If the epoxy coating is not dissolved by acetone, it’s still possible to remove it but it requires more patience. I used a combination of a hot air gun to soften the epoxy coating and a precision knife to scrape, piece by piece, the coating. The trick here is to find the sweet spot for the temperature:

Too low and it will have close to no effect and it will take you ages to remove the coating.

Too high and the solder will start melting, potentially making short circuits below the coating, the compound used on the packages of the components will also be softer and you may accidentally scrape it, revealing the internal die and breaking the bonding wires.

Empirically I found 285 degrees Celsius to be optimal for the keys I had to deal with: it softens the resin enough to allow you to scrape it while keeping the solder joints solid (lead-free solder has a higher melting point) and without softening the resin protecting the chips. Then patience is the key: scrape the epoxy gently without using strength or you will damage something. To hold the PCB while handling the hot air gun in one hand the precision knife on the other hand, I used my reliable PCBite. They are made of metal so they won’t be damaged in the process and they will hold the key firmly enough. At the end of the process I either let them cool down before removing the PCB or I used gloves to avoid burning my fingers.

Another trap on this process is to heat the USB key in a non uniform way. I found it particularly difficult on the models with a PIN pad because the sticker with the tactile domes creates some tension on the PCB and the first key I tried to clean was broken at the end because the PCB bent and some internal copper traces broke.

As a workaround, I paid extra attention to heat progressively the PCB and from time to time reheat the whole PCB and not only the spot I wanted to work on.

Desoldering the memory chip

Once the coating has been removed or if there was no coating at all, you may want to remove the memory chip, should it be a BGA or a TSOP. If the key was dipped into epoxy, it will require extra care because some of the compound will sit between the chip and the PCB, acting like glue. Lifting the chip with strength will damage either the PCB or the chip if not both.

Desoldering a chip always starts the same way:

If the PCB has components on both sides, secure the parts you don’t want to remove by using Kapton® tape Put some soldering flux on the component you want to desolder; it will help the solder joints to melt quicker and it will also soften the leftovers of the epoxy Preheat the PCB with the hot air gun in order to avoid bending it Heat the component and the surrounding area (keep moving the nozzle to avoid damaging anything with too much heat) until you see the solder melting If there was some epoxy, use the finest blade you can have on a precision knife and slide it very gently between the PCB and the component. This is very tricky to do because you don’t want to damage the pads of a BGA nor the copper traces of the PCB with the tip of the blade. Poke gently from time to time with a pair of tweezers to see if the component moves and lift it with the tweezers as soon as the component seems to move freely

As you can see in the following picture, even if I was super careful, many pads (fortunately, most of them were not used) were torn away from the PCB and you can see the epoxy that went down the BGA chip, gluing it to the PCB.