Another thing that might be ocurring is that when the potting absorbs
moisture it swells.
Proving that you adhesion isn't as good as you think it is.
If you are using silicone based potting material, then you MUST
contact your supplier and obtain the proper primer media. It also has
to be applied right (dipped, not sprayed), then cured, before the
potting is applied. Also, if you BAKE your potted assemblies, they
will settle in at the temperature your expect them to experience in
the field, or a median temperature between the most common cold
temperature, and the most common hot temperature expected.
The PCB assemblies MUST be absolutely clean, as in hot alcohol bath.
If water solubles are used, the assemblies MUST be baked out, if not
placed under vacuum for a period of time to get ALL of the water out
of them. If there is ANY sulfur or sulfur gassings occurring (such as
with heat shrink tubing), it WILL inhibit silicone potting curing with
many types of silicone based potting compounds.
I stated that conformal coating wasn't required. You DO need to
prime the assemblies though. I would be willing to bet that is the
issue. A properly primed assembly makes it through thermal cycling up
to 70C in our lab and production environments, and some of our gear
has a quart of potting in them each.
I now remember the only NASA approved potting compound for HV space
applications. It will NEVER delaminate, even at space level thermal
cycles (near absolute zero through 200+C) It is "CONAP". It is
expensive. It is guaranteed not to fail, it will stop bullets, and
you cannot even stab a knife through it more than a quarter inch with
a hard strike. It has better adhesion without priming than ANY other
material known to man that is not a firm solid. It is flexible. It
is polyurethane.
http://www.ellsworth.com/display/productlisting.html?vendorID=135&Tab=Vendors
You should talk to Ellsworth about your problem. If you are close,
they will send a technical rep to you to help you fix your problem,
and they KNOW what takes place in such settings.
They could also likely get what you use for you cheaper! :-]
Good luck.