Re: silicone grease

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."