Interesting failure mode after potting

[uwezi]

Epoxy resins have dimensional stability, a high degree och chemical resistance, and good mechanical properties, so that they have gained wide acceptance in a variety of applications. However, various problems can arise from the volume shrinkage that often occurs during the polymerization and curing processes. ... This shrinkage creates internal stress in the resin, and may cause failure by cracking or delamination.

(Leo-Wang Chen et al., Journal of Polymer Research, Vol. 4, No. 2, pages 65-72, 1997)

[mikeselectricstuff]

It is a soft compound - feels comparable to thick polythene tubing. You can cut through it with a scalpel (though applying a soldering iron to the blade speeds things up - it hardens and crumbles to granules at higher temps.

I've now reworked 17 failed strips and am absolutely certain that ESD is the only plausible explanation.
All failed strips had either one chip , or a pair of chips which shared a single connection fail, and the failures were all low-resistances to ground (5 to 100R) on a single pin.

In every case, the dead pin was either connected to a trailing wire, on the end of the PCB where there is a pad with minimal coating, or adjacent to a large bubble hole at the edge of the strip - all prime candiates for the easiest discharge path to the sharp edge of the metal mould about 2mm away

My best guess is that the strip is peeled out of the mould starting at one end, which builds up a charge, and discharge occurs towards the other end  of the PCB, which is still in the mould.

The failures at the locations of the edge bubbles is the "smoking gun" as this is  the easiest discharge path.
 

[mikeselectricstuff]

In my experience with potting products you are describing stress cracking. You may want to look into a softer encapsulant. there are specifically designed epoxy and polyurethane blends for electronics potting that give a little to allow the solder joints to remain intact. This situation is compounded if you have gold plated pads as well. We use a product from Aero Marine. Dow and loctite (Henkel) have a wide range of products that work quite well, some are even what I would describe as rubbery in texture for strain relief of your exiting wires. They also have a humiseal property for removing residual moisture from the components. In 25 years of using epoxy potting in sensitive electronics I am unaware of ever loosing a device to ESD from the potting process.

Did you ever do 1 metre long, fairly flexible strips (0.8mm PCB) though? The "long peel" to extract would be different than most typical operations. 

[mos6502]

I agree that the peeling probably creates an electrostatic charge. Like when you peel a long piece of tape off of a roll (no, I'm not talking about the triboluminescence effect mentioned earlier) you'll notice it becomes electrostatically charged and gets attracted to the thing you least need the tape to stick to.

In fact, I think this is a universal effect that occurs when large sheets of nonconductive materials are separated. Any small charge imbalance at the beginning gets amplified the more you separate the sheets.

[SeanB]

You considered adding moisture to the area during depotting to reduce charge build up? Easy enough to do so with just a swamp box air cooler to add the moisture. Likely it is a static build up, and a humid atmosphere, or simply spraying the moulds before removal with a mist spray of water and working damp will help.

[mikeselectricstuff]

You considered adding moisture to the area during depotting to reduce charge build up? Easy enough to do so with just a swamp box air cooler to add the moisture. Likely it is a static build up, and a humid atmosphere, or simply spraying the moulds before removal with a mist spray of water and working damp will help.

..or just connecting a ground wire when removing....

high humidity when curing is probably a bad idea - not sure about Epoxy but PU really doesn't like water.
And as the charge is created by seperating an air-free joint, on something surrounded by plasic, atmosphere may not make much difference

[SeanB]

Water when cured and out of oven cold. Resin and water during curing means a mess.