Path: ...!weretis.net!feeder6.news.weretis.net!usenet.blueworldhosting.com!diablo1.usenet.blueworldhosting.com!feeder.usenetexpress.com!tr1.iad1.usenetexpress.com!69.80.99.26.MISMATCH!local-2.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Mon, 01 Apr 2024 15:17:50 +0000 From: John Larkin Newsgroups: sci.electronics.design Subject: Re: silicone grease Date: Mon, 01 Apr 2024 08:16:14 -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: 83 X-Trace: sv3-EADfS4UqOTGlaHzQOsZ/OoMxCDO3yWJ39PpKvCDwvG8KwRRFUuE4jXCAzgLmA5seSDQHU7CymmZaNKf!MXt57h4+NcpWbaS95Az9aAfyVL/5e323HyBD5fyH4fyR2A/FoCC5Ei1ugthY+etUS+btC7ucBXuJ!oWXPYw== 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: 5548 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. > >https://materion.com/-/media/files/ceramics/articles/beo-still-a-force-in-rf-power-transistor-packaging.pdf > >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.