EFFECT OF Ag COMPOSITION, DWELL TIME AND COOLING RATE ON THE RELIABILITY OF Sn-Ag-Cu SOLDER JOINTS Mulugeta Abtew
Typical PCB Assembly Process PCB Loading Solder Paste Application Solder Paste Inspection (SPI) Component Placement Automatic Optical Inspection (AOI) Visual Inspection Wave Soldering Component Hand Load Automatic Optical Inspection (AOI) Reflow Soldering In-circuit Test Functional Test Final Assembly Research Domain 11/1/2012 2
State of PCB Assembly and Electronics Manufacturing Change in interconnect t packaging and Reliability requirements o o o Solder alloys being deployed into structural applications under demanding temperatures (room temperature is 0.5-0.8 Tm) and strain ranges. Miniaturization i i ti that t is stretching t the physical limits it of solder to provide sound and reliable solder joints. Reduction in cross section area and stand-off height of solder joints from the PCB resulting in a considerable increase in stress and strain. 11/1/2012 3
State of PCB Assembly and Electronics Manufacturing Continuous and dramatic increase in metal cost o Sn is up 30% in the past 2 years o Ag is up 55% in the past years o SAC305 solder is up 25% in the past years o SAC105 17% in the past years Source: Alpha Cookson 11/1/2012 4
Research Methodology 1. Full Factorial Design of Experiments Three factors Two Levels α = 0.05 FACTORS LEVELS High (1) Low (2) Ag Composition 3.0 wt.% (SAC305) 1.0 wt.% (SAC105) Dwell Time 10 minutes 60 minutes Cooling Rate 3.0 o C/s 1.0 o C/s 11/1/2012 5
Test Vehicle Configuration PCB assembly Test Vhil Vehicle 1.6 mm thick Epoxy-glass (FR-4) substrate 18 CBGA components per PCBA Single sided SMT Reflow Soldering In-circuit Test Daisy Chain Configurations 11/1/2012 6 CBGA Package with 360 I/O
Time Temperature Profiles SAC105 SAC305 11/1/2012 7
Accelerated Thermal Cycling (ATC) Testing Method 1. Accelerated Thermal Cycling (ATC) 2. Minimum Temperature: 0 o C 3. Maximum Temperature: 100 o C 4. 10 and 60 minutes dwell at Minimum and Maximum Temperature 5. Ramp Rate of 10 o C / minute 6. Vertical orientation of the assemblies in the Chamber during testing 11/1/2012 8
Results & Analysis 11/1/2012 9
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 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. Source DF Seq SS Adj SS Adj MS F P ANOVA Cooling Rate 1 5202 5202 5202 5.37 0.259 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 1 364 365 365 038 0.38 0650 0.650 Dwell Time*Alloy 1 613 613 613 0.63 0.572 Error 1 968 968 968 Total 7 242283 11/1/2012 10
Initial Microstructure before ATC Slow Cooled SAC105 Fast Cooled SAC105 Fast Cooled SAC305 Slow Cooled SAC305 11/1/2012 11
Initial Microstructure before ATC FAST COOLED SLOW COOLED SAC305 The microstructure of the SAC305 solder joints exhibited hb Sn cells with regions of binary eutectic decomposition at their boundaries. The Sn cell size is larger with the slow cooling rate. SAC105 The microstructure of the SAC105 solder joints exhibited Sn cells with regions of binary eutectic decomposition at their boundaries. The Sn cell size is also larger with the slow cooling rate. 11/1/2012 12
Initial Microstructure of SAC105 before ATC FAST COOLED SLOW COOLED SAC105 In addition, the microstructure of the slow cooled SAC105 exhibited: Relatively more Cu 6 Sn 5 IMC particles Wider areaofbinary eutectic at the Sn boundaries Intermetallic spalling* *Spalling is the result of solidification conditions that t promote a quasi peritecticreaction it ti ti in Sn based solders in the presence of Cu and Ni [L. Snugovsky, Materials Science and Technology, vol. 25, no. 10, 1296 1300, (2009)]. 11/1/2012 13
Initial Microstructure of SAC305 before ATC FAST COOLED SLOW COOLED SAC305 The microstructure of the slow cooled SAC305 exhibited: Relatively more Cu 6 Sn 5 IMC particles Wider areaofbinary eutectic at the Sn boundaries 11/1/2012 14
Summary of Weibull Data for ATC Testing 11/1/2012 15
Weibull plot of ATC Testing Results Slow cooled SAC305 solder joints showed slightly ybetter fatigue resistance sta cethan SAC105 solder joints at 10 minute dwell time during ATC. There was virtually no difference in fatigue resistance between SAC305 and SAC105 solder joints at 60 minute dwell time during ATC. 11/1/2012 16
Weibull plot of ATC Testing Results For the eutectic Sn Pbsolder joints better fatigue resistance was observed with fast cooling at both 10 and 60 minute dwell times. However, the over all fatigue resistance was significantly lower for the solder joints tested at 60 minute dwell time during ATC. 11/1/2012 17
Failure mode and crack location SAC105 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 mode SAC105 FAST COOLED SAC305 FAST COOLED 11/1/2012 18
Microstructure of SAC305 near failure site after ATC Testing FAST COOLED SLOW COOLED Backscattered electron images illustrating i the microstructural evolution in the SAC305 alloy. SAC305 11/1/2012 19
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 11/1/2012 20
Microstructure, Cooling Rate, Ag Composition & ATC Major Findings 1. Cooling rate was found to have little effect on the fatigue life of SAC solder joints. The fatigue life 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, SAC305 and Sn Pb (eutectic or near eutectic) solder joints. 4. Both SAC305 & SAC105 alloys exhibited slightly better fatigue resistance with slow cooling than fast cooling. 5. For the eutectic or near eutectic Sn Pb solder, fast cooled solder joints exhibited better fatigue resistance. 6. Both Pb free solders alloys show significantly higher fatigue resistance than eutectic SnPb solder. 11/1/2012 21
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 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. 11/1/2012 22
ACKNOWLEDGEMENT 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- Silver-Copper Ceramic Ball Grid Array (CBGA) Packages as a Function of Cooling Rate and Silver Content.