Thermal compound conductivity choice...rule of thumb?  Need to do the maths? - Page 1
Forum section: electronics / beginners

author: K5_489
author_rank: Regular Contributor
timestamp: October 03, 2024, 07:11:47 pm
content: Went to buy more thermal compound today....it's been MANY years since I last had to restock. I used to alway just go to Radio Shack or Frys electronics, look for the expensive silver stuff, and buy the biggest tube they had on the shelf.Today, I went to everyone's favorite first stop of Amazon, and realized there were many more choices. I had absolutely no idea you could get this stuff in tub form vs just the syringe tubes and noticed there were different "levels"? of thermal conductivity in watts per meter kelvin. Ok, Google time, where I learned that it's a rating of how much heat it will conduct in watts based on surface area of the compound. Ok, simple enough.But then how do I apply that to the parts I need the compound for? Such as the current project - replacing 2N3771 transistors on an Astron 30 amp power supply. I suppose the simple answer is "buy the highest rating you can find", but then these things are rated to dissipate 150W through the collector..clearly the tiny smidgen of paste I'm going to put on the case that's rated to transfer 3.17W/m-k isn't going to handle that, nor will the highest rating I've seen so far of 6.something watts.So clearly I'm missing something here. Or maybe looking at the wrong products altogether? The goop I'm always scraping off transistors and heat sinks have looked like dried up white lithium grease (I'm assuming it isn't actually simple lithium grease, just has the same appearance), whereas the thermal compounds I've been buying have looked like the gray stuff commonly found on CPUs. I was under the impression that the gray stuff is "better", but I wouldn't be able to tell you why.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2024, 09:08:01 pm
content: Wakefield 120. The basic, plain-yogurt, of thermal grease, and only a little less appetizing. (No please, don't actually eat it..)Thermal conductivity doesn't matter much, because a properly prepared joint has very little air gap to fill, and excess grease squidges out, leaving a minimum height joint. The transistor (RthJC), insulator (if applicable) and heatsink dominate overall thermal resistance.Perspective shift: I would say If grease is adding notable resistance, the joint is wrong, not the grease.AFAIK, lithium grease is a fairly ordinary petroleum or synthetic oil, gelled with a lithium soap (a low-solubility variant of the active ingredient in regular bar soap; basically, soap scum). The soap particles partially dissolve, thickening the oil, but also disperse, remaining in suspension, forming a gel that stays in place, and giving something solid but non-abrasive for the bearing surfaces to rest on, when not in (hydrodynamic) motion.Thermal grease is made much the same way, of course it isn't used under relative motion, heh. As a result, more abrasive materials can be used as the filler; ZnO is typical. (Indeed, wiping thermal grease off a bare aluminum heatsink, you'll find it leaving dark streaks, as the particles abrade and polish the metal.) A silicone oil is usually used, being chemically inert, and perhaps better at staying in place. Still, a common failure mode -- whether by improper formulation, application, overheating, thermal cycling, orit just does that-- is separation, leaving a dry powdery joint under the device, which needs to be cleaned and reapplied.So, yes indeed -- it has a similar appearance, for similar reasons, but using different materials.Anyway, high-K grease: the trouble is, this can only be done two ways: using higher-K filler, and using less oil. You can very well end up worse off, because the grease doesn't squeeze out, and now you have, sure it's 15 W/(m.K) or whatever, 20 times better than the plain white stuff, but maybe the white stuff crushes down to 20µm average, while you've got a 0.5mm thick blob under the device, and it's not even fully spread out, maybe the contact area is half. Or maybe the clamping force is so great, and uneven, that the device is cracked before it can sink any power at all(!). Applying those super-thick compounds is nontrivial.But, combining all of the above: if you're in a situation where the joints just aren't flat, there's big gaps, the faces are rounded, whatever, nothing fits right, and it's not practical to fix the surfaces; that might be a good place to use a higher-K grease, instead of the low-K stuff that'll end up filling large pockets and adding significant Rth.Soft rubber gap pads might also be attractive: these are available in fairly high K nowadays too (5 and up), and the softness means they fill voids very well. An excellent application is using them to clamp a PCB, components and all: they can fill in the gaps between SMT components, making contact all around, as well as to the board itself, vastly increasing power dissipation. Downside: they are solid rubber products still, not puttylike (well, some of them maybe?), so start to crack and tear when strained too far; they aren't great for rigid (screw and strong spring-clamp) mounting. On the upside, being soft rubber materials with some surface tack, they need hardly any holding force, and a soft spring clamp can do the job. Self-adhesive (firmer, very tacky) materials are also available.Tim

