Deformation Characteristics and Constitutive Relationship of SnAgCu Lead (Pb)-Free Solder Alloys Under Thermo-Mechanical and Mechanical Loading

In this paper is reported the mechanical and thermomechanical aspect of SnAgCu solder alloys that have been tested for their deformation behavior. Commercially available Sn3.8Ag0.7Cu was considered as base alloy. The constant stress and strain-rate tests were performed in tensile and shear configuration. Mechanical deformations were measured using bulk solder tensile specimens and grooved lap shear specimens which enabled a stress-state of nearly pure shear in the solder joint. The strain rate range was between 0.1/sec and 10^-6/sec, and test temperatures were 25°C, 75°C and 125°C. Both as-prepared and thermally aged samples were tested. The aging condition was 24hrs at 125°C. The measured tensile stress-strain data were utilized to determine the constants for the visco-plastic Anand's constitutive model. Thermo-mechanical properties like coefficient of thermal expansion (CTE) for those SnAgCu lead free alloys were measured in the temperature range of - 40 to 160°C using thermo-mechanical analyzer (TMA). The tensile and shear properties of 95.5Sn3.8Ag0.7Cu solder and solder joint specimens are highly dependent on test temperature and strain rate. The mechanical strength of SAC bulk solder and solder joint decreases with increase in temperature and increases with increase in strain rate. CTE for the SAC lead-free alloys were relatively lower compared with tin-lead solder. The steady-state creep test data for 95.53.8Ag0.7Cu solder was curve-fitted to a hyperbolic-sine creep model. The material constitutive parameters developed are in line with similar studies.