Voiding and Drop Test Performance of Lead-Free Low Melting and Medium Melting Mixed Alloy BGA Assembly

摘要

Low melting 57Bi42Sn1Ag (BiSnAg) was explored for replacing SAC solders as a low-cost solution. In this study, BGAswith SAC105, SAC305, and BiSnAg balls were assembled with SAC105, SAC305 or 57Bi42Sn1Ag solder paste. Jointmechanical strength, drop test performance, and voiding performance were evaluated against the reflow profile. SnPb wasincluded as a control. The findings are as follows: (1) The microstructure of solder joints showed that, among all of thecombinations, only BiSnAg-105 LT and BiSnAg-305 LT exhibited well-distinguishable alloy regions. For SAC-BiSnAgsystems, Sn-dendrites were noticeable at LT, while Ag3Sn needles developed at HT. The joints were homogeneous for therest of the combinations. (2) In the shear test, combinations involving BiSnAg solder were brittle, regardless of the Bi alloylocation and reflow profile, as evidenced by stress-strain curves and morphology of the ruptured surface. The strong influenceof Bi on the rupture site may have been caused by the stiffening effect of solder due to the homogenized presence of Bi in thejoint. With the stiffened solder, the brittle IMC interface became the weakest link upon shearing, although the brittle BiSncrystalline structure also contributed to the rupture. (3) In the drop test, all Bi-containing solder joints performed poorlycompared with Bi-free systems, which was consistent with shear test results. Drop numbers increased with increasingelongation at break of solder bumps as measured in the shear test. (4) Voiding was affected by flux chemistry and reduced bylow alloy homogenization temperatures and solid top factors, but was increased by low surface tension factor, meltingsequence factor, overheating factor and wide pasty range factor. Compared to SAC or SnPb systems, the BiSnAg system islow in voiding if reflowed at LT. In this study, voiding had an insignificant effect on shear strength and drop testperformance.