Physics of failure of die-attach joints in IGBTs under accelerated aging: Evolution of micro-defects in lead-free solder alloys

Abstract

Fatigue of die-attach solder joints is quite critical to reliability of IGBT modules. In the present work, physics of failure of die-attach joints in IGBTs was discussed based on the evolution of micro-defects in materials under accelerated aging through experiments and Finite element (FE) simulations. It shows that voids, cracks of Sn-Ag-Cu (SAC) alloys and detachment of Si/SAC interfaces are three major micro-defects in the die-attach solder. Voids could accelerate evolution of other two micro-defects. Cracking of SAC alloys is the major failure mode of die-attach solder under power cycling and detachment of Si/SAC interfaces turns to be the dominant failure mode under transient electrical/thermal shocks. FE simulations indicate that the shear thermal stress τshear at Si/SAC interfaces is the decisive factor for switching between the two failure modes. When ΔTj > 105 °C, the average shear stress τshear_ave at Si/SAC interfaces exceeds the bonding strength and detachment of Si/SAC interfaces plays a major role in failure of die-attach solder. Based on physics of failure, two physical life models for die-attach joints under various accelerated aging conditions were proposed.