Thermal Shock Reliability of Isothermally Aged Doped Lead-Free Solder With Semiparametric Estimation

This paper offers a statistical methodology for evaluating Pb-free solder alloy performance for high-reliability products using Cox proportional regression analysis and provides a detailed application of the technique based upon a 15-mm BGA package. Specifically, the effects of doping SAC305 solder balls with small amounts of Bi, Co, In, Mn, Ni, and Sb either directly or from the solder paste used for assembly have been studied. The test results have led to a down-selection of optimum material combinations that offer a potential solution to the wellknown material degradation of SAC305 solder alloys with aging. The test conditions had two different time/temperature periods: 10 days at 25 °C and an accelerated condition of 180 days at 125 °C. Each test board was temperature shocked 3000 times (-40 °C-125 °C) with a 15-min thermal profile (5-min dwell time and 2.5-min transition time). The electrical continuity of solder interconnect was examined during the tests, and the effect of solder doping and aging on the failure rate (hazard rate) were assessed at 90% confidence level using the proportional hazard model. We found that the solder doping combination, aging time and temperature, and the interactions between these parameters have significant impacts on the failure rate. BGA with Sn-Ag- Cu doped with Sb, Bi, Co, and Ni can significantly reduce the failure rate after aging with a 90% level of confidence. The failure analysis shows recrystallized assisted crack nucleation in the solder joints.