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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: silicone grease
Date: Mon, 1 Apr 2024 12:35:37 +0100
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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