Creep properties of soft solder die attach with Ni balls in 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, die tilt is usually one of the important issues and challenges during the soft solder process. The soft solder with Ni balls was introduced to reduce and control die tilt. During solder process, the Ni balls will not melt and will keep the ball shape, which would help control bond line thickness (BLT). The creep-fatigue failure is one typical failure mechanism for solder material under thermal cycling. So it is essential to investigate the effect of Ni ball on the material properties and reliability of the solder joint layer. In this work, tensile creep tests for solder wire with and without Ni ballss have been conducted at various temperatures and stress levels using Dynamic Mechanical Analyzer (DMA) to obtain the creep properties of the solders. The steady-state creep strain rates are similar for both solder typesl, which means that the Ni ball addition does not affect the creep behaviour of soft solder significantly. 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 can be implemented in finite element analysis (FEA) to investigate the creep behavior of solder in the power packages for evaluation of package reliability.