Study on creep fatigue behaviour of soft solders die attach for power package applications

Soft solder die attach is a widely used process for power and high-reliability automotive devices because soft solder has excellent heat-dissipation properties and outstanding robustness against delamination compared to traditional epoxy die attach. However, the creep-fatigue failure is one typical failure mechanism for solder material under thermal cycling. So it is essential to to understand the solder failure due to soft solder creep fatigue behavior. In this work, tensile creep tests for two soft solders, solder A and B, have been conducted at various temperatures and stress levels using a Dynamic Mechanical Analyzer (DMA). Creep constitutive models were developed and the material constants of the model were determined based on experimental results for both solders. The creep constitutive models were implemented in finite element analysis (FEA) to investigate the creep behavior of solder in the power packages for the evaluation of package reliability. Results show that solder A has better creep resistance than solder B, which indicates that the package with using solder A should have higher creep fatigue reliability than package with using solder B when subjected to thermal cycling. The storage modulus, liquidus temperature/solidus temperature for two types of solders were also measured by DMA and Differential Scanning Calorimetry (DSC), respectively.