Finite Element Simulation of Stress Relaxation in Fe- and Co- Nanoparticle Doped Sn-3.5Ag Solder Joints

Abstract

The lifetime of electronic parts is usually estimated by the weakest link approach. Owing to their low melting point, solder joints often represent the weakest link. Lifetime models are based on Finite Element computer simulations that determine the accumulated inelastic strain as a function of the applied thermomechanical loading conditions. Therefore, material models of solder alloys are needed to predict microelectronic reliability. Recently, it was found that the mechanical properties of Sn-3.5Ag solder joints can be influenced by iron (Fe) or cobalt (Co) nanoparticles. Doping with 0.5% Fe or 0.1% Co can suppress the formation of brittle intermetallic compounds at the interface between the solder alloy and the copper base material. Moreover, an increased fracture stress was observed at both room and elevated temperatures. Therefore, well-known material models for Sn3.5Ag must be updated to describe the improved material strengths of nanoparticle-doped solder alloys. In the present study, the stress-relaxation behavior of doped and undoped solder joints at 25°C, 100°C, and 150°C was compared with Finite Element computer simulations using the Anand material model. To determine the model's material parameters, the shear tests of a previous investigation were complemented by tensile tests conducted at different strain rates. In conclusion, the activation energy of creep could be determined along with the other material properties of the Anand model.