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NNTP-Posting-Date: Mon, 01 Apr 2024 15:17:50 +0000
From: John Larkin <jl@997PotHill.com>
Newsgroups: sci.electronics.design
Subject: Re: silicone grease
Date: Mon, 01 Apr 2024 08:16:14 -0700
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On Mon, 1 Apr 2024 09:32:20 +0100, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 31/03/2024 15:53, John Larkin wrote:
>> 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.
>
>It was quite a simple setup.
>
>Same heatsink, same stress test and note down the CPU core temperature 
>at equilibrium. CPUs are convenient in already being well instrumented - 
>the biggest difference was nothing vs anything else.
>
>There is an 80:20 rule at work here - you get 80% of the improvement by 
>eliminating the tiny air gap by wetting it out with a heat transfer 
>medium and the rest is incremental using ever more exotic materials.
>
>In the extreme they still use the near lethal BeO ceramic material in 
>some high power RF transistors since it is second only to diamond for 
>thermal conductivity whilst being an electrical insulator.
>
>https://materion.com/-/media/files/ceramics/articles/beo-still-a-force-in-rf-power-transistor-packaging.pdf
>
>In the bad old days you used to have to be careful of TO-3 can 
>transistors that had blown their top for that stuff. These days they use 
>inferior but much safer alternatives like alumina and aluminium nitride.
>
>You say that there are no numbers. Where have you been looking?
>
>> 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.
>
>If you are prepared to de-rate accordingly then there isn't really a 
>problem but if you want to run them at full power then they need to be 
>in intimate contact with their heat sink and that means wetted by some 
>sort of heat transfer medium. I was quite impressed with the bluetack 
>like stuff that came with my Raspberry Pi passive aluminium heatsink.
>
>I found the pads more annoying to handle than silicone grease YMMV.
>Getting them on square was much harder than just adding a dab of goo.

Yeah, that might need a fixture or something in production.

I'm getting quotes on custom AlN insulators, which would still need
grease.

I eyeballed several of my candidate TO-220 mosfets, The bottoms are
mirror finished but, sadly, not very flat. Placed on a flat surface,
and held edgewise with bright light in the background, I estimate
about 2 mils of daylight in places. I'll ask my machinist if he can
quantify that better, but it looks like I'll need grease or a
compliant pad. 2 mils is a lot of air.