The constant need for smaller and more powerful electronic devices has posed significant challenges in terms of thermal management. To cope with rising power densities, effective heat dissipation becomes critical and interfaces often are a thermal bottleneck. Thus, thermal interface materials (TIMs) are used to reduce the resistance to heat flow at these interfaces. Due to the variety of materials within the package, mismatches in the coefficient of thermal expansion (CTE) induce thermo-mechanical stresses on the TIM. These stresses are cyclic as the system heats up and cools down, leading to degradation of the TIMs such as pumpout failure for thermal greases. Past studies on failure generally focus on thermal greases applied between two flat surfaces. But this configuration does not always represent the real system as, generally, the silicon die often has some inherent warpage and the heat sinks are not always perfectly flat, especially for large cold plates in power electronic devices. The study aims to understand the impact of non-flatness in heat sinks on thermal grease degradation in power cycling environments. The grease is present between a flat metal heater (to mimic the silicon chip or package) and a lens (to mimic the heat sink side of an interface), where the curvature of the lens can be precisely controlled to be flat, convex, or concave. Three thermal greases are selected for testing due to their differing thermal conductivity and rheological properties. Power cycling causes voids, cracks, and movement of the thermal grease, and this structure evolution is recorded with a digital microscope through the transparent lens. Initial results demonstrate that a concave lens (such that the bondline thickness (BLT) is thickest at the center of the heater) generally reduces the grease degradation as it appears to retain the grease from flowing. On the other hand, for the same material, a convex lens (such that the BLT is thinnest at the center of the heater) squeezes out the grease with power cycling. Ultimately, these results demonstrate that controlling surface flatness or warpage is critical to controlling the degradation of thermal greases.