Thermal Interface Material (TIM)
The paste, pad or phase-change material that fills the microscopic air gaps between a power component and the heat sink. The thermal conductivity of air is about 0.026 W/mK while typical thermal paste is in the 1–8 W/mK band; this difference lowers the contact thermal resistance several times over and visibly reduces the junction temperature.
No matter how finely two metal surfaces are machined, the real contact area is a small fraction of the total area; the rest is air gap, and air is an excellent insulator. The interface material fills those gaps.
- Paste: Gives the lowest thermal resistance but must be spread thinly and evenly. Too much paste makes the result worse, since the paste itself is also a layer of resistance.
- Pad / silicone washer: Its application is clean and repeatable; it also provides electrical insulation. It gives a slightly higher thermal resistance than paste.
- Pressure: For the material to do its job the screw must be tightened adequately and evenly; a loose mounting increases the layer thickness.
The cost of a dry mounting is greater than supposed; at the same loss the junction temperature can rise by tens of degrees. When refitting a component removed during maintenance or replacement, clean off the old paste and apply new.
Where this term is used: DC Motor Driver Selection Guide