Potting Thumb Drives

I've potted a few thumb drives now, so I thought I'd update this post with some general advice.

Potted thumb drive.
( See: http://en.wikipedia.org/wiki/Potting_(electronics) )

First, how I got started coating bare thumb drives with epoxy: I had a thumb drive with an annoyingly large plastic case, so large that I couldn't have anything plugged into the usb port next to it on my laptop -- I'd have to unplug my mouse to use the thumb drive.  So I pried off the outer case to make it smaller.  There was an inner case, but after close examination I realized it was just hanging off the circuit board, not providing any strain relief -- every time I pulled this thing out of a computer, I was pulling it out by the board.

In electronics it's common to coat sensitive parts in epoxy so I thought why not just coat the whole thing?  I took off the inner case, painted the board in epoxy, and left it hanging to dry (thus the drop).  The first coat was 5 minute epoxy, and it didn't cure very evenly, so I added a second coat of "2 ton" epoxy, and finally a third blob to add support and structure behind the connector. I gave each coat 24 hrs to cure, and then left the whole thing over the weekend, and it felt indestructible. And wasn't taking up any more space than it needed to. The one drawback: the LED (bottom right in the photo above) was really freaking bright -- it was designed to shine through a layer of thick plastic, now it had nothing but clear epoxy above it.  Sitting at my desk at work, it was pointed right at my face.  But I'll get to that . . .
 

After I'd done this I figured I could hardly be the first one it had occurred to so I googled it. Sure enough.  Someone else who had done this pointed out that his potted thumb drives were now water proof, and better yet washing machine proof.

Not long after I made this first one I lost it -- it's probably still plugged into the back of some computer somewhere . . . but it was a small old thumb drive without much storage space.  I got some newer larger drives and eventually tore off the cases and coated them in epoxy as well.  Some general advice from my experiences so far:

• Don't use a thin epoxy (like casting epoxy) as the first coat unless you always keep the thumb drive vertical.  I had one at a 15 degree angle to horizontal, poured the epoxy over it, and the thin epoxy flowed up into the connector.  I didn't realize it had happened until I found the thumb drive glued to the magnet holding it the next morning.
• Don't use 5 minute epoxy -- both times I've tried, it hasn't fully cured in spots, I think it's just not possible to mix it well enough in the time you have before it gets too thick.
• The slow curing but still thick "2 Ton" epoxies seem to work best for the first coat at least.  Mix them well, and then pour into a second container and mix some more.  Use a thin epoxy for thinner later coats if you like -- thin casting epoxies tend to have fewer bubbles but can make for thin coats and long drops.
• The LEDs are awfully bright (in at least one direction) with no plastic over them, and even drives in solid black/metal/opaque cases often still have an LED on the board.  After the first coat, put a drop of something like white paint above the LED.  That will scatter the light from the LED, both making it much less bright if it's pointed at you, and it scatters the light within the epoxy.  I've had best luck with white epoxy, I can get a small drop about the same size as the LED.  I then put additional coats of epoxy over that once it's cured.

Potted drive #2 with small white dot above the LED.

I use magnets to hold onto the metal end, and hold it either entirely vertically for dipping into thin epoxy or somewhere between 15-45 degrees from horizontal for painting on thicker epoxy.  I can then turn the magnet to turn the drive over for coating the opposite side.  And if you let it cure in a near-horizontal position (but still tilted enough to keep any epoxy from flowing into the connector) the drops will form on the top/bottom instead of the end, giving you something to grab onto a little better for removing the drive from computers.  Although with some (thin) epoxies this can go a little too far -- that one is still at work, I'll add a photo of it later.

Magnets holding thumb drive during potting and curing.

Finally, if you're in a hurry you can speed the final stages of curing with heat.  I did this to finish coats that had already cured for 24 hours just to make sure they were done -- I'd put the drive in our oven (with magnets) and set it to 170 degF (lowest temp) and 2 minutes.  In 2 minutes the oven would get up to perhaps 130 degF and stay above 100 for over an hour -- two or three hours of that and a cooldown makes the epoxy good and hard (or in the case of 5 minute epoxy, convinces you it'll never cure).  Two cautions: if the epoxy isn't pretty much fully cured already, it may run and leave epoxy in your oven.  (If you were lucky it will have been a while since you cleaned your oven and there will have been just enough grease in the bottom to keep it from sticking.  Not that I know anything about this...)  Second, remember that it's in there!  One morning I was doing this, and by evening we forgot it was in there.  We started preheating the oven to 375 degF.  It was somewhere in the 200-300 range when I smelled smoke and remembered, and the thumb drive was literally smoking when I pulled it out.  But amazingly it works fine.