Re: silicone grease

On 31/03/2024 20:33, 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.
 

The recommended mounting torque is about 6lb-in and the plastic screws I am familiar with have max rated torques much less than half that.
Even if you find a plastic screw reliable for that torque including a conical belleville compression washer might be good to accommodate slippage and creep.
I suspect a good spring clip may be more consistent and quicker in production.
piglet