The purpose of the heat sink is heat dissipation. The compound enhances the heat dissipation between the heat sink built into the transistor and the heat sink attached to the frame. At the same time the transistor requires electrical isolation from ground. When a ground connection is required for the transistor it will be provided by a circuit board trace.
Just for grins, with no power check the resistance between the transistor and its heat sink.
BTW, remember that the polarity orientation of the bridge components is important.
Just for grins, with no power check the resistance between the transistor and its heat sink.
BTW, remember that the polarity orientation of the bridge components is important.