Wide band gap semiconductors such as GaN possess many favorable properties comparing to Si, which make them especially suitable for high power and high temperature applications. This brings about unique opportunities and challenges for the development of robust and reliable packaging technologies. In this talk, we will focus on the thermo-mechanical aspects of packaging reliability challenges, where thermal stress can be a main reason causing failure of power modules. We will discuss the source of the thermal stress from materials interaction during temperature excursion, and will discuss some of the stress induced failure mechanisms such as delamination and fatigue. Characterization techniques for high resolution and in situ measurement will also be reviewed. We will point out the similarity and unique challenges of these problems as compared with Si based devices to show that packaging reliability of wide band gap semiconductor devices presents an exciting opportunity to explore new packaging materials and designs.