HV tracking across flexible PCB strips

[Jules Richardson]

On 05/11/2015 12:02 PM, Tapley, Mark wrote:
> Surface contamination in a
> geometry like that is a pretty good suspect. Any hydrocarbons
> (coughnicotinecough) that deposited on the plastic between the traces
> could lower the effective resistance a long way; worse, once the first
> arc takes place, it’s not unlikely it burns a carbon filament across the
> contaminant which is even lower resistance. That might be a reasonably
> hard thing to clean off and could be narrow enough it’s hard to spot.

Aha, well it turns out that it *was* a sandwich - traces between two layers 
of plastic. It just didn't look like it the other day when I had the tube 
out. :/

In the spot where I estimate the arcing to have been taking place there was 
a definite failure of the glue which held the sandwich together, so there 
was certainly the potential for contaminants to have got in there. I've 
completely separated the layers (the glue was that ineffective after 40+ 
years) and de-gunked everything.

Question is, once it's all thoroughly dry, what would be the most 
appropriate way of re-sealing everything? Would silicone sealant perhaps do 
the job (dispensing with the top layer of the sandwich entirely and just 
coating the bottom layer/traces) and stand up to the HV OK?  Or some form 
of glue?

cheers

Jules