author: K5_489
author_rank: Regular Contributor
timestamp: October 03, 2024, 11:55:55 pm
content: Ok...maybe I'm misunderstanding the need for this stuff altogether? Or was that a long way of saying "buy the cheap white stuff unless you have identified a specific need due to something like mating surfaces having a poor fit up"?I've been under the impression that when attaching things like these high power transistors or CPUs to a heatsink, you absolutely positively 100% NEED to apply some sort of thermal compound to the mating surfaces, otherwise the magic smoke is gonna escape post haste?At least from an anecdotal standpoint, I've found that when I forget the paste on a CPU, the thing will throttle within seconds of starting up, sometimes not even completing POST due to overheating. I don't have much experience with transistors, as I'm just now starting my adventure in learning electronic repair.As for the current project, I just popped the four transistors off the back wall of the power supply along with the mica insulators, found the dried up greasy goo, and thought it would be wise to clean it up and put fresh goo down. If it wasn't for being out of greasy goo, I would have grabbed a tube of the same silver stuff I'd put on a computer CPU, put some on the transistors and been on my way. It wasn't until I started looking at specs of different greasy goos that I realized there may far more to this, and I always like learning better ways of doing things.

author: Smokey
author_rank: Super Contributor
timestamp: October 04, 2024, 01:45:33 am
content: Paste with diamond in it! Look it up!

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2024, 02:04:34 am
content: Quote from: K5_489 on October 03, 2024, 11:55:55 pmBroadly speaking, thermal grease is bad for heat conduction, except for the alternative, which is worse.When you try to mate two metal surfaces together, if you look under a microscope, you will find they only touch at a few points, and most of the gap between them is filled with air, which is a very good thermal insulator. This is why not using thermal paste leads to bad results.The job of thermal paste is to fill the gaps, and replace the air with something more thermally conductive. It only has to be more conductive than air, not be as conductive as metal (not likely). The reason it doesn't have to be as conductive as metal is that the space to be filled is very thin, and therefore not much thermal paste should be in the conduction path.If you wanted to go totally crazy, the best thermal result would be to solder the part to the heatsink, but this is rarely practical or feasible. One time it is totally feasible is with surface mount parts that have a metal pad underneath to be soldered to the copper heat sink layer on the PCB.The short summary is that any kind of filler is better than no filler, even toothpaste or plain oil has been proposed! But in reality, thermal paste is thermal paste, and good enough is good enough for most ordinary purposes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2024, 02:04:39 am
content: Yes, "cheap stuff is fine".But also that, yes, there are reasons to use various products.Without paste, there's only air (or perhaps fingerprint oils, dust and such) in the gap, a particularly poor conductor for one, but it gets worse than that: on a microscopic scale, both surfaces are rough, and make contact at very few points (surface asperities). Heat can flow directly between surfaces where they touch, but these points are few and far between. Around contact points, the separation might be some nm, and they can't "feel" each other anymore: no conduction. In fact, since the mean free path of molecules in air itself is only about 50nm, for gaps less than this, there's effectively vacuum between faces -- only radiation carrying heat, between (probably reflective metal) surfaces. But most of the gap, for average surface finish and flatness, will be in the 10s of µm, where a classical air filling is accurate enough. Which, with so little air space, and temperature drop, convection is negligible, and it's like a single layer blanket.So yeah, just a little filling -- anything at all, it could be plain oil, but it's better with a conductive (solid) filler, and stays in place better as a grease, paste, or rubbery material -- improves things dramatically, and further improvement (in grease properties) makes very little change in the average case.Outside of the average case, this may be less true, and more benefit may be had from better greases.Tim

author: showman
author_rank: Contributor
timestamp: October 04, 2024, 02:24:45 am
content: Quote from: T3sl4co1l on October 04, 2024, 02:04:39 amDon't you think that the heat transfer could actually be better in that case? If the distance is smaller than the mean free path then effectively the energy can be directly transported from the hot to cold side, i.e. a molecule gains energy from the hot side and instead of a long process via random collisions with other air molecules, it is likely to hit the cold side next and lose some of its energy.Also, the figures provided by thermal compound manufacturers who focus on computers are not to be trusted:https://www.arctic.de/en/faq/detail/why-doesn-t-arctic-communicate-thermal-conductivity-valuesQuote

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2024, 03:56:01 am
content: Quote from: showman on October 04, 2024, 02:24:45 amHonestly, I don't know the dynamics of heat transfer in such a regime -- that's an interesting question.My (hand-waved) understanding of it is, there's simply far fewer molecules in the gap at all -- if there were more, they'd be colliding with each other as well, right? Or put another way, think of how often the surfaces are hit by "bouncing balls", and where; the amount of area covered by those hits (over a given time frame), decreases as the surfaces come together and there's less gas in the gap. At the diffusion limit (say 50nm, up to some ~mm where convection can maybe start to help), conductivity is as expected, but below, in this pseudo-vacuum limit, it drops...perhaps proportionally with separation? Until it goes back up again when the surfaces can "feel" each other (Van der Waals forces, overlap of atomic orbitals).Also to be clear, heat transport in still air is mediated by diffusion, so, it's kinda not very effective, and conductivity scales with... sqrt(distance), probably?(I don't think we ever covered thermal transport in gasses in school -- maybe chem eng would? -- and I don't recall working any problems about the interface between matter in Stat Mech. Oh god, imagine how messy those problems must be... And, not that I remember much of it these days, it's been so long, lol. Can't say I've read much about transport in gas or (near) vacuum since then, either; it's not really a very important topic in general anyway, with applications probably like, outer space... or a curiosity like Crooke's radiometer. Which come to think of it, that the latter has been such a mystery for centuries, is probably suggestive in part of how "rarefied" (if you'll excuse me) coverage of this topic is, given that the currently-accepted explanation of adsorption-driven heat transport. Neat, huh?)Tim

