Long Term Isothermal Aging of BGA Packages Using Doped Lead Free Solder Alloys
Authors: Anto Raj, Sharath Sridhar, Ph.D., Seth Gordon, John Evans, Ph.D., Michael Bozack Ph.D., Wayne Johnson, Ph.D. Company: Auburn University Date Published: 10/14/2018
Abstract: 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 . 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.
BGA, High Reliability Solder, Doping, PCB, Reliability, Solder, lead free