightly better fatigue resistance than SAC105 s g t y bette at gue es sta ce t a S C 05solder joints at 10 minute dwell time during ATC.
There was virtually no difference in fatigueThere was virtually no difference in fatigue resistance between SAC305 and SAC105 solder joints at 60 minute dwell time during ATC.
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Weibull plot of ATC Testing Results
• For the eutectic Sn‐Pb solder joints betterFor the eutectic Sn Pb solder joints better fatigue resistance was observed with fast cooling at both 10 and 60 minute dwell times.
• However, the over all fatigue resistance wasHowever, the over all fatigue resistance was significantly lower for the solder joints tested at 60 minute dwell time during ATC.
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SAC105 SLOW COOLED SAC305 SLOW COOLED
Failure mode and crack locationSAC105 SLOW COOLED SAC305 SLOW COOLED
• The location of the first failure for all test cells was in the outer row of balls, at or near a corner position of the package.
• Failure mode was inter‐granular crack along Sn cell boundaries
• All samples exhibited the same failure• All samples exhibited the same failure mode
SAC105 FAST COOLED SAC305 FAST COOLED
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Microstructure of SAC305 near failure site after ATC Testing
FAST COOLED
SLOW COOLED
• Backscattered electron images ill i h i lillustrating the microstructuralevolution in the SAC305 alloy.
SAC305
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Microstructure of SAC105 near failure site after ATC Testing
FAST COOLED
SLOW COOLED
• Backscattered electron images illustrating the microstructural evolution in the SAC105 alloy.
• The intermetallic spalling did not appear to have an impact on fatigue crack propagation.
SAC105
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Microstructure, Cooling Rate, Ag Composition & ATC
1. Cooling rate was found to have little effect on the fatigue life of SAC solder joints. The fatigue life
Major Findings
of SAC solder joints was not significantly affected by the range of cooling rate used in a typical
PCB assembly reflow soldering process.
2. The fatigue life of SAC solder joints was not found to be affected by Ag content. At higher dwell
time (60 minutes), the fatigue life of SAC305 and SAC105 were found to be nearly identical
independent of cooling rate.
3 Longer dwell time during ATC testing dramatically reduced the fatigue life of the SAC105 SAC3053. Longer dwell time during ATC testing dramatically reduced the fatigue life of the SAC105, SAC305
and Sn‐Pb (eutectic or near eutectic) solder joints.
4. Both SAC305 & SAC105 alloys exhibited slightly better fatigue resistance with slow cooling than
ffast cooling.
5. For the eutectic or near eutectic Sn‐Pb solder, fast cooled solder joints exhibited better fatigue
resistance.
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6. Both Pb‐free solders alloys show significantly higher fatigue resistance than eutectic SnPb solder
.
CONCLUSIONS
I. Cooling rate and Ag composition were found to have no significant effect
on the fatigue life of SAC solder joints.
II Magnitude of Stress dwell time during ATC was found to have a significantII. Magnitude of Stress, dwell time during ATC, was found to have a significant
impact on fatigue life of solder joints.
III. The fatigue life of SAC305 and SAC105 were found to be far more superior
to that of the eutectic or near eutectic Sn‐Pb solder joints.
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ACKNOWLEDGEMENT
This work was a joint project between R. Coyle from Alcatel-Lucent and M. This work was a joint project between R. Coyle from Alcatel Lucent and M.
Abtew and R. Kinyanjui from Sanmina-SCI. The work has been published in the
SMTA proceedings, 2011 under the title “Solder Joint Reliability of Pb-free Tin-p g y
Silver-Copper Ceramic Ball Grid Array (CBGA) Packages as a Function of
Cooling Rate and Silver Content”.