Thermal interface materials (TIMs) are an essential part of managing the thermal performance of electronic assemblies. Knowledge of the mechanical properties of these materials is required in order to have a robust design that will perform as required over the life of the product, including many thermal cycles, without causing damage to electrical components. In this paper, we report on the mechanical properties of three putty TIMs and four pad TIMs, showing that the stiffness of the TIMs is proportional to the square of the initial shape factor over the range of shape factors from 1 to 18. Since the putties can flow more readily under pressure than the pads, the putties had a lower measured stiffness at a given shape factor compared to the pads. From these relationships, designers can predict loads with various geometries (i.e., shape factors) and loading rates (i.e., shock loading versus temperature cycling), which can impact their design. While all of the materials were tested at compressive strain rates of 20–70% strain/minute, one putty was also tested at a 10× higher rate to determine the effect of a relatively high strain rate on the peak stress. In that case, the peak stress was approximately 3× higher than measured at the lower strain rates. However, the relaxed load at each strain rate tested was unaffected by strain rate, indicating that hardware assembly conditions can be adjusted to minimize stress on components and yet, still achieve an interface having low thermal resistance.