On the does the epoxy crack on it's own or does the inside swell and crack it question, I had emprical evidence many years ago. This was postthe first knownspate of RIFA failures I knew of which was in BBC B computer PSUs.? I bought a quantity of equipment and parts that had beed stoered in a garage for years. There were several carbboard boxes / "trays" of NOS RIFA self healing capacitors. Some had sealed plastic bags over the whole box, some didnt. None of sealed capacitors showed any sign of craking or swelling. The open ones were swoolen and cracked. I noted two other things,
Where the capactor "roll" was not centered in the moulding the cracking and swelling was assemetric being worse on the side with the thinnest expoy
On a box without a bag that was sandwiched between two boxed ones there was a clear progression of damage with the ones on the outside being very swollen and those in the middle undamaged.
From this I posit that the epoxy is not impermeable to moisture and the paper used is hygroscopic. In damp conditions moisture gets into the paper making it swell an cracking the epoxy letting more moisture in. This results in a accelerating failure.
The only inlet filter RIFA failures I've seen were in ex MOD Solartron 7150 DMM's This could be due to a bad batch or poor storage between withdrawal from service and eventual fail.
Robert G8RPI.
