Just a thought, reflow soldering modern Intel CPU to heatsink - Page 1
Forum section: electronics / general

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 29, 2015, 09:43:27 am
content: I've recently had some thermal difficulties with my PC, so I've tried many different thermal pastes, application techniques, heatsinks and fans to find the best cooling performance for my setup ($2000 of parts in a mini-ITX case). Which made me think of - and actually consider - the 'ultimate' thermal solution - to directly couple the CPU case to the heatsink with metal. i.e. to put a layer of solder paste between them and reflow it in an oven. Even the best of thermal pastes are far inferior to metal, we're talking max. 2 W/(mK) for pastes vs. 60-72 W/(mK) for SnAgCu/SnAg alloys.Take my Intel i5-4670k as an example. Would a modern Intel/AMD CPU survive a reflow cycle? Anyone willing to give it a try? Anyone seen it done? Would solder even reflow on the CPU case?

author: dr.diesel
author_rank: Super Contributor
timestamp: August 29, 2015, 10:04:19 am
content: Part of your problem is the TIM between the actual die and the metal cover lid, on Haswell it isn't bonded like the previous Sandy Bridge.You can De-lid it and use better stuff that makes a big difference, search around, or visit the experts on HardOCP etc.

author: kony
author_rank: Regular Contributor
timestamp: August 29, 2015, 10:19:58 am
content: Too high melting temperature on normal SAC paste. For such sensitive stuff, indium solder is commonly used (but such things are not cheap).How good is the surface finish and flatness of both your CPU top lid and heatsink? The less heat "conductive" paste is there, the better - lapping can do quite a lot for this, but is extremly tedious and time consuming to do.Alternativelly you can try using pyrolytic graphite foil, then we are talking about 20-30W/m.K in transverse and ~ 1500 W/m.K in parallel plane to foil surface.

author: helius
author_rank: Super Contributor
timestamp: August 29, 2015, 11:04:35 am
content: I pasted a video from 3M in the "Vapor Phase Soldering" thread:https://www.eevblog.com/forum/projects/vapour-phase-soldering/msg591750/#msg591750The condensing vapor FC-40 was used to reflow preform pads of a low MP solder, a bismuth alloy. Bismuth is cheaper than indium.When reflowed, the pads attached the CPU package to a copper heatsink.The purpose behind the video is to prepare parts for a system that uses boiling inert fluid as a coolant. The copper heatsinks were sprayed with a boiling enhancer, a kind of copper foam. The CPUs were then loaded into a machine that sat submerged in Novec, transferring all their TDP into boiling it.

author: max666
author_rank: Frequent Contributor
timestamp: August 29, 2015, 11:47:09 am
content: If I'm not mistaken your CPU is still one with the fucked up Thermal Interface Material (TIM) between the chip and the heat spreader (IHS), introduced with Ivy Bridge. Because with Ivy Bridge Intel replaced the previous fluxless solder with "crappy" thermal paste as TIM&. It wasn't until Haswell Refresh that they improved the TIM with a so called next-generation polymer thermal interface material (NGPTIM). Your i5-4670k is Haswell but not Haswell Refresh, I believe?Also while you're right that 08/15 thermal pastes only has up to 2 W/(mK), silver thermal compounds may have a conductivity of 3 to 8 W/(mK), or even16 W/(mK). And beyond that there is the crazy liquid metal shit, for which I sadly haven't found thermal conductivity values yet.Where am I going with all of this? I think the bigger issue with your CPU running hot isn't the transition IHS to CPU cooler (I would guess 3°C with 2 W/(mK) TIM), but the transition chip to IHS (10°C). So I think it would actually be less risky and you would gain more, if you delidded your CPU and fixed the TIM's.

author: max666
author_rank: Frequent Contributor
timestamp: August 29, 2015, 12:04:23 pm
content: Quote from: kony on August 29, 2015, 10:19:58 amDo you have some practical information on where to obtain a pyrolytic graphite foil for that purpose?

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 29, 2015, 12:09:10 pm
content: Quote from: max666 on August 29, 2015, 12:04:23 pmhttp://ca.mouser.com/search/refine.aspx?Ntk=P_MarCom&Ntt=135626005this?

author: BradC
author_rank: Super Contributor
timestamp: August 29, 2015, 12:12:33 pm
content: Quote from: max666 on August 29, 2015, 11:47:09 amDelidding is a 10 second process with a bench vice and a hammer. Big gains for not much work.

author: kony
author_rank: Regular Contributor
timestamp: August 29, 2015, 12:29:03 pm
content: Quote from: TheAmmoniacal on August 29, 2015, 12:09:10 pmYes, Panasonic and few other manufacturers make them - in various flavours - with or without insulating foil, added silicon padding, glue and such - beware trough, that any added material will decrease its performance.I should perhaps put emhasis on note that the magical conductivity is really only working in two directions, so your heatsink interface still sees about 20-30W/m.K thermal resistance, but with all hotspots eliminated - heats get wery well spread across the whole contact surface. That is the wonder of it (and quite obvious afterwards, if you look how graphite lattice looks like).

author: max666
author_rank: Frequent Contributor
timestamp: August 29, 2015, 12:30:39 pm
content: Wow, I didn't think this stuff would be that readily available! How did I not stumble onto this yet myself?!Why would anyone mess around with electrically conductive liquid metal, when there is something like this available?PS: Love the commercial:

author: kony
author_rank: Regular Contributor
timestamp: August 29, 2015, 12:58:03 pm
content: It's going widespread mostly thanks due to ever harder thermal management of miniaturized mobile electronics (as advertised), but it has MANY cool uses in other areas. From what I had seen so far, my favourite one is using chunk of such foil for geometry calibration on TEM/AFM.

