Influence of hard encapsulation onto reliability of soldered die-attach in power modules

Abstract:
The standard approach for encapsulation of power modules is potting with soft silicone gels, a viable alternative is the usage of epoxy-based resins either applied by potting or large-area transfer molding. Within the study to be presented, standard frame-based automotive power modules with sixpack topology, soldered bottom-side and Al-wire bonded top-side die-attach are encapsulated with standard soft silicone gels as well as different epoxy-based resins, allowing for direct assessment of encapsulants reliability influence. Lifetime tests by means of active power cycling according to automotive qualification guideline AQG324 have been carried out, failure modes and failure characteristics are subsequently assessed via scanning acoustic microscopy and optical microscopy on cross sections. Focus has thereby been set onto the soldered die-attach. Experimental results show, that for the evaluated power module package the usage of epoxy-based encapsulants, which are mechanically stiffer compared to silicone gel, leads to stabilization of the chip solder layer, completely eliminating failures due to solder fatigue. Hard encapsulation shifts the failure location in power cycling for the assessed module setup completely to the top-side die attach. Assessment of different epoxy-based encapsulation materials has shown that the endured number of power cycles is directly related to their thermo-mechanical stiffness. Examinations do show that full potential exploitation of reliability-increase by fatigue reduction of soldered and/or planar interconnects by hard encapsulation is only possible with altered top-side die attach, such as soldered Cu-clips.