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NNTP-Posting-Date: Sun, 31 Mar 2024 14:54:39 +0000
From: John Larkin <jl@997PotHill.com>
Newsgroups: sci.electronics.design
Subject: Re: silicone grease
Date: Sun, 31 Mar 2024 07:53:05 -0700
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On Sun, 31 Mar 2024 13:25:02 +0100, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 30/03/2024 18:14, John Larkin wrote:
>> Give a nice flat mosfet package and a flat heat sink, I wonder how
>> much benefit accrues from adding silicone grease. It's really messy in
>> production and it's hard to confirm proper application. A little
>> googling didn't provide hard numbers.
>> 
>> I'm thinking a big-die TO-220 fet, bolted to a copper CPU cooler, AlN
>> or mica insulator, no grease, 40 watts. I guess I'll have to try it.
>
>ISTR on one of the overclocking hacker CPU cooling sites someone tried 
>everything from dry to cooking oil and engine oil. The marginal best was 
>some exotic "liquid metal" silver loaded brand I have never heard of and 
>the worst by a long way was dry.
>
>The biggest change was from dry to some sort of heat exchange medium is 
>by preventing an air gap. It was a significant difference too.
>
>The problem is that your flat surfaces are not exactly flat so that the 
>direct metal contact area can actually be quite small if there is any 
>surface roughness. Air is a rather good insulator and metals don't 
>radiate well at all. Silicon grease prevents air gaps and anything 
>similar will do the same job. It is just that silicon oils and greases 
>are less inclined to evaporate or go rancid and corrode your parts.

There's a lot of opinion on this but few or no numbers. Some people
seem to think that their music sounds better, or their gaming scores
improve, with some expensive grease.

A TO-220 footprint with a 100 micro-inch air gap, assuming zero
metal-metal contact to the heat sink, calculates to 0.65 K/W. I
wouldn't mind 0.65. A 2 mil mica insulator gets that up to about 1,
which is still fine for my application.

The TO-220 FullPak insulated transistors are appealing but have awful
thetas.

Digikey keeps getting worse. They mostly show gap-pad thetas that are
about 5x worse than reality. They seem to use the TO-3 specs for
TO-220 parts. And they show obsolete parts with zero stock if I
specify "usually stocked."