Thermal derating of components in potting compound - Page 1
Forum section: electronics / beginners

author: e100
author_rank: Frequent Contributor
timestamp: November 22, 2015, 05:08:07 pm
content: Does anyone have any rules of thumb for the thermal derating of components once they are potted?The published specs for the thermal conductivity of air and silicone rubber/epoxy don't take into account the convection and radiation cooling effects in air so it's difficult to do calculations.

author: Simon
author_rank: Unknown
timestamp: November 22, 2015, 05:22:08 pm
content: This is the problem I have too. It will depend on what you are potting with. when you think about it air is one of the worse conductors so surrounding a part in potting compound could help remove more heat than air would. Take your tiny 1206 resistor, it has a certain surface area and typically can dissipate 250mW up to 50 or 70C, but now you make it bigger by putting it in compound that takes heat off it faster than air would and gives it more surface area. Of course the next problem is how fast will the potting compound conduct heat and give it up to the air and other parts in the compound will make heat too.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: November 22, 2015, 06:37:28 pm
content: natural convection cooling is not linear - the effect becomes stronger at higher delta temperature which causes more air motionradiation cooling is very nonlinear ~ T^4 but doesn't usually dominate until dT between the hot surface and everything it "sees" is getting to 100 C or thereaboutsso no, the formulas aren't going to be simpleyou may end up on mostly relying on conduction in any copper planes - some military/avionics has thermal contact features in the frames holding the modules by their edges

author: Simon
author_rank: Unknown
timestamp: November 22, 2015, 06:59:23 pm
content: If your worried you can have aluminium backed PCB'sthere are highly thermal conductive potting compounds, if there is enough potting surface are surely potting amounts to a moulded heat sink.

author: tautech
author_rank: Super Contributor
timestamp: November 22, 2015, 07:18:17 pm
content: AFAIK the better potting products have thermal conductivity specs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 22, 2015, 09:58:53 pm
content: Often, the ratings go up, not down! Air is a very poor thermal conductor, we just have a lot of it around...You do want to keep the amount of potting small (or the thickness low), so that heat can get out quickly -- because, unfortunately, it probably has to be dissipated in air at some point. You are putting a blanket over everything, and the average internal temperature will necessarily rise. (The peak temperatures of small components may be lower, due to better heat spreading in the bulk material.)And if you can get any of the potting up against a heatsink of some sort, you'll be doing very well. A metal enclosure, an aluminum bracket, heatsinks bonded into the potting... Better still if the assembly, in turn, mounts on a metal frame or chassis or something, so it gets conduction cooling to an even larger surface. This is part of the reason automotive parts are as reliable as they are (well, with some notable exceptions I'm sure, due to poor design).Tim

author: Simon
author_rank: Unknown
timestamp: November 22, 2015, 10:40:25 pm
content: Surely a suitable amount of potting of good thermal conductivity will amount to a heat sink, if you have a small device and put potting compound over it that conducts heat well and take heat from a small part and takes it to a larger surface area it will actually aid cooling. Of course it has to be highly conductive compound and putting a heat sink in is a good idea.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 23, 2015, 10:06:59 am
content: Quote from: Simon on November 22, 2015, 10:40:25 pmYeah, but compared to what? Comparing to the unpotted case, the solid block has less surface area (not necessarily less boundary layer* area), so will dissipate more poorly in the end, plus you're adding a thermal resistance between air and components. Both act only to increase internal temp.*The region of stagnant, low velocity air near a boundary or wall or surface, where heat is transported more by diffusion than convection. For passive convection, this layer is fairly thick (hence the fins on a passive heatsink are quite fat apart). Conversely, glass wool works because, if enough damping/turbulence is added to a volume, the entire volume can basically be turned into a boundary layer.But like I said, it can be better, perhaps more in terms of worst-case than average-case. If the air boundary layer is dominant (say, in the middle of a forest of vertical THT parts,where air just doesn't circulate), and especially if some conduction or heatsinking is provided, it could be all-around better with potting. (The assumptions I'm using above aren't at all solid, but intentionally much looser than assumptions I usually work with.Such tends to be the case, when geometry gets complex: you can make many rigorous statements about simplified situations, like heat dissipation from a flat surface; but putting together many things generates complexity that defies such short explanations.)Tim