Transient Liquid Phase Sintering (TLPS) is a novel high temperature attach technology. It is of particular interest for application as a die attach in power electronic systems because of its high melting temperature and high thermal conductivity. TLPS joints formed from sinter pastes are comprised of metallic particles embedded in matrices of Intermetallic Compounds (IMCs). Compared to conventional solder attach, TLPS joints contain 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 modeling of paste-based TLPS sinter joints that possess complex microstructures with heterogeneous distributions of metal particles and voids in IMC matrices. Thermo-mechanical simulations of the post-sintering 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 principal stress within the assembly was found to be a poor indicator for prediction of vertical crack formation. In 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 principal stresses, but large voids induce higher interfacial stresses, which explain why the vertical cracking failure was only observed in joints with large voids.