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Re: [Amps] SSPA & Heatsink Compounds

Subject: Re: [Amps] SSPA & Heatsink Compounds
From: Manfred Mornhinweg <>
Date: Thu, 12 Dec 2013 14:43:10 +0000
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Don, and all,

while it's very true that a THIN layer of thermal compound is desirable, it's ESSENTIAL for the layer to be continuous, that is, to avoid having air pockets there. And the techniques to achieve these goals vary.

Most people recommend applying a very thin layer, spread evenly over the mounting surface. But I happen to disagree with that method! The fact is that it's impossible to apply a perfectly even and flat layer. So, at the moment of contact when you mount the part, some areas will touch while some others will remain with air between them. When you tighten down the device, hopefully this air will be expelled and the tiny space between the device and the heatsink will completely fill up with thermal compound. But if the applied layer is too thin, this might not happen, and you can end up with air trapped under your expensive power transistor!

For that reason I use a radically different technique: I apply a single thick round blob of thermal compound to the middle of the mounting surface, or if the device is long, I apply a single nice smooth string of compound to it. Then I mount it, pressing it down strongly but evenly, to make the compound flow out and fill the whole space. In this way the formation of air pockets is avoided. When thermal compound appears squeezed out all around the device, I can be sure that no air remains anywhere under the part. From this point on, additional pressure can be used to thin out the layer of thermal compound. This pressure should be well distributed over the device, to avoid deforming it. Just tightening the mounting screws is not a good idea. A clamp in the middle of the device is ideal.

If necessary, the device and heatsink can be warmed up to soften the thermal compound and make it flow out more easily. Otherwise it's a good idea to re-tighten the mounting screws during the first test run, when the equipment and specially the power devices get warm.

Since I'm a hopeless cheapskate, I scoop up the excess thermal compound that squeezes out, and use it for the next device.

This method has worked well for me, in 35 years building electronic equipment, and is easy and quick.


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