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 29, 2015, 01:02:03 pm
content: If anyone buys a sheet, cut me a square and mail it over! (I'll pay. Get the pure PGS Type S, not padded/isolated or with adhesive)

author: BradC
author_rank: Super Contributor
timestamp: August 29, 2015, 01:07:52 pm
content: Quote from: TheAmmoniacal on August 29, 2015, 01:02:03 pmIt's not going to help. Your thermal limitation is the TIM that lives under the heat spreader. Until you fix that you are chasing fractions of an improvement by working on the spreader to heatsink interface.

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 29, 2015, 01:18:43 pm
content: Quote from: BradC on August 29, 2015, 01:07:52 pmI do think you are right, but I want to test without de-lidding first (a bit extreme for an expensive CPU like that). If it still isn't good enough I could always just try the sheet directly on the die after de-lidding. Curious to try it anyway.

author: BradC
author_rank: Super Contributor
timestamp: August 29, 2015, 01:33:34 pm
content: I guess its relative. I did it to an i5-3570k and then an i7-3770k. Gotta pay to play.

author: senso
author_rank: Frequent Contributor
timestamp: August 29, 2015, 01:35:12 pm
content: Delid, cover the test pads with some nail polish, and if you really want metal, use CLU:http://www.coollaboratory.com/en/products/liquid-ultra/NO aluminium near that, it contains galium, aluminium just crumbles in minutes.But delliding is simple and you can use much better thermal paste, for reference:http://www.overclock.net/t/1568432/expreview-i7-6700k-delidded-temperature-measurementsAnd there is no thermal pad in earth that will outperform thermal paste, because they are always much thicker, they impose a worse thermal interface..

author: BradC
author_rank: Super Contributor
timestamp: August 29, 2015, 01:38:17 pm
content: +1 for CLU for the die to spreader interface.

author: max666
author_rank: Frequent Contributor
timestamp: August 29, 2015, 01:40:23 pm
content: Quote from: TheAmmoniacal on August 29, 2015, 01:18:43 pmSays the guy asking about reflow soldering the CPU directly to the heatsink

author: BradC
author_rank: Super Contributor
timestamp: August 29, 2015, 01:44:15 pm
content: Thinking about it, you could easily de-lid, solder the IHS to the heasink and then put it all back together again with a liquid metal interface (CLU) between the die & IHS. No risk of damaging the CPU while soldering in that case.

author: max666
author_rank: Frequent Contributor
timestamp: August 29, 2015, 02:05:54 pm
content: That does indeed sound like an interesting idea. I just want to know what the thermal conductivity of the liquid metal interface (CLU) is?

author: ConKbot
author_rank: Super Contributor
timestamp: August 29, 2015, 02:21:01 pm
content: Seems like a great application forhttp://www.indium.com/nanofoil/but too bad the availablity is going to be next to nil

author: senso
author_rank: Frequent Contributor
timestamp: August 29, 2015, 02:22:08 pm
content: See the results for yourself:http://www.tomshardware.com/reviews/thermal-paste-performance-benchmark,3616-17.html

author: BradC
author_rank: Super Contributor
timestamp: August 29, 2015, 02:23:07 pm
content: Quote from: max666 on August 29, 2015, 02:05:54 pmApparently the spec is 38.4 W/(m*K)

author: rs20
author_rank: Super Contributor
timestamp: August 29, 2015, 02:28:36 pm
content: Quote from: BradC on August 29, 2015, 02:23:07 pmWhich specific product is that? Coollaboratory Liquid Pro claims 82 W/(m·K). It's a Gallium alloy with a melting point of 8 degrees C. Keep it away from Aluminium, there are some fun YouTube videos on the interations between Gallium and Aluminium.

author: BradC
author_rank: Super Contributor
timestamp: August 29, 2015, 02:32:32 pm
content: Not the Pro, the Ultra (which is what I have here).

author: amyk
author_rank: Super Contributor
timestamp: August 29, 2015, 02:41:57 pm
content: Quote from: max666 on August 29, 2015, 11:47:09 amI think they just want to reduce cost - the IHS used to be soldered to the die, but that takes more time and equipment than just putting some TIM on there and gluing it on.Of course if they wanted to reduce cost even more they would just have a bare die like in the days of the PIII and Athlon XPs... but maybe the possibility of damaging the die (chipped corners were common) made them add the IHS.

author: IanB
author_rank: Super Contributor
timestamp: August 29, 2015, 02:51:41 pm
content: Quote from: amyk on August 29, 2015, 02:41:57 pmAnd this seems ridiculous...saving a few cents on a part that sells for ~ $500?

author: BradC
author_rank: Super Contributor
timestamp: August 29, 2015, 03:10:54 pm
content: Quote from: IanB on August 29, 2015, 02:51:41 pmI get the impression the soldering process was not quite a few cents. The reality is the TIM is good enough as long as you leave power dissipation as stock. It's when you want an extra GHz or so things start getting marginal.

author: max666
author_rank: Frequent Contributor
timestamp: August 29, 2015, 03:26:48 pm
content: Quote from: BradC on August 29, 2015, 03:10:54 pmI agree that the TIM is probably sufficient as long as you don't overclock, but having it on the K series which is intended for overcklocking is just deliberately crippling potential performance.It's also about long term reliability, your CPU will age faster at higher temperatures. Let me get my tinfoil hat and talk about planed obsolescence ...