Insulating Washer / Sil-Pad
A thin layer placed between the metal body of a power device and the heatsink that insulates electrically while passing heat. Mica, silicone-based pad and ceramic types exist. While providing insulation it adds typically 0.3-1.5 °C/W to the thermal resistance; this addition must be included in the loss budget calculation.
On most power MOSFETs the metal back surface is electrically connected to the drain terminal. If more than one device is to be mounted on the same heatsink, these surfaces must be insulated so that there is no short circuit between them.
- Mica washer: Its thermal resistance is low but it does not work on its own; a thin layer of thermal paste must be applied to both faces. It is brittle and needs care while tightening.
- Silicone pad: Requires no paste, its application is clean and repeatable; its thermal resistance is slightly higher than mica.
- Insulating bush: Just as important as the washer is insulating the mounting screw as well. If the metal screw touches the body of the device, the washer is of no use at all.
After mounting, verify with an ohmmeter that there is insulation between the device body and the heatsink. This check takes seconds and prevents a short circuit at first power-up.
The tightening torque matters too: a loose mounting raises the thermal resistance, while overtightening can pierce the washer and break the insulation.
The context in which this term is used: DC Motor Driver Selection Guide