Effect of voids on crack propagation in AuSn die attach for high-temperature power modules

Eutectic AuSn solder is increasingly used in high reliability and/or high temperature applications where conventional Pb-free solders exhibit insufficient strength, creep resistance, thermal conductivity, and resistance to corrosion. Excessive void presence and growth and irregular phase formation are critical factors governing the solder joint reliability as long as they act as crack initiation sites. As a whole, the thermal resistance of test vehicles assembled using AuSn solder has experienced significant variation during aging [-55/+175]°C. On the other hand, inspection of as-prepared die-attachments by X-ray and SEM (observation of cross-section) showed that the initial voids sizes were increased and a propagation of transverse cracks inside the joint between voids had appeared. In this paper, the joint strength and fracture surface of AuSn solder power assemblies after thermal-aging testing were studied experimentally. Finite-element method (FEM) simulations were performed on the joint to study the influence of the voids size and position on the initiation and propagation of cracks. Simulation results were in good agreement with the experiments. Additional challenges, possibilities, and recommendations for how the reliability of a high-temperature AuSn bonding can be improved are also discussed.