Reliability of New SAC-Bi Solder Alloys in Thermal Cycling with Aging

Drive towards lead-free electronics began in the early 2000s. Solder pastes based on tin (Sn), copper (Cu), and silver (Ag) were the initial replacement for the traditional SnPb solder. With the SAC alloys, several researches reported that one year of aging consumed more than 50% of the component life. Once the detrimental effects of aging were discovered, the industry started the search for better solder paste materials. The SAC based pastes were made better by adding elements such as Bismuth (Bi), Antimony (Sb), Nickel (Ni). Recently, all the leading manufacturers have introduced new solder materials that claim to have high reliability in harsh environments. Extensive tests are required to filter the best solder pastes. In the study, three high reliability solder materials from leading manufacturers have been selected and used for the test vehicle assembly. SAC305 paste is also included for comparison with the new materials. The test vehicle is a printed circuit board (PCB) of FR-4 laminate material with three CABGA208s (15x15mm) with SAC305 spheres, three LGA36s, and six SM resistors. Three surface finishes, namely electroless Nickel immersion Gold (ENIG), immersion Silver (ImAg), and organic solderability preserve (OSP), have been considered for the study. Immediately after assembly, all boards are aged for a period of twelve months at 125C. All the boards are then thermally cycled for 5000 cycles from -40C to +125C with a ramp time of 50 minutes and dwell times of 15 minutes at high and low temperatures. Two parameters Weibull analysis is used to quantify the performance of the different alloy materials. ANOVA analysis involving the different composition and surface finish is also done in order to get insight into the most influential factors on the component reliability. Generally, all the new alloys were found to outperform SAC305 paste. Materials with a high content of Bi, Sb, and Ag performed the best in the lot. The microstructure analysis showed that bulk solder failure was the typical failure mode with the crack propagating in bulk along with the intermetallic compound layer on the component side.