Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Bill Sloman Newsgroups: sci.electronics.design Subject: Re: silicone grease Date: Mon, 1 Apr 2024 22:08:57 +1100 Organization: A noiseless patient Spider Lines: 43 Message-ID: References: <050658@dontemail.com> <29ej0j5staom8amnb5lkfde7uc94m99hsf@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Mon, 01 Apr 2024 11:08:58 +0200 (CEST) Injection-Info: dont-email.me; posting-host="18aab1eec5384e230cd19593004747ce"; logging-data="2581426"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19VmetjK93jWDaBDlFaUCVHcRK9oq2Pq+E=" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:c2+9wwavuf5KfD/9sz17ENhfXwM= In-Reply-To: Content-Language: en-US Bytes: 2949 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 wrote: >> >>> On Sat, 30 Mar 2024 11:14:10 -0700, 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. >>> >>> 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