How much does potting help with thermals / hotspot management?  - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: August 04, 2025, 02:05:53 am
content: I'm looking into potting a design.How much potting helps with thermal hot spots on ICs. (assuming a sealed box full of still air vs potting compound.)I'm not looking for anything specific. Obviously it will depend on the thermal properties of the potting compound being used.I'm just looking for a general rule of thumb at the moment. I will get to testing later on. Just curious what's typical.It will obviously be better when potted, since potting compound is always going to be more thermally conductive than air. Any hot IC should have more heat removed faster from its case and the PCB should have heat removed faster into the block of potting compound.If I have an IC case that sits around 70C at 25C ambient and I pot the box what sort of improvement to the IC case temp can I probably expect?10, 20, 30 degC less etc..

author: coppercone2
author_rank: Super Contributor
timestamp: August 04, 2025, 02:41:51 am
content: thermal conduction means little because you have flow. Air cooled systems get flow cooled. This is whhy its so hard to estimate differencesLow thermal conductivity can mean little if there is flow of fresh lower temperature 'coolant'.Air always moves. You would need to some how freeze mechanical motion to make the air metric accurate. Perhaps this is true for a very fine bubble aerogel or some thing.A air cooled sealed enclosure can reach a steady state. But there is still gonna be flow in there.this is happening for your air cooled parts

author: Psi
author_rank: Super Contributor
timestamp: August 04, 2025, 03:03:55 am
content: Ok, will just have to test it then.The IC has I2C and build-in die temp sensor so I can get some actual data once the PCB is done.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 04, 2025, 03:19:05 am
content: I don't think there is a good rule of thumb. It varies so much based on the situation. It can make things better or worse depending on how much convetion was helping. Or it can make no difference at all if heat transport was mostly through the copper ground plane already.Luckily while airflow is rather difficult to calculate, solid heat conduction is pretty easy to estimate.

author: floobydust
author_rank: Super Contributor
timestamp: August 04, 2025, 03:36:27 am
content: It is wrong to assume potting compound cools a circuit board.I found this out the hard way. Thermal IR cam seeing a hot diode running 40°C above the potting compound temp.I learned that these potting compounds act as a thermal insulator! Some are low density. You lose all convection cooling a component would get on the PCB in free air, on your workbench.Electronic potting compounds have a thermal conductivity spec. - check it.ExampleSylgard 170Silicone potting compound is 0.4-0.48W/m.K on par with plaster lol.

author: Psi
author_rank: Super Contributor
timestamp: August 04, 2025, 05:03:41 am
content: I'll be able to test it in a few weeks, I was just curious since there are implications about how many copper layers the PCB might need, and how thick they need to be.The chip has thermal ramp-down/shutdown, and you can tweak the threshold over i2c to kick in at a lower temp, so I have plenty of options.It's just good to get the brain churning over the issues and options sooner rather than later.Quote from: floobydust on August 04, 2025, 03:36:27 amI wasn't expecting cooling, just spreading the heat out faster than with air and hence reducing hot spots a bit.

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 04, 2025, 12:31:50 pm
content: Quote from: floobydust on August 04, 2025, 03:36:27 amThermal conductivity of still air is around 0.025 W/m-K. Makes plaster look quite good.Of course air is rarely still. But accurately modelling natural convection is extremely challenging.If potting yields an acceptable result, it's not unreasonable to take that route, especially if the boss wants to start manufacturing transfer next month.

author: floobydust
author_rank: Super Contributor
timestamp: August 05, 2025, 06:26:30 pm
content: Hard to know exactly what caused the silicone potting compound to keep parts... cozy hot.I fully assumed the potted brick would conduct and spread out the heat- but it was the opposite I found.It was funny because the chinese PSU brick manufacturer had removed the synchronous output rectifier (present in the golden samples) and replaced it with a small Schottky diode, which of course ran really hot. Under potting compound, who looks in? I do, I pried it all apart to reveal that scam.Heat is transferred in any of three ways - conduction, convection, radiation.What about heat loss due to radiation? If the potting compound reflects any of that it would explain what I observed.