When you need really good cooling: Mikrotik hAP ax3 - Page 1
Forum section: electronics / general

author: golden_labels
author_rank: Super Contributor
timestamp: February 28, 2024, 08:34:12 pm
content: A friend found this on a main board of a Microtik hAP ax3. Experts on applying thermal paste, this is your time!

author: Nominal Animal
author_rank: Super Contributor
timestamp: February 28, 2024, 09:55:37 pm
content: What the actual fuck? That image is the Monty Python dead parrot sketch of incorrect thermal paste use.If you use enough paste to form a layer between the two surfaces the thermal performance is often worse than without any paste at all.If the surfaces are not flat, use a thermal pad designed for the purpose; thermal paste is not that.

author: thm_w
author_rank: Super Contributor
timestamp: February 29, 2024, 12:11:35 am
content: I'm thinking this IC is normally fitted and the paste is on the empty side of the board, to conduct to the aluminum. With the IC there it would not spray through.Although it is a hilariously large amount as Norminal says.Heres another photo where no paste is visible:https://www.reddit.com/r/mikrotik/comments/10i74a7/hap_ax3_disassembled_possible_expansion_slot/There seems to be white thermal pad for the RAM visible also.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 29, 2024, 01:46:54 am
content: I don't understand the picture.

author: Nominal Animal
author_rank: Super Contributor
timestamp: February 29, 2024, 09:05:24 am
content: Quote from: thm_w on February 29, 2024, 12:11:35 amYep; and a thermal pad should be used there instead. Similar cooling construction (from the opposite side of the board than the IC itself) is sometimes used for example for stepper driver ICs. Some Trinamic ones for example allow a lower junction-to-heatsink thermal impedance that way (compared to heatsink on top of the IC).Some SBCs likeOdroid M1are designed so that one entire side is covered by a heatsink (basically forming one side of an enclosure), with the most heat-generating ICs put on that side. It works well, but any height differences must be taken up by different thickness thermal pads, not thermal paste. Thermal paste is only effective if the heatsink surface is machined flat, the components the same height, and sufficient pressure (additional screws between components) is applied between the heatsink and the PCB. Thermal pads are much more forgiving, although even their use must observe the limits of how much "give"/"crush" a specific pad allows before its properties are affected.In this case, I think it is an one-off, an error by someone who thought they could replace a thermal pad they didn't have and didn't know the specific type or properties of, with gobbets of thermal paste. "The parrot isn't dead, it's just resting."

author: golden_labels
author_rank: Super Contributor
timestamp: February 29, 2024, 10:11:45 am
content: The author of the photo can’t tell, what’s on the other side. That would require removing the heatsink and I understand they don’t want to damage their device.I foundsome photos of ac3from another person. It seems on the other side there is only the heatsink, pressing directly against the PCB. Unlike in ax3, there seems to be a daughterboard present over this area. So it may make sense to provide thermal paste on both sides. But the amount of it is hard to explain.If the daughterboard is present during manufacturing, the paste would stay mostly on the radiator side, but then they wouldn’t need to use so much of it. And on the other photos we see the paste is somehow present on both sides, which isn’t aligning well with applying with the expansion already in place.If the daughterboard is absent, we get these magnificent noodle. After the card is placed, they would cover everything. Again, not consistent with what we see on the other photos or any logic.SiliconWizard: the gray thing is thermal paste oozing through vias from the other side, in ridiculous amounts.

author: Nominal Animal
author_rank: Super Contributor
timestamp: February 29, 2024, 05:38:46 pm
content: It does remind me of a macaroni pasta machine. Yummy. (The pasta, not the dead parrot, nor the thermal paste.)

author: audiotubes
author_rank: Regular Contributor
timestamp: February 29, 2024, 07:16:22 pm
content: Quote from: Nominal Animal on February 29, 2024, 05:38:46 pmThere was a similar thread recently where somebody showed a picture of goop all over a board. Both that picture and this one remind me of silly putty.Still looks like people are bringing their 5 year old kids to work probably a bit too often.https://www.eevblog.com/forum/chat/someone-went-crazy-with-the-silicone/

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 29, 2024, 08:51:43 pm
content: Someone must have been playing Play-Doh a bit too much in their childhood.I'd be concerned about long-term reliability, the thermal paste used may not be conductive (hopefully), but having so much of it in between pads and vias does not inspire confidence. It's bound to carry all kinds of particles over time. Yuck.

