Achieving high reliability low cost lead-free SAC solder joints via Mn or Ce doping

In this study, the reliabilities of low Ag SAC alloys doped with Mn or Ce (SACM or SACC) were evaluated under JEDEC drop, dynamic bending, thermal cycling, and cyclic bending test conditions against eutectic SnPb, SAC105, and SAC305 alloys. The Mn or Ce doped low cost SAC105 alloys achieved a higher drop test and dynamic bending test reliability than SAC105 and SAC305, and exceeded SnPb for some test conditions. More significantly, being a slightly doped SAC105, both SACM and SACC matched SAC305 in thermal cycling performance. In other words, the low cost SACM and SACC achieved a better drop test performance than the low Ag SAC alloys plus the desired thermal cycling reliability of high Ag SAC alloys. The mechanism for high drop performance and high thermal cycling reliability can be attributed to a stabilized microstructure, with uniform distribution of fine IMC particles, presumably through the inclusion of Mn or Ce in the IMC. The cyclic bending results showed SAC305 being the best, and all lead-free alloys are equal or superior to SnPb. The reliability test results also showed that NiAu is a preferred surface finish for BGA packages over OSP.