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2024, 04:22:15 am
content: Quote from: T3sl4co1l on October 04, 2024, 03:56:01 amAs it happens I'm a chemical engineer, and for sure, heat transfer is one of the core topics. However, most industrial treatment of heat transfer addresses large scales, where the behavior of molecules can be averaged out to bulk phenomena. Much treatment of the subject where fluids are concerned is empirical, where investigators do lots of experiments, collate the results, and fit correlations to the data. This is because first principles, analytical solutions are hard to come by. The most accurate predictions come from CFD modeling, but this is still a numerical solution to the governing macroscopic equations.If you reach low enough pressures, or small enough scales where the mean free path of gas molecules becomes significant, then different theory is needed, and I am not familiar with this area. It is unusual, and not normally encountered in everyday engineering.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2024, 06:26:41 am
content: Yeah, we did thermo as well, basic fluids and heat stuff, but all very macroscopic (and empirical).As you can see, most cases it's a bad thing easily avoided -- chemists usually want more pressure to shrink reactor size, raise boiling point, etc.; heat transfer you just use the grease and don't care about the air-gap case. But that inevitably leaves the useless situation something of a mystery.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: October 04, 2024, 06:59:07 am
content: lower is better always, lets say a bolt loosens and contact gets worse. The better the compound the more durable the thermal interconnect will be. Lower thermal resistance means cooler parts.You have the choice of cheap or expenisve bolts, cheap or expensive thermal pads and cheap or expenisve thermal compounds. It formulates into a quality decisionyou can also measure it with a good micrometer of the correct type to see how thick your bond line is, the question is, is the design compatible with the correct micrometer? It may require a special fixture to verify in many standard PCB cases. A mechanical micrometer can do 0.0001 inches resolution, meaning 2.5 micrometers.

author: Sorama
author_rank: Frequent Contributor
timestamp: October 04, 2024, 07:07:08 am
content: It doesn’t really matter what soort of paste (or any liquid for that matter) is used.There are only 3 ways of energy transfer:ConductionRadiationConvectionSo in this case, even radiation is no great because of the poor distance.Conduction how ever is your best friend in this application and for that, the more space is covered with paste/compound ( more contact between component and heat sink) the better.The energy flux depends of the temperature delta, the surface and the thermal resistance.Even toothpaste works fine, as long is doesn’t get burned.

author: Phil1977
author_rank: Frequent Contributor
timestamp: October 04, 2024, 07:20:11 am
content: Having done many FEM simulations about that topic and also experimentally tested the results I can contribute:- Thermal conductivity in W/mK of the thermal paste does NOT much influence the results as long as you use stuff that´s called thermal compound. I wouldn't advise to use toothpaste, but every cheap ZnO-filled grease can do this job.- A thin interface is always better than anything thick. Avoid silicone-foils for that reason.- Mechanical stability of the connection is super important. If the interface moves only a few um then even the silicone foil can be of advantage.- A copper heat spreader inside an Al-heatsink reduces component temperature much more than all other experiments with different thermal compound. It seems to be counter-intuitive to improve cooling by introducing more material and an additional interface, but heat transfer is 3-dimensional and not 1-dimensional as in a resistance chain. FEM simulations well confirm this.

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 04, 2024, 07:20:54 am
content: Unless I'm misreading your original post, it seems you're mixing up W/m.k with W. They don't have to be equal or higher or something. Here's an intuitive way to understand what W/m.K actually means.A 1W/m.k rating means that if you take a cube of 1m x 1m x 1m of material and you inject 1W of heat on one side (over 1m²) that cube it will cause a delta T of 1K over that blocks thickness of 1m. After that, you can play around with that cube. Per example, you probably don't have an interface of 1m² but, say, 1cm². That means that for the same 1m thickness of material, you now have a delta T of 10 000K. But obviously, you won't be using a 1m thick material but, say, 1mm. So, the delta T becomes 10K.Now, thermal paste is not supposed to have much thickness at all. It's there to fill microgaps and uneven surfaces. Per example, you might have a layer thickness of 25um. For the same 1W/mK material as above over 1cm² that would be a delta T of 0.25K. For the 150W you mentioned: 37.5K.A few pointers:- thermal conductivity for paste is important but controling surface roughness is at least as important- often the importance of the paste is secundary to other parameters like die-to-case thermal resistance etc- the 150W dissipation in datasheets should be taken with a considerable grain of salt. Ussually only achievable if half a dozen vestal virgins pee a continuous stream of liquid nitrogen on it.

author: showman
author_rank: Contributor
timestamp: October 04, 2024, 04:28:20 pm
content: Quote from: T3sl4co1l on October 04, 2024, 03:56:01 amNeither do I, but yes it is indeed interesting.I need to think about your interpretation a bit, draw some figures, maybe write a small simulation. Yes, if there are fewer bounces then I think you could either directly or effectively interpret it as lower pressure and therefore as some type of vacuum and lower heat transfer, but I'm not totally convinced that it is the case. But neither am I convinced by my very simplified view mainly because those direct bounces from one surface to the other do not necessarily imply energy transfer. It does not help that most common literature I can find is about this subject is for nanopourous materials (aerogels etc), but in those cases they seem to be considering the gaseous heat transfer along the pores, not across those.

