Precipitation of Bi and SbSn Phases in Next-Generation Pb-Free Solders
Authors: S.A. Belyakov, K. Sweatman, T. Akaiwa, T. Nishimura and C.M. Gourlay Company: Imperial College and Nihon Superior Co., Ltd Date Published: 9/22/2019
Abstract: There has been substantial industrial and scientific interest in adding bismuth and/or antimony to next-generation Pb-free solders, as these additions can significantly improve performance in thermal cycling. In this paper, we study the influence of Bi and Sb on solid state precipitation and coarsening in Sn-3.0Ag-0.5Cu-4.0Bi/Cu and Sn-3.0Ag-0.5Cu-5.0Sb/Cu (wt%) solder joints. The focus is on the location and kinetics of (Bi) and SbSn nucleation, growth and coarsening during room temperature storage. (Bi) plates nucleated in tin with two preferred crystallographic orientation relationships (ORs) and, during coarsening, one OR became dominant leading to aligned particles in the tin matrix. SbSn particles initially precipitated in the tin matrix without a preferred OR but, during aging at 125°C, an OR developed with the tin matrix. For short holding times, (Bi) and SbSn particles were small and closely spaced but, for both phases, the room temperature coarsening kinetics are found to be relatively rapid leading to a risk that prolonged storage of electronics could over-age solder joints that are designed to be precipitation strengthened by (Bi) or SbSn.