Path: ...!weretis.net!feeder6.news.weretis.net!newsfeed.hasname.com!usenet.blueworldhosting.com!diablo1.usenet.blueworldhosting.com!feeder.usenetexpress.com!tr1.iad1.usenetexpress.com!69.80.99.22.MISMATCH!local-2.nntp.ord.giganews.com!news.giganews.com.POSTED!not-for-mail NNTP-Posting-Date: Mon, 01 Apr 2024 21:30:24 +0000 From: Joe Gwinn Newsgroups: sci.electronics.design Subject: Re: silicone grease Date: Mon, 01 Apr 2024 17:30:24 -0400 Message-ID: References: User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 92 X-Usenet-Provider: http://www.giganews.com X-Trace: sv3-meVr0/fXupkAukdH0SmX0J5VsjtAODYWCTRgN4Aaqyy3J0m9gvpt7A6vsg5mQBLMJTeBAhP3WvZH6hp!X2TacW1RKtMgQEtHjfjN46giTsvIJc0bPyGlU6w8C8RsGfVFUXDRFv0IXCgtIelcAUdOf50= X-Complaints-To: abuse@giganews.com X-DMCA-Notifications: http://www.giganews.com/info/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: 6016 On Mon, 01 Apr 2024 08:16:14 -0700, John Larkin wrote: >On Mon, 1 Apr 2024 09:32:20 +0100, Martin Brown ><'''newspam'''@nonad.co.uk> wrote: > >>On 31/03/2024 15:53, John Larkin wrote: >>> 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. >> >>It was quite a simple setup. >> >>Same heatsink, same stress test and note down the CPU core temperature >>at equilibrium. CPUs are convenient in already being well instrumented - >>the biggest difference was nothing vs anything else. >> >>There is an 80:20 rule at work here - you get 80% of the improvement by >>eliminating the tiny air gap by wetting it out with a heat transfer >>medium and the rest is incremental using ever more exotic materials. >> >>In the extreme they still use the near lethal BeO ceramic material in >>some high power RF transistors since it is second only to diamond for >>thermal conductivity whilst being an electrical insulator. >> >> >> >>In the bad old days you used to have to be careful of TO-3 can >>transistors that had blown their top for that stuff. These days they use >>inferior but much safer alternatives like alumina and aluminium nitride. >> >>You say that there are no numbers. Where have you been looking? >> >>> 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. >> >>If you are prepared to de-rate accordingly then there isn't really a >>problem but if you want to run them at full power then they need to be >>in intimate contact with their heat sink and that means wetted by some >>sort of heat transfer medium. I was quite impressed with the bluetack >>like stuff that came with my Raspberry Pi passive aluminium heatsink. >> >>I found the pads more annoying to handle than silicone grease YMMV. >>Getting them on square was much harder than just adding a dab of goo. > >Yeah, that might need a fixture or something in production. > >I'm getting quotes on custom AlN insulators, which would still need >grease. > >I eyeballed several of my candidate TO-220 mosfets, The bottoms are >mirror finished but, sadly, not very flat. Placed on a flat surface, >and held edgewise with bright light in the background, I estimate >about 2 mils of daylight in places. I'll ask my machinist if he can >quantify that better, but it looks like I'll need grease or a >compliant pad. 2 mils is a lot of air. Two mils sounds quite good for such a package. Your machinist can definitely make a more precise measurement. The official definition of the package type likely has a flatness requirement, and it's this that you should design for or at least be prepared to deal with. Joe Gwinn