Phase Formation and Solid Solubility in High Reliability Pb-Free Solders Containing Bi, Sb or In
Authors: S.A. Belyakov, B. Arfaei, C. Johnson, K. Howell, R. Coyle and C.M. Gourlay Company: Imperial College, Binghamton University, Ford Motor Company, Nokia Bell Labs, Nihon Superior Co., Ltd Date Published: 9/22/2019
Abstract: Sn-Ag-Cu (SAC) solders generally have better thermal cycling performance than eutectic Sn-Pb. However, their performance deteriorates significantly as the harshness of the thermal cycle increases, and the high-Ag SAC solders that perform best in thermal cycling have relatively poor drop impact properties. Therefore, there is a drive to develop a new generation of Pb-free solders that have improved thermal cycling performance under the temperature ranges relevant to emerging applications (e.g. in automotive, avionics, and defense), while also having acceptable performance under drop, shock and vibration loading. The new Pb-free solder alloys entering the market have taken a variety of alloy design approaches. However, a common theme in most is the addition of one or more of Bi, Sb and In to existing Pb-free compositions. Often the Bi, Sb and In additions are the most concentrated addition in the alloy and lead to the formation of new phases as well as dissolving in the ß-Sn. In this work, the development of phases, microstructure and the distribution of Bi, Sb and In are investigated in a range of third generation Pb-free solder joints. The focus is on the influence of Bi, Sb and/or In combinations on the intermetallic layers on Cu and Ni-based substrates, the primary and eutectic solidification phases, phase formation due to solid state precipitation, and the overall ß-Sn grain structure of BGA solder joints.