H. Grevea, S. A. Moeinia, P. McCluskeya, and S. Joshib
aCenter for Advanced Life Cycle Engineering (CALCE), Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, United States
bToyota Research Institute of North America, Toyota Technical Center, 1555 Woodridge Avenue, Ann Arbor, MI 48105, United States
Transient liquid phase sintering (TLPS) is a novel high-temperature attach technology. It is of particular interest for application as die attach in power electronics systems because of its high-melting temperature and high thermal conductivity. TLPS joints formed from sinter pastes consist of metallic particles embedded in matrices of intermetallic components (IMCs). Compared to conventional solder attach, TLPS joints consist to a considerably higher percentage of brittle IMCs. This raises the concern that TLPS joints are susceptible to brittle failure. In this paper, we describe and analyze the cooling-induced formation of vertical cracks as a newly detected failure mechanism unique to TLPS joints. In a power module structure with a TLPS joint as interconnect between a power device and a direct bond copper (DBC) substrate, cracks can form between the interface of the DBC and the TLPS joint when large voids are located in the proximity of the DBC. These cracks do not appear in regions with smaller voids. A method has been developed for the three-dimensional (3D) modeling of paste-based TLPS sinter joints, which possess complex microstructures with heterogeneous distributions of metal particles and voids in IMC matrices. Thermomechanical simulations of the postsintering cooling process have been performed and the influence of microstructure on the stress-response within the joint and at the joint interfaces have been characterized for three different material systems (Cu+Cu6Sn5, Cu+Cu3Sn, Ni+Ni3Sn4). The maximum principle stress within the contrast, stress levels at the interface between the TLPS joint and the power substrate metallization are good indicators for this failure mechanism. Small voids lead to higher joint maximum principle stressed, but large voids induce higher interfacial stresses, which explain why the vertical cracking failure was only observed in joints with large voids.