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2024, 05:48:42 pm
content: To get a good result with a real thermal grease (if electrical insulation is not required) you need decent metal surfaces (watch out for burrs on drilled holes), complete coverage of the area, and a very thin layer of grease, with good compression force on the transistor package.Insulating spacers (e.g. mica) should have a thermal spec and need two grease layers.

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2024, 05:57:49 pm
content: Quote from: showman on October 04, 2024, 04:28:20 pmTemperature in an ideal gas corresponds exactly to the average kinetic energy (speed) of the gas molecules. What happens when a gas molecule bumps into a hot surface is that it gets a little kick from the vibrating molecules at the boundary, and bounces off faster than it arrived. In the process it takes a little energy away from the hot surface. If it then bumps into a colder surface, it imparts a little jolt to those surface molecules, losing some energy in the process.When we look at the situation on a large scale, there are so many molecules that the different speeds of all the molecules average out, and heat transfer can be looked at as a continuous process, rather than a discrete process of individual kicks and jolts.What we perceive as temperature does not actually exist on a quantum scale. It is just rotational, vibrational and kinetic energy of molecules. Thermodynamic temperature only becomes a thing when we average out the energy macroscopically.So, if you had thinly spaced trapped molecules bouncing back and forth between hot and cold surfaces like tiny rubber balls, they would still be transferring heat from one surface to the other, and nothing would really change about what is observed. Air molecules are so small, and so numerous, that even in a vacuum, with a tiny gap, there is still an uncountably huge number of molecules present. So macroscopic thermodynamics will still apply, and heat transfer by conduction will still happen just the same (although the measured thermal conductivity may change).

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2024, 06:03:02 pm
content: Quote from: TimFox on October 04, 2024, 05:48:42 pmI think if you have a package like a TO-220, and if electrical insulation is not required, you can solder such packages to the heat sink. Especially if the heat sink is a large copper area on a PCB. You could even solder them to an aluminum heat sink, if you can overcome the challenge of soldering to aluminum, and getting solder to melt on a device purposely designed to suck the heat away

author: K5_489
author_rank: Regular Contributor
timestamp: October 04, 2024, 07:01:37 pm
content: Quote from: Ice-Tea on October 04, 2024, 07:20:54 amAll I know is that up until yesterday, all I ever knew of thermal greasy goo was that I looked for a tube that said silver on it on the shelf at the local electronics place, because I figured more expensive was better and for the tiny amount I was using the difference between the cheapest and the most expensive was insignificant. I then smeared a thin layer on my part, occasionally wondered why the tiny bit of dried and crusty greasy goo I was removing was white and I was putting gray down, and went on my merry way.It wasn't until yesterday that I even knew there was such a rating of W/m-k on greasy goo, and I certainly didn't know just WTF W/m-k meant. I also then learned I could get this stuff in tubs that were MUCH cheaper than the prefilled syringe type tubes on a per gram basis, and wondered if I was maybe using the wrong products or needlessly throwing money away on stuff that was much better than what I really needed, because I noticed I could get 200g of the cheap stuff for the same price as only 50g of the "better" stuff. Though honestly, based on the rate that I've been using it, 50g would probably last me the rest of my life, lol.

author: darrellg
author_rank: Unknown
timestamp: October 04, 2024, 07:07:07 pm
content: Quote from: K5_489 on October 04, 2024, 07:01:37 pmIf you really want to "save" money, you can buy a 30 lb (13.6078 kg) bucket from Grainger.https://www.grainger.com/product/SUPER-LUBE-Silicone-Heat-Sink-Compound-44N789

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2024, 07:22:55 pm
content: Since it’s impracticable to control the exact thickness of the grease layer, its thermal conductivity value is only useful when comparing different greases to each other. Mica washers don’t compress, so their conductance (not conductivity) value is relevant.

author: K5_489
author_rank: Regular Contributor
timestamp: October 04, 2024, 07:39:59 pm
content: [quote author=darrellg link=topic=441503.msg5665995#msg5665995 date=1728068827If you really want to "save" money, you can buy a 30 lb (13.6078 kg) bucket from Grainger.https://www.grainger.com/product/SUPER-LUBE-Silicone-Heat-Sink-Compound-44N789Can that actually be used? Funny enough, I have a few of the 3oz tubes of that stuff for welding and soldering to keep adjacent parts from getting too hot.

