Lead-free solder material characterization for thermo-mechanical modeling

Abstract

Systematic mechanical characterization has been done on lead-free solder alloys, i.e., SAC105, SAC205, SAC305, SAC105Ni0.02, and SAC105Ni0.05 as promising alternatives of SnPb solder. Solder joints in service stay in high homologous temperatures that have profound effects on mechanical properties such as creep and the apparent elastic modulus. Mechanical deformation behaviors of bulk solder alloys at temperatures between 298 K and 398 K, as well as strain rates between 10-5 to 10-1 under isothermal conditions, have been studied for the above 5 solder alloys with a designed bulk test vehicle. Isostress loading conditions on the solders were also conducted to study creep behaviour. Based on substantial experimental measurement results, the elastic modulus, yield stress and UTS of the various SAC solder alloys show direct relationships with Ag content at temperatures between 298 K and 398 K. The relationships of strain rate, temperature, Ag content and material properties were developed and material constants are presented. Basic material parameters with viscoplastic constitutive relations for solders encompassing temperature and strain rate dependence have been obtained through curve fitting with experimental data. The creep constitutive model has also been established. By implementing the developed creep or viscoplastic constitutive models of solders into FEA models, the life of an electronic package under thermal cycling and drop performance in board level tests can be evaluated. Material compositions such as Ag content and Ni dopant effect are also studied for their influence on mechanical properties. It was reported that Ni dopant could improve solder joint drop performance by changing the microstructure of the material.