Authored By:
Anto Raj, Sharath Sridhar, Ph.D., Seth Gordon, John Evans, Ph.D., Michael Bozack Ph.D., Wayne Johnson, Ph.D.
Auburn University
AL, USA
Summary
This work considers the deterioration in characteristic lifetime and mean time to failure (MTTF) under isothermal aging at 75°C for 6, 12, and 24 months for 15mm and 17mm BGA assemblies on 0.200” power computing printed circuit boards. The solders were doped, low creep, lead free alloys designed for high-temperature reliability: SAC doped Sb, Sn-4Ag-0.5Cu-0.05Ni, Sn-3.8Ag-0.7Cu-3Bi-1.4Sb-0.15Ni and Sn-3.8Ag-0.7Cu-3Bi-1.5Sb-0.02Ni. The board substrate was Megtron6. The assemblies were subjected to thermal cycles of -40°C to +125°C with a 120-minute thermal profile with a 15-minute dwell time and 45-minute transition time. We find that the solder pastes have lower degradation as measured by characteristic lifetime after 24-months of aging at 75°C compared with Sn-3Ag-0.5Cu (SAC305) and tin-lead (SnPb) solder pastes [1]. Sn-4Ag-0.5Cu-0.05Ni has shown the least degradation compared with the other solder pastes. The design of experiments (DOE) method was performed to analyze the factors key to the degradation, such as the solder paste, BGA package, and aging time. We find that all of the factors have significant effect on MTTF with a 90% level of confidence. Failure analysis indicated that crack propagation occurred at the top and bottom of the solder joint.
Conclusions
The thermal cycling reliability of 15mm and 17mm BGA packages with different lead-free solder pastes exposed to 6, 12, and 24 months of isothermal aging at 75°C has been studied. The performance of the lead-free solder pastes was compared with SAC305 and SnPb [1]. The pastes show reduced degradation in characteristic lifetime after 24-months of aging at 75°C compared with SAC305 and SnPb. Considering the doped lead-free solder pastes that were tested, Sn-4Ag-0.5Cu-0.05Ni has shown the highest characteristic lifetime followed by SAC doped Sb. A DOE analysis was performed to analyze the effects of solder paste type, package configuration, and aging time. It was found that, within a 90% level of confidence, all these factors, including their interactions, have significant effect on MTTF. Failure analysis showed crack propagation in failed interconnects at the top and bottom of the solder joint. Intermetallic thickness measurements showed that the growth curve was least for Sn-3.8Ag-0.7Cu-3Bi-1.4Sb-0.15Ni and Sn-4Ag-0.5Cu-0.05Ni solder paste compared with Sn-3.8Ag-0.7Cu-3Bi-1.5Sb-0.02Ni and SAC doped Sb. The Sn-4Ag-0.5Cu-0.05Ni solder paste was significantly better than other lead-free solder pastes, with the least deterioration after 24-months of aging at 75°C.
Initially Published in the SMTA Proceedings
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