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2024, 08:00:51 pm
content: Grainger's page says: "These compounds dissipate heat during welding, brazing, and soldering. They are spread onto adjacent pipes and absorb heat so as to protect the equipment that they are plumbed up to."As Hamlet said, “There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy.”

author: showman
author_rank: Contributor
timestamp: October 04, 2024, 08:09:44 pm
content: Quote from: IanB on October 04, 2024, 05:57:49 pmWell no. This logic definitely breaks down, for example aerogels that contain air can have significantly lower thermal conductivity than air itselfhttps://en.wikipedia.org/wiki/Aerogel#Knudsen_effecthttps://en.wikipedia.org/wiki/Knudsen_diffusionAs I said, I think that this is because the thermal conductivity decreases along the pores, not across it, but that might be my wrong interpretation of the Knudsen effect. Also, I'm much more familiar with solids and at least for those the interfaces themselves have thermal resistance. For example if you put two dissimilar metals in (perfect, as in atoms touch each other without any vacuum or gas) contact there is a sharp drop of temperature at the interface although the common logic would dictate that the temperature drop is continuous and depends only on the thermal conductivities of the two metals. That is in simplistic terms because some of the energy gets refected back because of the discontinuity.

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2024, 08:31:58 pm
content: Quote from: showman on October 04, 2024, 08:09:44 pmNo, there is no breakdown in the logic.Aerogel is an insulator in part because it traps air in pockets and prevents convection from occurring, which is another mechanism of heat transfer that is very important (probably most important) in free and open air. Heat sinks dissipate a lot of heat by convection, which is why fans (forced convection) make such a difference to the rate of heat transfer.But in this thread we are talking about conduction, not convection. Conduction happens in small spaces where the air is not free to move. Thermal conductivity is a physical constant of air and does not change when the air is in pockets or otherwise (unless the pockets are microscopically small, apparently). In fact,aerogeltypical air pocket insulators have ahigherthermal conductivity than air because the thermal conductivity of theaerogelsolid matrix material is greater than that of air. (For example, air has a thermal conductivity of 0.026 W/m-K, while typical solid materials are in the order of 50-100 times more conductive than that.)If you have two dissimilar metals in perfect molecular contact (for example soldered or brazed), there is not, in fact, a sharp drop of temperature at the interface. The temperature gradient is smooth and continuous, although there will be a sharp change in thegradientat the interface due to the different thermal conductivities.Edit: Updated to correct a misunderstanding about the special properties of aerogels.

author: showman
author_rank: Contributor
timestamp: October 04, 2024, 08:41:29 pm
content: Quote from: IanB on October 04, 2024, 08:31:58 pmAnd if you actually click on the link you'll see that the thermal conductivity of that particular aerogel given as an example is 0.005 W/m-K. Much lower than the air's 0.024. This has nothing to do with convection, why even bring it up?QuoteIt is very well known that the interface itself has thermal resistance, first random work from googlehttps://www.mdpi.com/2073-4360/12/10/2409. Not metal-metal, but the idea is the same.

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2024, 08:56:21 pm
content: Quote from: showman on October 04, 2024, 08:41:29 pmVery well. I stand corrected. I did not realize aerogels fall into such a special category. See the comment I made before:Quote from: IanB on October 04, 2024, 04:22:15 amI mention convection, because preventing convection is an important reason for the performance of all air-pocket based insulating materials, including aerogels.QuoteIf you have dissimilar materials in contact, like polymer in an injection mold, they are not molecularly bonded (otherwise you would not get the polymer out of the mold). When there is physical contact, but not molecular bonding, then yes, there is an interruption to the heat transfer path at the interface. Granted.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2024, 08:56:34 pm
content: I mean, there's a small change at the interface between materials -- but it's very small indeed, and microscopic in nature: grain boundaries between crystals, and different types of crystals (e.g. pure iron to brass to copper, etc.).On a macroscopic scale, these all average out as the bulk conductivity of the material. You get a very small anisotropy with directional solidification (actually, I wonder how much, for what kinds of materials; I assume it's *something*), more in single-crystal materials (take graphite for an extreme case, ~5 W/(m.K) thru-plane, >2000 in-plane).Basically nothing to worry about, interface is dominated by the boundary layer (i.e., fill it with grease to be sure), and bulk conductivity and heat equation applies.Tim

author: showman
author_rank: Contributor
timestamp: October 04, 2024, 09:07:01 pm
content: Quote from: IanB on October 04, 2024, 08:56:21 pmThis is a strawman argument. There is nothing about getting the polymer out of the mold. It is a simulation paper (i.e. idealistic case) where there is as perfect molecular contact there can be.I'll copy the image here that actually shows the structure and the temperature plot.Also the change in this particular case is not insignificant, but almost 100 degrees.

