Re: silicone grease

On 1/04/2024 7:45 am, wmartin wrote:

On 3/31/24 12:33, John Larkin wrote:

On Sun, 31 Mar 2024 10:45:17, Wanderer<dont@emailme.com> wrote:
>

On Sat, 30 Mar 2024 11:14:10 -0700, John Larkin <jl@997PotHill.com>
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.

>
Silpads. I used silpads since the 80's. I don't know if they are better
or worse than grease, but they are good enough and most importantly they
are consistant. What happens with testing on the prototypes, happens with
production units. Shorts were always with the little vinyl grommets, you
use to keep the screws from shorting to the part. You can squeeze those
down to tight and cause a short.

>
Right, I want to avoid the mess and uncertainty of grease.
>
Bergquist has a TO-220 pad, their 1500ST material, that should be
about 1.5 K/W for a TO-220. That would give me a Tj max about 170c,
which ain't great but is survivable.
>
I'm considering using good 6-32 plastic screws to mount the fets to
the cooler.
>

I expect they will stretch when heated, so you might see increasing failures over time. Not an easy problem...

It's not stretching that's the problem, but cold flow or creep "the tendency of any solid material to move or deform over a period of time under the influence of persistent mechanical stress, with no recovery of shape when the stress is removed."
--
Bill Sloman, Sydney