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I'm experiencing a strange failure of electronics after potting which I'm hoping to get some more insight and explanation to. The device is a custom, rugged USB drive in an Aluminium/Titanium body (see www.viud.net for pictures and more details). I'm potting the USB board (a variety of off-the-shelf boards) in Aremco 526N epoxy and curing in the oven. The USB drives are verified to work fine before potting but after potting I'm getting a high failure rate (17% Aluminum body, 78% Titanium body).

Possible explanations for the failures I've considered are:

  1. Heat - Since I'm curing the devices in the oven I thought that maybe they were getting too hot. The first batch was cured at 90C for 2 hours but after testing I found the oven does get considerably hotter than that. The next batch I did at 55F for 4 hours and confirmed that nothing gets hotter than 80C but the failure rate didn't change.
  2. Epoxy - The Aremco 526N is not technically advertised as a "potting" epoxy although its electrical properties are better or similar to other potting epoxies I've used with no issues.
  3. USB Drive - The failures occurred with 4 different USB boards so it is unlikely related to that.

None of these possibilities explain why the Titanium models have a much higher failure rate than the Aluminium. If it was heat related I would expect the Aluminium models to have a higher failure rate anyways (thermal conductivity of the Al is x36 greater than the Ti).

The only possible cause of the failure I have left is related to internal pressure of the epoxy while it is curing in the oven. The epoxy is essentially sealed completely into the body with only a very small escape through/around the USB connector. Since the epoxy around that area likely sets first the rest of the epoxy in the body is likely under some pressure due to thermal expansion. This also explains why the Titanium models fail more than the Aluminium: the Al conducts the heat faster through the body which means the epoxy sets more evenly reducing the internal pressure.

Unfortunately, I have no way of verifying this theory as to open the drives I have to destroy the USB. I also cannot believe there is enough internal pressure from the epoxy to actually damage the circuit board. I can try curing the epoxy at only room temperature but it then takes weeks to set and the epoxy is not as strong.

Any ideas, theories or comments on my issue are more than welcome.

MrPhooky
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uesp
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    As a WAG, how about too much pressure inside. The Titanium is stronger and doesn't give as much as the epoxy expands. I think sometimes people put a pliable coating over the electronics first. But I've never done any potting of electronics. – George Herold Sep 04 '14 at 14:10
  • I was thinking of trying a conformal coating of the board before potting but have never done it and know nothing about it really. Looking at the USB board the memory chip does have space under it which might let it crack if there is too much pressure on it from the outside. – uesp Sep 04 '14 at 14:17
  • I've only expoxied two drives over the years with the basic hardware store epoxy, but both failed before their time. So the theories above do seem correct. – grabbers Apr 06 '17 at 16:56

5 Answers5

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After a good deal of testing I believe the problem is actually a combination of three things:

  • Temperature
  • Sensitive USB flash drives
  • Air pockets

On the initial batch the temperature was definitely a little too high which may have contributed or exacerbated the issue.

I've used several different models of USB flash drives and one in particular seems extremely sensitive to temperature and shock.

However, it seems the root cause of the issue is large air pockets. If any significant portion of the circuit board is not covered by epoxy it seems to fail very easily. For example, one unit failed after just falling 1 meter to the floor and was confirmed to have a large air pocket at the top of the flash drive. Another unit test with no epoxy at all similarly failed after a low speed drop test.

I've changed my potting procedure to try and eliminate air pockets and ensure the entire flash drive is covered. This is a little tricky as the design is completely enclosed and there is no (easy) way of confirming how much or little air is left inside. But recent tests seem to confirm that careful assembly to minimize air pockets leads to a more reliable product with no premature failures.

uesp
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At an internship quite awhile ago, we would pot connectors with some kind of black two part potting compound. We would outgas the epoxy in a vacuum chamber, the vacuum doesn't have to very good. Maybe there is some trapped gas causing your problems.

ACD
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Mike F
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  • While I don't think there is trapped gas I'm willing to entertain any possibility. I don't have definitive proof (only a few samples) but it seems there is more failure rate when there is more epoxy in the case that leaks out during curing. – uesp Sep 04 '14 at 15:25
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At a 78% failure rate, I don't understand your hesitation to open a drive and see what's going on. For two-hour soaks, I wouldn't think that the thermal conductivity of the case has anything to do with things -- though maybe thermal expansion does, creating an interesting shear on your PCB. No way of knowing without getting in there. Can you xray through the case??

Scott Seidman
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  • I'd like to keep the cases as these were a pre-production run and they are expensive. I actually just managed to open one from the last failure while keeping most of the USB board intact. There were no signs of any of the ICBs cracking or other obvious damage. – uesp Sep 04 '14 at 15:21
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    @uesp -- If, as you describe in your blog, the epoxy expands as it cures, there's your answer right there. Dollars to donuts, it's causing shear of the circuit elements with respect to the PCB. I'd venture that expanding epoxies are not suitable potting agents – Scott Seidman Sep 04 '14 at 18:50
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According to the datasheet for that range of epoxies, the 526N you're using has among the highest cure shrinkage in the range.

0.01 in/in may not sound like much, but its more than enough to, at the very least, cause hairline fractures in joints on the PCB. I have experienced this exact phenomenon when potting accelerometer sensor PCBs into an aluminum enclosure.

It you're set on using that epoxy, you could give the PCBs a conformal coating of some sort before potting - but make sure its flexible. I used a thin layer of low viscosity RTV silicone (Dow Corning 3140) and my failure rate dropped from over 80% to well under 1%.

brhans
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Simplify your life and pot with syntactic foam EFF15 from Henkel. It is a powder that expands greatly and gets everywhere then has low CTE so doesn’t stress items. If you want to stay with the 526N you must vacuum de aerate after mixing then again after filling the part or at least gently vibrate the assembly to get better wetting. Another trick is to use a brush to scrub on a layer prior filling. Despite this being a low CTE media at just 18 for F or 33 for C, it is setting up at cure temp then shrinking differently that the housing or boards and components as it cools. This stuff is pretty hard at shore D 89 so it’s going to push HARD in a CTE competition. The aluminum shrinkage and compressive stress might be helping somehow to reduce the destructive stresses or it’s as you say that the cure is different due the conductivity. But the EFF 15 avoids most of that by being very weak as a structure after cure so peak forces are always low, tremendous gentle growth upon heating, gets around all corners, and very low cured CTE so better matches your component and the board.

Richard Kudrna
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