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2024, 09:23:37 pm
content: The paper addresses in great technical detail the same thing that I wrote. I am glad we are agreed.

author: showman
author_rank: Contributor
timestamp: October 04, 2024, 09:43:15 pm
content: Quote from: IanB on October 04, 2024, 09:23:37 pmNot sure about that. First I think you overestimate/emphasize the importance of "bonding". You don't need any attractive interaction to transfer heat. As you yourself pointed out earlier, energy is transferred by collisions (does not really matter if its gas, liquid or solid), so it is the repulsion that matters the most. Yes, attraction matters too, but it is secondary. Secondly if you believe the same effect occurs at a solder-copper interface, yes we agree, but I don't think you do because "bonding". I'm pretty sure that you are sure that electrical energy reflects at impedance discontinuities, but for some weird reason it seems unbelievable that heat energy can reflect at atomic structure discontinuities.Non-infinite thermal conduction in crystalline materials like metals occurs (leaving electrons aside for now, but similar arguments as below will apply as well) because phonons which transport energy collide with each other. When they travel inside a solid, the motion is essentially determined by the mechanical, if you will, characteristic impedance of the material, normally called the phonon dispersion relation. This characteristic impedance is different for every material. Now when you put two materials together, even if they bond very strongly, for the sake of argument let's say even more strongly than the atoms within the two materials themselves, there is an impedance discontinuity at the boundary which will reflect the phonons back to the material it came from. The only way to make it more efficient to transport energy across the interface is to impedance match the two materials, but just stronger bonding of the interface does not do it, at least not to the degree that all reflections are removed. In fact very strong bonds (as well as very weak ones) at the interface likely increase the mismatch so there are more reflections (larger temperature drop).Note that whereas this effect is also real in a solder-copper interface, it can also be somewhat (but not completely) impedance matched, when for example you don't have an atomically sharp interface, but some diffusion of tin into copper/vice versa that makes the interface/material properties variations more smooth.

author: IanB
author_rank: Super Contributor
timestamp: October 04, 2024, 10:54:04 pm
content: Thanks for the information about phonons.I have never delved that deeply into heat transfer at the microscale and the associated physics. There also appears to be a difference between metals and non-metals, with electrons playing a role in heat conduction in metals.A subject for further reading when I have spare time.

author: showman
author_rank: Contributor
timestamp: October 04, 2024, 11:00:10 pm
content: Quote from: IanB on October 04, 2024, 10:54:04 pmYes, at room temperature the electronic thermal conductivity is typically much larger in metals than the lattice (phonon) thermal conductivity, but to actually handle that there is really no way around quantum mechanics so things get significantly more complicated. Also because of that complexity I don't know if anyone has done similar calculations as above with both electrons and phonons. Whereas the same argument applies about discontinuities, it could be that the electronic part is less sensitive to that in which case the temperature drop could be even smaller than with the somewhat impedance-matched one for the solder-copper case (but still non-zero).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 05, 2024, 12:01:09 am
content: Yeah, phonons are just the quanta of sound (acoustic) waves; the wavelengths relevant for thermal purposes are quite high indeed, and easily scatter, and absorb and emit spontaneously (as you might assume from being thermalized..!), so it's not like you're going to hook up a piezo disc and listen to its temperature; but the physics is the same as regular LF acoustics. So we can think of it in terms of impedance matching between bulk substances of varying speed-of-sound and density. (Also, not that these parameters are exactly the LF values either; I believe there's some skew to them, approaching cutoff? And of course, the upper cutoff is given by, essentially, the mode where every other atom in the crystal is moving oppositely, the spacial Nyquist limit if you will.)(Which, such a cutoff is obvious enough in solids, and, I believe was sufficient to explain the heat capacity of materials? But applying the same reasoning was quite the quandry for EM radiation. Circa turn-of-the-last-century. The same wavelength vs. degrees of freedom argument, applied to the EM field, leads to the "ultraviolet catastrophe" because there's simply no such limit to the classical EM field: the further up you go in frequency, the more modes fit in a given volume of space, and those modes should all fill equally with thermal energy (equipartition theorem). Setting an energy-based cutoff, and solving a fairly nasty integral, delivered the black-body spectrum perfectly; but no one quite believed the method, or really understood why it worked. IIRC, Planck himself disapproved of QM and always considered his trick a hack; mind, this was at a time when convenient hacks were popular in physics, Rutherford's "plum pudding" model, or Bohr's (literal) atomic orbit model of hydrogen, for examples. It took some decades before we had QM developed well enough to reasonably believe, yeah this weird shit seems to actually be what's going on, and to justify those hacks via direct solution or suitable approximation from the equations of motion, or state.)Tim

author: showman
author_rank: Contributor
timestamp: October 05, 2024, 01:12:38 am
content: I think it happened in reverse order, so Einstein and Debye essentially used Planck's EM theory to derive the roughly correct heat capacity for solids thereby further validating QM. The key for both is not the upper cutoff, but the quantization of energy itself. So for phonons it is the lower cutoff that really matters. I.e. classically they can have as low energy (above the ground state, which classically is 0, but in QM not) as you wish, but quantum mechanically as you approach 0 K the degrees of freedom start to be confined to their ground (lowest energy) states as you no longer have sufficent (thermal) energy to excite those, so equipartition does not hold any more and the heat capacity drops to 0 as opposed to the classical 3kB. Or in other words if you add a tiny amount (below the Planck's constant times the phonon frequency) of energy to a classical crystal at 0 K, its temperature will rise by a tiny amount, but trying to add the same small amount of energy to a quantum crystal will result in nothing. For light I think (but not quite sure) it is roughly the same argument, so at small wavelengths the gaps between energy levels get increasingly large so essentially those degrees of freedom or modes get frozen out as you don't have enough energy to excite those beyond the ground state as opposed to the classical case where the energy is not confined to specific levels.

