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NNTP-Posting-Date: Mon, 01 Apr 2024 21:30:24 +0000
From: Joe Gwinn <joegwinn@comcast.net>
Newsgroups: sci.electronics.design
Subject: Re: silicone grease
Date: Mon, 01 Apr 2024 17:30:24 -0400
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On Mon, 01 Apr 2024 08:16:14 -0700, John Larkin <jl@997PotHill.com>
wrote:

>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.

Two mils sounds quite good for such a package.  Your machinist can
definitely make a more precise measurement.  The official definition
of the package type likely has a flatness requirement, and it's this
that you should design for or at least be prepared to deal with.

Joe Gwinn