Thermal interface material (TIM) is a crucial part of any thermal management solution. Since it’s physically a small portion of most applications, it’s an easy component to overlook. But thermal interface material can make or break a device and its associated product.
Many engineers focus more on heat sink design but often forget to spend time considering how heat gets from the heat source to the heat sink. Even the smoothest surface has some level of surface roughness. The roughness of two surfaces in contact with each other creates air pockets. Since air is a good thermal insulator, air pockets from mating surface roughness impedes heat transfer from one surface to another.
In the image below, heat coming from the black surface can only conduct through to the grey heat sink at the points highlighted in red. Orange arrows help visualize that heat is coming from the entire black surface but is restricted through these contact points. Air pockets are represented in light blue. Heat can transfer through the blue air pockets, albeit highly inefficient and very little compared to the level of conduction through the red contact points.
This is applicable for any electrical device, commonly enclosed in a case, that transfers heat to a metal heat sink without a thermal interface material.