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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: silicone grease
Date: Mon, 1 Apr 2024 22:08:57 +1100
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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