Path: Xl.tags.giganews.com!local-2.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Sun, 31 Mar 2024 14:54:39 +0000 From: John Larkin Newsgroups: sci.electronics.design Subject: Re: silicone grease Date: Sun, 31 Mar 2024 07:53:05 -0700 Organization: Highland Tech Reply-To: xx@yy.com Message-ID: References: X-Newsreader: Forte Agent 3.1/32.783 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 44 X-Trace: sv3-qhP0NHGUOIS4ItHQw2B6ZNjhWPJ1e1Imx46RSqBHNO2fQxXI91YepmOwSBC5XrELlu2uI/WSr3aaZpn!Lj8lMZkT2vjPc/iq7DCaKJEzK0U7gi834pOlYgmnlibmxCDzKtozLeH4o9WiaT+dO5USVMudF85i!/F0eNQ== X-Complaints-To: www.supernews.com/docs/abuse.html X-DMCA-Complaints-To: www.supernews.com/docs/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 3220 On Sun, 31 Mar 2024 13:25:02 +0100, Martin Brown <'''newspam'''@nonad.co.uk> wrote: >On 30/03/2024 18:14, 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. > >ISTR on one of the overclocking hacker CPU cooling sites someone tried >everything from dry to cooking oil and engine oil. The marginal best was >some exotic "liquid metal" silver loaded brand I have never heard of and >the worst by a long way was dry. > >The biggest change was from dry to some sort of heat exchange medium is >by preventing an air gap. It was a significant difference too. > >The problem is that your flat surfaces are not exactly flat so that the >direct metal contact area can actually be quite small if there is any >surface roughness. Air is a rather good insulator and metals don't >radiate well at all. Silicon grease prevents air gaps and anything >similar will do the same job. It is just that silicon oils and greases >are less inclined to evaporate or go rancid and corrode your parts. There's a lot of opinion on this but few or no numbers. Some people seem to think that their music sounds better, or their gaming scores improve, with some expensive grease. A TO-220 footprint with a 100 micro-inch air gap, assuming zero metal-metal contact to the heat sink, calculates to 0.65 K/W. I wouldn't mind 0.65. A 2 mil mica insulator gets that up to about 1, which is still fine for my application. The TO-220 FullPak insulated transistors are appealing but have awful thetas. Digikey keeps getting worse. They mostly show gap-pad thetas that are about 5x worse than reality. They seem to use the TO-3 specs for TO-220 parts. And they show obsolete parts with zero stock if I specify "usually stocked."