Power supply potting compounds

[jr852]

I am packaging a power supply to be used in a high vibration environment on an aircraft. Right now we are considering potting compound since serviceability is NOT a concern. Reliability is critical. We also need to use a low complexity design for the PCB mounting scheme and thermal transfer. Some leading particulars:

- Shock mounting is not an option
- Through hole components on a PCB, both sides
- Needs to be tough and/or flexible so it can withstand high vibration stresses and wide temperature swings
- Will be mounted in an aluminum enclosure
- Some of the components are heavy and the solder connections will not withstand the vibration levels without additional support.
- Since it is a power supply, good thermal conductivity to the case is important
- The PCB will be crowded, not much space between components.
- We have some latitude on the enclosure design, including custom machining (from billet) since it is a low production item.

I have identified several heat activated two-part epoxy compounds that look like they will work, including products from Loctite, Permatex, and Aremco. I am looking at of-the-shelf epoxies since I cannot live with a long lead time.

Questions:
1. Some of the epoxies have a high viscosity at room temperature. Can I expect a significant reduction in viscosity when I heat the mixed epoxy to curing temperatures (on the order of 350 Deg F)? I want to get good component wetting for good heat transfer.
2. We will be minimizing the extra volume in the enclosure and are considering potting the entire volume of the enclosure (minus the components and PCBs). The thickness of the epoxy will be on the order of 1-2 inches. Are there notable potential pitfalls here, like curing shrinkage damaging the components? Curing shrinkage with one product is 0.003 in/in. Is this typical? Not all the products have this spec on their tech sheets.
3. Any suggestions for potting compounds I may have overlooked?

I know, I will be calling the epoxy supplier's applications engineers as soon as I can I have also looked through this site and located some good information.

Thanks, this is a great forum.

 

[jr852]

Another piece of information. We are modeling the components using solid modeling software, and will also be running a thermal (steady-state) analysis to assess localized temperatures to check for component overheating.

Thanks

 

[Compositepro]

Dow makes some silicone potting compounds that you should condider. They can be relatively easily removed to allow repair as well. Pulling vacuum after potting your circuit will reliably get rid of air bubbles so you don't have to be very careful about mixing and pouring to avoid bubbles.

 

[jr852]

Hey that was a quick response Compositepro, thanks. I have not looked at silicones very much yet but I'll check out what Dow offers. With silicone, do I need to be concerned with outgassing or corrosion of electrical conductors?

 

[Compositepro]

Corrosion wouldn't be an issue. You'll have to check with the vendor regarding out-gassing and compare to your requirements. Space hardware has some very stringent rquirements and would require a qualified product.

 

[GregLocock]

Raychem make a whole bunch of somewhat expensive potting compounds, with fantastic properties, such as sticking to PTFE.

You might also consider a polyurethane. They tend to be cheaper than epoxies. You can adjust the stiffness by adding microballoons.

I'd be very concerned about heat build-up since you seem to be keen to use gallons of the stuff.

Finally, down here in Australia we had a lot of help from our local CIBA-GEIGY rep, who was prepared to make up 1 litre batches of custom mixes to give us particular properties.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kontiki99]

I had to perform a reliability investigation on an auto throttle position sensor (an RVDT) a few years ago. It tested fine in the shop, but if it was returned to service it would fail again. There were just a few units that did this.

It was a no go item when it faulted and the value of the shop testing process was under scrutiny.

After two weeks of trying to make one fail I finally got lucky. A wire or solder joint somewhere in the potted assembly had cracked, but the but the soft flexible potting compound held the components in place to make the circuit when the vibrations stopped.

So, it would pass the screening tests on the bench and get placed back in service.

Back in service on the engine, vibration would momentarily shake the cracked joint, the computer would sense the momentary interruption and fault the sensor again.

I don’t know what the circumstances of your application is, just be aware.

We never picked through the potting to see exactly what was broken.

When we determined that they had in fact failed and it was not just a troubleshooting removal, they went back to the vendor for overhaul anyway.