author: helius
author_rank: Super Contributor
timestamp: October 05, 2024, 01:50:05 am
content: The performance of a thermal paste is not only about the R_k of its filler and the amount of oil used. Even if the oil fraction gets reduced to zero, a uniform powdered ceramic like zinc oxide can't conduct heat nearly as well as its bulk solid form, because there is a limit to the density by which uniform spheres may be packed of 63%. To get high performance from a thermal paste,densificationmust be used, which combines different particle sizes in the necessary ratios to achieve a nearly 100% packing density.TechIngredients did a good video about this years ago:

author: coppercone2
author_rank: Super Contributor
timestamp: October 06, 2024, 10:10:27 am
content: small gaps mean thermionic coolingyou might wonder, why are phonons hard to understand? well it was introduced by a soviet physicist during stalin's reign

author: K5_489
author_rank: Regular Contributor
timestamp: October 07, 2024, 07:15:39 am
content: Good lord....what did I start???

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 07, 2024, 07:19:42 am
content: I feel your pain.

author: 5U4GB
author_rank: Super Contributor
timestamp: October 07, 2024, 12:44:28 pm
content: Quote from: T3sl4co1l on October 03, 2024, 09:08:01 pmJust looked it up and, at least from Mouser, they won't ship anything but the tiniest quantity outside the US ("Shipping Restricted / USA Only"). Element14 doesn't seem to stock it, but they have their Multicomp-branded stuff, MPGCS-014, which seems to be roughly equivalent, and the data sheet for it covers all of the characteristics and test methods used unlike most thermal greases which quote a W/mK value and that's it.

author: bostonman
author_rank: Super Contributor
timestamp: October 10, 2024, 06:25:45 pm
content: After reading through this thread, I have a question.Early on it was loosely stated the grease heats and conducts the thermal heat into the heat sink, therefore an oil could technically be used.Until now, my thought was that it "pulls" heat away from the part into the heat sink and different types exist that do this better than others.Does it conduct heat and pulls heat away or does it actually act as a liquid, heat, and the heat sink pulls the heat out of that? I understand the thermal grease fills the gaps between the non-perfect metals (the heat sink and the metal tab of a component), but, as I stated above, I always thought it was transferring heat rather than it heating before the heat is pulled out of that.

author: TimFox
author_rank: Super Contributor
timestamp: October 10, 2024, 06:28:52 pm
content: In this application, heat is transferred by conduction (as opposed to convection or radiation), from the hotter surface to the colder surface.Therefore, the transistor or IC will be warmer than the heat-sink surface, which will be warmer than the surrounding air.The grease is one part of the conductive path: the manufacturer adds higher conductivity powder (e.g., ZnO or Al2O3) to increase the net conductivity of the grease.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2024, 12:11:43 am
content: "Pulling" implies some active mechanism. This seems a poor word choice, as there is no heat pump here, only the passive diffusion of heat through materials.But in the sense of "heat diffuses through it at some rate", yes. An air gap is terrible; something denser, like a liquid, is an improvement, but the diffusion rate (bulk conductivity) is usually rather low (particularly for silicone oil, heh). Making the liquid gaps as small as possible (by filling the liquid with granular substances that interlock to some extent, and have high conductivity) improves things even further.The liquid spaces are far too small, and viscosity high, for convective transfer.You can always model heat flow as an incremental transfer between infinitesimal shells/layers across the system. In this case, perhaps (zillions of layers in the component's Si, die attach, Cu bulk, Sn plating), (thousands of layers of alternating oil film and particles), (zillions of layers of heatsink), etc.In the sense that, compared to an insulator in that place, a conductor "pulls" heat out, I guess, but it's worth emphasizing there's no action here, simply the Fourier heat equation as applied to respective materials and boundary conditions.Tim

