Thermal Analysis Of Potted Electronics

I'm looking for a book on thermal design, does anyone offer a book which covers potted assemblies? I've read through the table of contents on every book I can find on electronic thermal design and none of them call out such a popular method as potting the components in a material with a better thermal conductivity than air. Why is this? Every place I have ever worked has potted assemblies of one type or another. Seems like it might be important in a competitive environment like aerospace where every ounce of weight counts reliability is king and the competition is fierce. Maybe it's the fierce competition? Anyhow if anyone knows of a good book which covers this topic please let me know.

I do not know of any books.

But several years ago I looked into potting for a particular high reliability project for a very harsh wet environment, and was advised VERY STRONGLY against potting circuit boards.

The big problem as I understand it, is the exothermic effect during curing is not really controllable, and can reach temperatures high enough to cause serious future reliability issues if not outright failure at final testing.

A much better and more popular technique for electronics is a conformal coating, which military, areospace, and the mining industry, are very fond of.
That is what we ended up doing, and it worked very well for our circuit boards.

Potting seems to work fine for underground cable joints and things like that, but for fragile electronic components it has proved to be very troublesome.

Sorry for veering off topic, but just though I would share my own experiences with potting.

Warpspeed said:

I do not know of any books.

But several years ago I looked into potting for a particular high reliability project for a very harsh wet environment, and was advised VERY STRONGLY against potting circuit boards.

The big problem as I understand it, is the exothermic effect during curing is not really controllable, and can reach temperatures high enough to cause serious future reliability issues if not outright failure at final testing.

A much better and more popular technique for electronics is a conformal coating, which military, areospace, and the mining industry, are very fond of.
That is what we ended up doing, and it worked very well for our circuit boards.

Potting seems to work fine for underground cable joints and things like that, but for fragile electronic components it has proved to be very troublesome.

Sorry for veering off topic, but just though I would share my own experiences with potting.

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I would advise to test samples to get the correct results. Use an IR camera to see temperature levels before potting and afterwards. Avoid power dissipation of more than 0.1 W on any component. Such hot components should not be potted.
Small components are designed to heat-sink in air and through the PCB. I have not seen any components specified for potting, so you must experiment to get your data.

Hi,

It's not thermal conductivity only but airflow also.
If you pot them you have no airflow around the parts.

Another problem with potting is that the potting mass shrinks during curing. If it is too hard, the you get mechanical stress, especially on the solder joints of SMDs.
Potting HF electronics is problematic because of the high relative capacitance of the potting material. Especially two component potting materials have high thermal drift of capacitance.
Pottion mass including silicone often cause problems with connectors and switches because uncured residuals act like penetrating oil covering the contact material and thus increasing contact resistance.

But putting has benifits also. It may solve problems with humidity or mechanical vibrations...

Klaus

Thanks everyone. Potting is pretty standard for this application and there are devices which dissipate a few watts involved and power density is important. Maybe some FEA software is in order.

Dust and moisture absorption does much more leakage and capacitance, as Water has a d=80 vs epoxy, silicone, polyurethane types 2~4, proportional to thickness.

Automotive boards we made for Chrysler all had to be masked and dipped in thin conformal coating. A spray is less uniform but may be adequate.

Hydroscopic materials will rise in capacitance due to 20x higher dielectric constant in water but like transmission lines without the distributed inductance, if the distributed capacitance has ground tracks between sensitive lines on outer surface it should be ok.

Material thickness adds to Rja (thermal jcn to ambient resistance) so thick RTV is preferred when exposed to flaming external harsh military environments, which adds more design work for passive thermal conduction..

THin dips or sprays are a good compromise for Industrial Electronics to avoid the dust bunny failures.

I worked at a TWTA power supply place and all their 1kw smps's are potted. Due mainly to the high voltage I believed. You can buy transparent liquid pre-potting compound which you literally pour (its at room temp) over your board as it lies in a transparent box. To see what it works like before actually potting it "proper". Another HV company showed me it. They were making kit to stop esd in production lines.

Low pressure moulding, used in automotive a lot...
As to thermal design, expensive simulation software, a thermal camera, best option planning it out as you design the product, i.e. think about heat removal at the concept point of design.

treez said:

I worked at a TWTA power supply place and all their 1kw smps's are potted. Due mainly to the high voltage I believed.

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At one time I was involved with high voltage power supplies for pumped laser flash tubes, and the whole thing was immersed in a tank of oil.
I suppose that should also work well on a much smaller scale for very high energy density switched mode power supplies.
Although messy, it would at least make future maintenance a possibility.

According to my experience, potting with thermal conductive epoxy resin is valuable option for high power density electronics.

For simple geometries like toroid transformers, the thermal behavior can be easily calculated with FEM methods.

Agreed, the classic use of potting is when you have a pcb inside a metal enclosure and you have no fans. The potting is then the best way to move the heat from the pcb to the metal enclosure.

But it then becomes a throw away item.
Not so bad for a five dollar wall pack, pretty serious for a thousand dollar high end power module.

agreed, but of course, most consumer <100W SMPS's are throw away, nobody ever tries to fix them. I imagine every laptop power supply in the world is potted, they feel pretty solid. Then again, ive had enough of them fail on me.

thermally conductive potting compounds have problems and much higher cost for very little increase in heat removal.

heat sinks as the primary means to remove the heat.

Silicone dip or spray is effective isolation from automotive contaminants at the expense of rise in thermal resistance.

If you must pot, consider this **broken link removed**

I see your point, but they should have filled one puck with a non potting compound and compared it. (ref the link above)
The thing is, it says to use a heatsink instead of potting.....and of course, we all would do that if possible, .....its just that sometimes you don't have room for heatsinks, and in any case, in a totally enclosed enclosure, there is nowhere for the heatsink to convect heat away to....so surely having potting compound round the components, so that the heat can be conducted away to the metal enclosure is a very worthwhile thing.?...I mean its effectively like having gap pad between a hot component and a metal enclosure, and the use of gap pad is well respected.

We had a 3kw battery charger bought OTS, and it had gap pad from lots of the components which weren’t heatsinked so as to get the heat to the metal enclosure…the components that were “gap-padded” to the metal enclosure were….

The power film capacitors of the LLC converter
The ferrite transformers.
The electrolytic capacitors
The input film capacitors to the PFC.
The PFC inductors
The LLC output capacitors.

So surely this means this principle of using an intermediary substance such as potting compond or gap pad to get heat conducted away to a metal enclosure is valid?

So surely this means this principle of using an intermediary substance such as potting compond or gap pad to get heat conducted away to a metal enclosure is valid?

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Surely. You get the answer by calculating thermal resistances and achieved heat transfer in detail. And compare to natural or forced air cooling solutions.

It's correct to say that thermal conductive potting isn't the solution for everything, pros and cons have to be balanced for specific applications. But it also seems to me that some posts in this thread are lacking actual experience with potting solutions.

I think potting is more useful when thru hole components are used, as they tend to have a bigger surface area which can interface to the potting compound surrounding it.
But yes, it would be nice to have a "league table" of potting compounds, so we can see which has the best thermal conductivity. No such info exists.

I bet somewhere in the world somebody has developed a thermal conductive/elec insulative potting compound which is as good as solid metal (almost).
The fluid potting compounds which never set are good as they can have convection currents in them aswell, but you need to be careful the enclosure is tightly sealed