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| really proper way to apply thermal compound? |
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| coppercone2:
If you really wanted to do this, is there a better method then the 'baboon fishing for ants' way we normally do it by smearing it around with a fine brush? If both surfaces are super clean, would it not be better to do something like place a frozen sphere of the compound (or hemisphere) in the center of the die, warm it up so it flows, then compact it warm and use a precise jig to clamp it down to some known seperation distance? What is the crazy semiconductor manufacturers way of doing this? Maybe spinning a layer on it like a photomask? also can vacuum degassing possibly get rid of air bubbles if combined with warming? or possibly replace voids that are normally air filled with voids filled with more thermally conductive solvent present in the thermal compound? |
| IanB:
The purpose of thermal compound is to displace any air between the mating surfaces (air is a better insulator than the thermal compound, which we hope is a thermal conductor). Therefore the best way to apply thermal compound is as a small blob in the center of the mating surfaces and then compress the surfaces together so that the blob spreads out sideways displacing air as it goes. (If the surfaces are large then maybe one or more beads might be better than a single blob.) It also wouldn't hurt to smear the surfaces first with a very thin layer of compound to remove any pockets of air in the surface pores and assist the spreading process. Overall, the thinnest layer of compound possible would be the ideal as thermal compound does not conduct heat as well as metal does. (Note, it is not a good idea to spread the compound before mating the surfaces as this will cause pockets of air to be trapped and will reduce the heat transfer efficiency.) |
| coppercone2:
i wonder if the lapped surface should be measured first with a surface texture gauge, then a thermally conductive compound of grit size appropriate to fill the voids is used (non appropriate for the larger connection), lapped off, then the proper thermal compound applied that possibly has different properties since it works to fill a different bulk geometric profile rather then micro cavities. |
| coppercone2:
--- Quote from: IanB on December 09, 2018, 11:13:38 pm ---The purpose of thermal compound is to displace any air between the mating surfaces (air is a better insulator than the thermal compound, which we hope is a thermal conductor). Therefore the best way to apply thermal compound is as a small blob in the center of the mating surfaces and then compress the surfaces together so that the blob spreads out sideways displacing air as it goes. (If the surfaces are large then maybe one or more beads might be better than a single blob.) It also wouldn't hurt to smear the surfaces first with a very thin layer of compound to remove any pockets of air in the surface pores and assist the spreading process. Overall, the thinnest layer of compound possible would be the ideal as thermal compound does not conduct heat as well as metal does. (Note, it is not a good idea to spread the compound before mating the surfaces as this will cause pockets of air to be trapped and will reduce the heat transfer efficiency.) --- End quote --- I thought this was the best method but I thought I was advised against it some where before. I did not understand why, I thought maybe its because if you have a giant blob rather then a thin layer you get some kind of point stresses that end up causing the compound to separate out under the increased pressure that would not be present if spread out more. I wonder if a domed mold used with chilled compound could spread it out more from the beginning with less peak pressures. |
| DaJMasta:
The recommendations I've seen for lapped surfaces and optimal cooling are generally: Smear a very small amount of the compound, less than opaque thickness, all over each side, then connect (optionally with a small amount in the center). This does require very flat surfaces and every even pressure applied, because if either isn't right there will be air gaps with no contact, but since TIM is generally less thermally conductive than the metals it sits between, the idea is to minimize the amount while still getting maximum contact area. Generally, the surfaces aren't exactly smooth so you need a little leeway in the application, I've seen a fair number of techniques, but some options are: Smear around and dot in center Pea sized dot in the center X over the contact area X with dots in each quadrant Dots over each die (if you know where it is) with or without a smear Even opaque application (if you just want to guarantee complete contact) Not 100% sure on what would be the "best", but there are some reasonable options depending on the surfaces. Worth noting that different TIM compounds probably prefer different applications - more viscous compounds will not spread as much under the same pressure, so more frequent smaller dots or similar would be preferable. Nice thing about the even smear is that it doesn't have to move around much. |
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