author: bostonman
author_rank: Super Contributor
timestamp: October 11, 2024, 01:21:08 am
content: Probably correct using "pull" is the wrong word.The reason I used "pull": thinking of just a metal tab of a component and a metal heat sink, the cold heat sink is pulling heat away from the hot tab.Guess this is a freshman way of thinking about thermal transfer.In the case of grease (and continuing to use the incorrect term of "pull") I was visualizing the grease acting as a material helping to pull the heat away onto the heat sink. Or does it heat the grease before the heat is transferred to the heat sink?Not sure if I'm asking correctly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2024, 01:35:39 am
content: Not sure if you're asking about steady-state or dynamics (or, that's precisely part of the confusion).Heat flow depends on the temperature drop across a given volume. So, the component heats up, which heats up the grease, which heats up the particles, which heats up the heatsink. No heat flows until the temperature rises, at least by small increment, and so on until equilibrium.Thermal impedance does depend on the heat capacity of materials involved. The grease layer of course is very thin so can be ignored for this purpose, but in general everything has thermal mass and conductivity, and therefore some diffusivity which describes the rate at which heat spreads out over time:https://en.wikipedia.org/wiki/Thermal_diffusivityThere are two kinds of flow here:Heat flow is a steady-state condition, when one thing is hot and another is cold and there's power dissipated. The power and temperature values are static, but we still say a power is energy in motion, a flow. In thermodynamics terms, this is a quasi-steady-state condition, because it's nonequilibrium (heat is still flowing), but nonetheless no variables are changing."Thermodynamics" itself is a bit confusing or unintuitive, because what's "dynamic" is the "therm", the heatenergy-- not heat power. Nowadays we mostly use "dynamics" for time-varying systems (e.g. differential equations), so it can seem strange that thermo deals with steady equilibria. Typical thermo problems consider a start and end condition (e.g., pressure, volume, temperature, energy input/output), and don't care about how the system evolves between those end points, so long as the end points are stable equilibria; or more particularly, that the system can only evolve in specific (and most importantly, continuous) ways between those points, and in such a way that the rate doesn't matter (expanding a volume of gas in a perfectly insulating cylinder, changes its temperature adiabatically, no matter how fast or slow you do it).The other flow here, is the change of temperature over time; at least when the conditions are first imposed, starting from an equilibrium (say, cold) state. This involves heat capacity of the materials, and diffusibility and all that.So that's probably part of the confusion.Tim

author: bostonman
author_rank: Super Contributor
timestamp: October 11, 2024, 02:54:07 am
content: Certainly a great explanation.Maybe I was thinking more on the idea the grease is a "conductor" where it allows the heat sink to "touch" the tab as the tab gets hot. Similar to wire conducting electricity.Seems more that the grease is touching the tab, so the grease needs to heat before the heat sink sees the heat coming from the tab.So the grease would need to be made of material that gets hot quickly so it can transfer the heat from the tab to the heat sink. If the grease was say dense dried chalking, then it would take longer to transfer the heat.

author: IanB
author_rank: Super Contributor
timestamp: October 11, 2024, 02:58:31 am
content: The grease is a conductor, a thermal conductor.There is so little grease that the time taken for the grease to heat up is insignificant.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2024, 03:13:58 am
content: I mean, it's both. They're touching, and conducting heat, so when the temperature rises on one side, it rises on the other side too.Whereas a dry compound is full of air spaces, and air is bad. Bulk conductivity principles apply at most any scale, at least until quantum effects come into play.Tim

author: bostonman
author_rank: Super Contributor
timestamp: October 11, 2024, 03:24:52 am
content: I am guilty of using too much.I thought more grease meant more heat was transferred but also the extra just squishes out.Also, I get worried when I smooth the grease because I tend to get scrap lines that remove a small track of grease.

author: IanB
author_rank: Super Contributor
timestamp: October 11, 2024, 04:03:51 am
content: More grease is more badConsider that either side is a metal surface. The thermal grease is a worse conductor than metal, but is a better conductor than air. So you need just enough grease to fill the air gaps, but no more.The grease spreads out a lot when you apply pressure to the cooler when you lock it down.An interesting experiment you can do is to get two glass plates like microscope slides, put a tiny blob of thermal grease between them, and then press them together. It is amazing how far a little dab of grease expands.

author: bostonman
author_rank: Super Contributor
timestamp: October 11, 2024, 12:17:25 pm
content: QuoteI've seen or have done similar experiments, and you're correct, it spreads out quite a bit more than assumed.Using more grease may go back to what I was trying to explain on how I interpreted what exactly it does. Let's assume two pieces of metal are mounted together (the tab of a component and a heat sink) and a pit located exactly at the same points on both pieces of metal. Let's say they are half spheres on both, and, when pressed together, create a perfect sphere.Obviously this sphere is an air gap. My thought on grease was that the grease fills the sphere and transfers the heat by the heat going through (I know, poor word to use) it and onto the heat sink. Better quality grease transfers the heat better/faster.From what I'm understanding is that the grease heats and transfers its heat to the heat sink.Technically both my thoughts are the same because the heat sink would be warming slower than the grease and the grease slower than the component, but to be a bit ridiculous and completely incorrect: I thought the grease doesn't heat (that's the completely incorrect statement that I'm using for purposes of discussion) and is just a compound that transfers the heat from one object to another.Therefore, more grease, the more cooling.Yes, I understand (now) that more grease is more wrong.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2024, 01:01:01 pm
content: Yeah, you've got itA sphere obviously won't carry much heat across it, as having a spherical bubble in the middle of a metal sandwich; but a very flattened spheroid sort of shape might well describe a lot of the bumps and warpage that real mating surfaces have, and it is that gap that grease helps the most at.Tim