Under the heatsink is 3 layers of thermal paper. It is there to insulate the heatsink from the chassis as the heatsink has full drive amperage traveling through it.
The heatsink compound is used under the q1 transistor to prevent it from welding (melting) itself to the heatsink.

That previous post about q1 never blowing and the card always being the culprit is BS! Stupid rookie 5 years in my ***. What, were the first 4 watering batteries??? All the q1 is is just 8 mosfets glued in a plastic housing you dodo and your gonna say mosfets never blow... I'm wowd by your knowledge... not. Ez is super sensitive to static hahaha I have seen so many forks arc out to racks over the years and all on running trucks. They are not. I have never heard of a battery creating static that's new to me also please explain how that is possible I would love to hear that story.