Re: silicone grease

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