Effect on Creep Rate of Alloying Additions to Ni-Stabilised SN-CU Eutectic Solder
Authors: Keith Sweatman, Tetsuya Akaiwa, Tetsuro Nishimura Ph.D. Company: Nihon Superior Co. Ltd. Date Published: 10/14/2018
Abstract: While solder based around the Sn-Ag-Cu eutectic has dominated the first phase of Pb-free implementation the dramatic evolution in the design and application of electronics since July 2006 is forcing alloy developers to look at other alloying options. The realisation that the strengthening effect of Ag fades rapidly with ageing is another factor prompting the exploration of strengthening mechanisms other than particle hardening with the Ag3Sn intermetallic compound. Some of the strengthening additions now being evaluated form different phases, metallic and intermetallic, that provide particle strengthening but there is also renewed interest in the mechanism that worked so well for Sn-Pb solder, solid solution strengthening. This mechanism is based on the addition of elements that remain soluble in the solid Sn matrix of the solder at normal operating temperatures. Because they are larger or smaller than the Sn atom they replace, the crystal lattice around the solute atoms is distorted, impeding the movement of the dislocations that make deformation of the crystal possible at relatively low stress. In the experiments reported in this paper the effect of various alloy additions on the creep rate and time to failure at 125°C of a widely used Sn-Cu-Ni solder was measured. The performance of these alloys was benchmarked against the widely used Sn-3.0Ag-0.5Cu alloy. Since the stresses to which solder is subject in service is usually below the nominal flow stress, creep, in particular the damage accumulated as a result of creep, is the main driver of solder joint failure. The alloying additions evaluated in various combinations include up to 21wt% Bi, up to 4wt% Ag, up to 6wt% In, up to 10wt% Sb. Alloys with the greatest resistance to creep deformation were those with certain levels Bi, Bi+Ag and Bi+Sb. The creep performance is interpreted in terms of the current understanding of alloy strengthening mechanisms.