The continual increase of device power and package integration levels has driven the development of advanced power electronics packaging solutions. This study will focus on a numerical modeling approach to design analysis and material selection to improve solder joint reliability in one of these advanced solutions — a thermally integrated power electronics package that aims to dissipate hot-spot heat flux (5 kW/cm2) via mini-contact based thermo-electric (TE) cooling in addition to removing background heat flux (1 kW/cm2) by manifold-microchannel cooling. The methodology used for performing the structural reliability modeling is a non-linear finite element analysis (FEA) approach. Combined thermal and mechanical analyses were run to obtain stresses and strains in the solder joint used to integrate the TE cooler with the mini-contact and the mini-contact with the Silicon Carbide (SiC) chip. To predict the Mean Time to Failure (MTTF) of SAC305 at various levels of integration, a Physics of Failure (PoF) based methodology was applied using Engelmaier’s failure model.
In this paper, we will discuss the results of analyses of tapered, t-shaped, and lofted shaped mini-contacts made out of SiC, copper and diamond. Both structural design and material selection affect hot-spot heat dissipation and solder joint reliability. SiC has a good thermal conductivity at room temperature (RT), however, with increase in temperature, its thermal conductivity drops, and this can adversely affect device performance in high temperature applications. On the other hand, one can take advantage of high conductivity materials like copper, diamond or silver-diamond composite to keep the device cool and thus, improve package life time. However, for such high conductivity materials, one will need to take into account the cost of manufacturing complex shapes without any compromise in package thermal or reliability performance.
It was found that a ductile mini-contact material will share the thermal mismatch strain with the solder interconnection, while a brittle mini-contact material will shift the failure site inside the TE cooler. It was determined that a mini-contact structure tapered near its top base and lofted (constant cross-sectional area) near the chip (bottom base) would provide the best reliability results. Application of high conductivity composite material (silver-diamond composite) to enhance structural reliability is discussed.