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Thermal grease?
Hi Guys

It is a common practice to use silicone thermal grease when mounting power semiconductors on a chassis or heatsink. The grease itself is fine to use except that it is universally NOT APPLIED CORRECTLY. The grease is designed to fill MICRO voids in the surfaces of the semiconductor mounting surface, the insulator, and the surface to be mounted on. A tiny amount of grease would be applied to the surface then scraped off with a straight edge. When applied correctly, you should not be able to see any grease without a magnifying glass or possibly a microscope!

Does anyone use thermal grease that way?

NO. No one does. You always see huge globs of the white stuff all around the devices and this means much more has been used than should have been. The reason this is unacceptable is not merely because of the permanent mess; rather, the problem is that the grease compresses over a short time and then the device is loose. Torquing the devices carefully makes no difference to this situation, it just means you are fooling yourself that you are being careful. Unfortunately, the care you are taking not to warp the mounting planes of the device or the heat sink is swamped by the inevitable loosening of the mounting and the loss of heat sinking that results, ultimately leading to device failures.

Because if this, we recommend NEVER USE THERMAL GREASE.

Way back when I started building solid-state amps, I bought a little tube of thermal grease. I was serious about building these things for life. Then I read an article in the Hewlett-Packard Journal (which the engineers where I worked received courtesy of the employer) and went home and threw away my tub of grease.

In our Power Scaling kits, the mosfets are bolted to the chassis for heat sinking. Thermal pads are supplied rather than mica insulators as the pads travel better. Even were we still using grease with mica we would not use it with the pads as the pads are designed to compress during the initial mounting. Ideally, if you unbolt the device for some reason or replace it, a new pad should be installed.
I have say that wholeheartedly agree with Kevin on this. I would use thermal grease from time to time on repairs and it was nothing but a mess and trouble. I favor the aluminium oxide ceramic pads which seem to work really well....

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