author: ConKbot
author_rank: Super Contributor
timestamp: March 01, 2024, 04:03:12 pm
content: Thermal paste aside, that package footprint?(partially) triple row QFN? Because double row QFN while a coworker kicks you in the rear isnt enough pain?

author: nctnico
author_rank: Super Contributor
timestamp: March 01, 2024, 04:20:02 pm
content: Quote from: Nominal Animal on February 28, 2024, 09:55:37 pmI find it shocking how thermal paste gets abused. I've found it between rubber silicone pads where you don't need it. I've found it used as a big blob to conduct heat into an NTC near a heatsink. But the worst offenders was a thick layer of thick thermal paste (aimed at the overclocking crowd) which was used between a module (in an aluminium cast) and a heatsink. The module got physically damaged because it was warped way too far due to the thick layer of thermal paste. I have not been able to convince the people resposible to change their applicator technique or use thermal pads. You can buy large thermal pads for thyristor and PSU bricks... BTW I got some leftover Sigraflex (graphite based) thermal interface material. That is also very nice stuff to use.Bottom line: thermal paste is not a very good heat conductor.

author: Karel
author_rank: Super Contributor
timestamp: March 01, 2024, 04:43:38 pm
content: Quote from: nctnico on March 01, 2024, 04:20:02 pmNothing wrong with that. It's common practise in every Japanese hifi stereo amplifier since the sixties or seventies until today.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 01, 2024, 09:00:48 pm
content: Quote from: nctnico on March 01, 2024, 04:20:02 pmYes, absolutely. It's just meant to fill up the tiny air gaps between the heatsink and component due to surfaces not being perfectly flat and smooth. That's all. Ideally, there should be a thin layer barely thicker than the max height of said gaps. That's very little.

author: Nominal Animal
author_rank: Super Contributor
timestamp: March 02, 2024, 10:15:58 pm
content: Quote from: SiliconWizard on March 01, 2024, 09:00:48 pmIf we look at typical thermal pastes, they are basically a suspension of ceramic and metal (oxide) particles in sillycone or sillycone oil.Their thermal surface properties are much better than their bulk properties. In a thin enough layer, basically just filling the microscopic pockets, they're stable and have very nice thermal conductivity.Compare to how surfactants work, say soap on top of water. (They form a single-molecule thick layer at the surface, be that surface to air or to a dirt particle.)As a macroscopic blob or a layer thick enough to no longer be "surface only", thermal paste thermal properties differ, because surface and bulk can have wildly different properties. Typically, the suspended particles start to clump, and the oil leak out or evaporate. The clumping does not really happen in a 2D layer.If we get down to the molecular scale, surface properties and bulk properties can beextremelydifferent. A good example of this, one I'm well familiar with (from simulation and modeling side, not from using in practical applications), is stainless steel and other ferrochrome alloys (iron + chrome). Even at room temperature, the chromium atoms slowly diffuse closer to the surface. When the average chromium content is between 13% and 25%, the chromium density is much higher near the surface, forming clusters and "dendrites" towards the bulk. (These were first imaged after the turn of the millenium, too; the surface structure just forms too easily in normal conditions.) This is what makes these alloys resistant to oxidization. This surface layer is hundreds of nanometers thick, too, and just won't show up in simulations unless you have millions of atoms, so you can see the difference in bulk and surface.And, as we all know, nanoparticles are extremely useful in e.g. LEDs, and are nothing but clusters of atoms (typically metals), with the clustersizecarefully controlled. There is no bulk at all, and the surface properties and the size of the cluster determines its properties, especially phonon spectra, including the visible color (visible photon absorption and emission spectrum due to surface phonons limited to specific modes due to particle size, the good ol' quantum stuff).Rubbery potting compounds and silicones do not dry out even when used in bulk, but even there, if you cut a blob open and poke it with a sharp pokey thing, you can observe the difference in the surface and bulk properties (until the bulk also oxidizes et cetera, and the sliced part becomes a new "surface"; this was long the reason actual cross-section images at the atomic scale of stainless steel and ferrochrome alloys were so hard to obtain). Even if a decade old, the innards are more "liquidy" than the surface.

author: 5U4GB
author_rank: Super Contributor
timestamp: March 03, 2024, 10:51:16 am
content: Quote from: golden_labels on February 28, 2024, 08:34:12 pmIf anyone's wondering what sort of tool applies paste like this, it's this: