Heat sink backing plate
The heat sink backing plate is a plate that helps secure a heat sink to a motherboard, improving the heat removal process and making a more secure mount for the heat sink. The backing plate mounts to the underside of the motherboard. After the plate is attached, the heat sink attaches to the backing plate with four screws. To reduce any vibration from the heat sink and minimize motherboard cracking when tightening the screws, rubber washers sit between the motherboard and heat sink brackets.
Backing plates come in many sizes. A heat sink and fan assembly should include different backing plates to accommodate multiple processor socket sizes on the motherboard. When using and installing a backing plate, make sure you use the correct size. Sizes are usually designated by the processor socket type, like LGA1366, LGA1156, Socket AM3+ or Socket FM1.