The Effects of Aging on Doped Lead-Free Solder under Thermal Shock and Thermal Cycling
Type of DegreePhD Dissertation
Industrial and Systems Engineering
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In this experiment, the thermal shock performance of various doped Pb-free solder pastes on Ball Grid Array (BGA) packages and resistors were observed in order to determine their reliability. This investigation is based on evaluating the potential substitutes to replace the SAC305 solder paste which has deleterious effects in long term isothermal aging conditions. The effects of doping SAC305 solder balls with small amounts of Bismuth (Bi), Indium (In), Manganese (Mn), Nickel (Ni), and Antimony (Sb) either directly or via the solder paste used for assembly have been studied. Packages used for testing were SMR 2512, QFN’s, 6mm BGA, 15mm BGA, 31mm BGA, and 35mm BGA. The test results have led to a downselection of optimum material combinations that offer a potential solution to the well-known material degradation of SAC305 solder alloys with aging. The test conditions had two different time/temperature periods: After assembly (No aging) and an accelerated aging condition of 6 months at 125°C. Each test board was temperature shocked 3000 times (−40°C to 125°C) with a 15-minute thermal profile (5-minute dwell time and 2.5-minute transition time). The electrical continuity of solder interconnect was examined during the tests. The failure analysis shows recrystallized-assisted crack nucleation in the solder joints. Solder materials that showed higher thermal shock reliability and reduced degradation in characteristic lifetime after aging compared with SAC305 were chosen for the next phase of the test which is Phase II. Phase II considers the long term aging effects of Pb-free solders with higher thermal shock reliability. Isothermal aging at 75°C for 6, 12, and 24 months were used. The board substrate for Phase II was Megtron6. The test vehicles were comprised of surface mount resistors, 5mm, 6mm, 13mm, 15mm, 17mm, 31mm, 35mm, and 45mm BGA packages. 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. 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 . A high level of degradation was seen for the SAC305 solder paste throughout the test from Phase I. Failure analysis indicated that crack propagation occurred at the top and bottom of the solder joint. The only material from Phase II to outperform the materials in TC-Phase I (SAC305 and SnPb) for the 24-month aged group was Sn-4Ag-0.5Cu-0.05Ni solder paste which exhibited a much lower degradation rate (17%). According to these results, Sn-4Ag-0.5Cu-0.05Ni solder paste performed best across all BGA design. The materials performed much differently when used with the 2512 resistors. Sn-3.8Ag-0.7Cu-3Bi-1.4Sb-0.15Ni and Sn-3.8Ag-0.7Cu-3Bi-1.4Sb-0.15Ni pastes were the top performing materials for the 2512 resistors but performed poorly for BGA components. Sn-4Ag-0.5Cu-0.05Ni proved to be the worst material for 2512 resistors.