Thermal paste - Page 1
Forum section: electronics / beginners

author: fabiodl
author_rank: Frequent Contributor
timestamp: November 27, 2021, 04:59:00 am
content: I am going to replace a regulator on a CRT. This has the usual white goo.That is thermally conductive silicone grease, isn't it?Do you have any recommendations? Is something with this specs "0.84W/m·K temperature range -50 +200" good?What are the differences with (usually gray) thermal paste for cpus? When is one better than the other? Is it only a cost/performance tradeoff or are there reasons not to use CPU's thermal paste for heat dissipation of components screwed to heat sinks?I also saw some thin, paper like sheets of thermally conductive material. How do they perform in comparison to the silicone grease/thermal paste?

author: Siwastaja
author_rank: Super Contributor
timestamp: November 27, 2021, 12:47:31 pm
content: 0.84W/(mK) is OK for thermal grease. These greases are never too great at conducting heat (compare to aluminum at around 200W/(mK)), the key is they are much better than air gaps, so first make sure the surfaces are flat, smooth and free of dirt particles, then apply modest amount of thermal grease, and enough pressure so that excess grease squeezes out.Better thermal pastes exist but definitely not used in general purpose electronics. They only make a small difference in special cases, some computer overclockers might benchmark some minor advantages.By paper like thermally conductive material, you need to make a distinction between materials which are just thermally conductive, but still require the interface material such as the grease, as they are "hard" and don't fill the tiny gaps themselves; classic example would be a thin mica sheet. And then there are so called "thermal pads"/"silpads"/"gap pads", which are sheets of soft, sticky material. With those, you don't need the paste. They (not all!) offer proper electrical insulation at the same time, something which grease alone does not. As a compromise, thermal conductivity is not that great because the layers are thick (0.5mm would be fairly typical - way thicker than any grease layer, ever!).For CRT repair, the most important aspect is to understand whether you need electrical isolation or not. If the original solution really was just grease between the component and heatsink (i.e., there is no mica sheet), then there is no isolation, and applying new grease is the correct solution.If the original solution uses insulation layer, you need to work carefully, understanding what goes into maintaining that insulation.

author: fabiodl
author_rank: Frequent Contributor
timestamp: December 12, 2021, 06:21:30 am
content: Thank you very much. Is this electrical insulation? On the same boars, all the others ICs have the "goo", so either this requires a better thermal conduction(but is the sheet better?) or this is the only one that required electrical insulation.Do you think it's a mica sheet? (size is 30x40mmx0.4mm). It's quite soft, you can easily leave marks with the fingernails. And it's not transparent. Searching online, it seems mica is usually transparent. It is however not electrically conductive, but I guess that would be true for silicone likethisas well.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 12, 2021, 07:34:23 am
content: It's electrically isolating SILPAD.If you look at the transistor package, you'll notice the area around the screw (on the backside, too!) is fully plastic, so that special measures for isolating the screw are not needed. But likely the transistor has some metal on the backside exposed, which is why you need electrically isolating thermal interface material, in this case a typical silpad.If you replace the transistor, replace the silpad as well. Use glassfiber reinforced type rated for mains voltage isolation purposes, then pick one of the better thermal conductivity variants (like some 3.5...4 W/mK - it's very unlikely the original is better than that).The best thermally conductive silpads (like 10W/mK) are not good for electrical isolation because they are too soft and lack physical reinforcement.

author: fabiodl
author_rank: Frequent Contributor
timestamp: December 12, 2021, 07:59:37 am
content: Thank you for this ton of info!Do you thinkthiswould be ok?I searched for cheaper alternatives on digikey and so forth, but it seems they carry only pads for TO-220, while in my case it's quite bigger, so I guess I have to get a larger sheet and cut it down.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 12, 2021, 09:37:57 am
content: That's fine. Even with glass fiber reinforcement, make sure there are no sharp edges or dirt particles that could puncture through.You can make custom shapes with required cutouts with X-acto knife. Best to do that before removing the protective films.