Are there designs that don't use thermal paste? As I understand it regardless of solution you need paste or live with significantly reduced conductivity. That goes for any system - fan cooled, liquid cooled, passive,...
Normal thermal paste is conductive also, including the AS5 that the author uses in the article. You can get electrically nonconductive paste too, but it's not as thermally conductive.
>> Are there designs that don't use thermal paste?
Yes. Glue. Paste is for when you might want to one day separate part from heatsink. If you don't care about repairs, it is perfectly reasonable to just glue them together with thermal adhesive.
> since modern thermal pastes are non-conductive, there is no real downside to an enthusiastic application of paste except for making a mess, so you want to default to putting too much on rather than having some dumb intern kill a PC with an insufficient application. There is virtually no effect on thermals.
The effect on thermals are absolutely not insignificant. Thermal pastes are actually extremely bad at transferring heat and too much of it really has negative effects.
Yes, it is safer to put too much than too little. Using too much the worst case is just that the CPU throttles and performance is sluggish and that rarely leads to an RMA.
For example, the thermal conductivity of a high-grade thermal paste is 8.5 W/mK, and the heat conductivity of copper is 385 W/mK, or for aluminum 205 W/mK. You can see that the thermal compound is actually a poor heat conductor and that is exactly the reason why you only need a very thin layer of paste to fill the micro imperfections between the IHS and heatsink.
> you can use too little, so no reason to err in that direction in mass manufacturing
Exactly. The thermal conductivity of air is 0.026 (W/m/K), your average thermal paste (non-metal) is at 2. For manufacturers it's way better to guarantee uniformly meh conductivity by using too much thermal paste than risking air bubbles by using too little. Most manufacturers will lean this way when they don't just use those POS thermal stickers.
Same with average versus silver: silver provides a gain but a limited one, and it's conductive so you have to make very sure not to leak any onto circuits or you risk a short. That's why manufacturers don't bother.
8W/m.K is pitiful as they are much 'thicker' than thermal paste; good pastes should have higher thermal conductivity to boot as well. If a solid pad is needed in place of thermal paste - carbon pads, e.g. "Carbonaut" is a lot better solution. Carbon sheets have higher base thermal conductivity than copper, of course not so high when polymerized.
> For example, the thermal conductivity of a high-grade thermal paste is 8.5 W/mK, and the heat conductivity of copper is 385 W/mK, or for aluminum 205 W/mK. You can see that the thermal compound is actually a poor heat conductor and that is exactly the reason why you only need a very thin layer of paste to fill the micro imperfections between the IHS and heatsink.
This has always been my rule of thumb whether it's metallic or non-metallic paste. I throw on a pair of vinyl gloves, squirt a dab on the heatsink (not the CPU), and spread an even layer with my finger. I go back and fill in low spots with a smaller dab, trying for an even surface. This way, it's thin enough that it doesn't squirt out the sides when it's installed, but it's a uniform spread that covers any gaps between the heatsink and the CPU surface. Also, by putting it on the heatsink instead of the CPU, I don't risk accidentally getting paste on the non-contact parts of the CPU.
I've also used the semi-solid thermal pads similar to what comes from the factory on most OEM heatsinks, but they are far less efficient than paste.
>...and the properties of thermal paste (typically) are just barely better than air.
I am not certain how you have managed to come to such a conclusion. Thermal conductivity of air is around 0.03W/(m·K)[0]. Good thermal, non-conductive paste is like 12.5W/(m·K)[1] (or 400 times better than air). Conductive ones are in the region of ~40-80 W/(m·K) and Aluminium is 237W/(m·K). Also air also expands pushing the cooler and CPU away.
Normally you if choose between "too much" and "too little" paste, you pick the former. The pressure pushes out the unneeded amounts.
There are thermal pads that perform equally or better than thermal pastes, due to their design. In particular, something like https://thermalmanagement.honeywell.com/us/en/products/therm... can be better than a good thermal paste like https://www.thermal-grizzly.com/en/products/16-kryonaut-en. The thermal conductivity is improved due to the use of a phase shifting material. Reliability is improved due to the fact that it will naturally return to solid without cracking during heat cycling.
Somewhat unrelated, but maybe you can shoot me down since you seem to have some experience?
Metal is an incredibly good conductor on its own, and the properties of thermal paste (typically) are just barely better than air. So long as your cpu and heatsink are fairly flat surfaces and mashed together physically, it seems like either forgoing or having the absolute minimum amount of paste is ideal. I've used a razor to leave an absolutely minimal layer of paste (e.g. filling in sub-millimeter surface structure) on my latest build, and cpu temperatures are well within a reasonable range. But I'm also not trying to OC the cpu or anything.
You want to use as little paste as possible while fully covering the heat spreader.
Most modern coolers will provide sufficient pressure to spread a pea sized amount of thermal paste to cover the whole cooler.
There are also thermal pads, both reusable and single use, that perfom well and don't require any guesswork.
If you want to paste, noctua has the best paste in terms of thermal resistance, but mx4 or mx5 both perform well, as does cryorig and a bunch of others.
Don't, you need to multiply the raw conductivity by the linear distance occupied by the thermal paste? I presume that distance will be at least two orders of magnitude larger than that occupied by air in a metal contact only setup.
I would be extremely surprised if increased pressure due to air at higher temperature played any role whatsoever unless the bolts connecting the heatsink and cpu were very loose. If anything, I'd expect the increased conductivity of air at higher temperatures to dominate.
I'd also expect there to be effects at the metal-paste and paste-metal interfaces which reduce the effective system conductivity (i.e. phonons are much more likely to reflect in this scenario than in a metal-metal interface).
> I expect that getting a heatsink paste rated for 2200 C is ... challenging, but, conveniently, you'd do better if you just skipped the paste
Liquid metal is some of the best performing thermal paste around. In computer applications that's normally an alloy made from Gallium, Indium and Tin, but at 2200C the majority of metals should work. Maybe Gold to reduce oxidation.
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