Carl
Despite Earl's elogious remarks (ta, much), I can hardly add anything to
what Peter has eloquently, sorry, elephantly said. Remember, ALL
synthetic materials are more or less like a sponge with very small pores
and epoxies are no exception. Furthermore, the pores are smallest only
under stoichiometric conditions, which produce the largest MW molecules.
How do you ensure stoichiometry? In my experience, the most critical
part are the connections, as this is the interface between the
electronics and the outside world. The best and most expensive way is a
hermetically sealed metal box with glass-to-metal seals (may be cheaper,
though, than having failures and returns). The worst way is the
elephant's trunk method, or a bundle of insulated cables coming out, as
these will form an effective pipe, straight to the electronics. In
between, you have some form of connector which, if not pre-treated to
ensure excellent adhesion of the epoxy, will soon leak salty wheel spray
straight into the animal's intestines. Remember, despite popular
thought, epoxy and some metals (including copper) are not really
compatible for adhesion (this is why the foil used for PCBs is very
rough and treated to promote adhesion). Rigid plastics are always
suspicious too, unless you can be sure there is no, no, no mould release
and they do not have a glossy surface. Flexible plastics are simply a
no-no. Finally, the epoxy for potting must be flexibilised and a good
electrical grade from a reliable manufacturer (mechanical grades have
reduced chloride elimination).
Please read Peter's message very carefully: everything he says is
important, especially his 2nd last paragraph, about qualification.
My 0.2 c worth.
Brian
Earl Moon wrote:
>
> Folks,
>
> Are there those of you out there doing automotive type packaging using
> various sealing techniques/processes ensuring high reliability and
> survivablity under hood? If so, I have an application needing similar
> packaging especially in areas of sealing "lids" on top of boxes containing
> electronic assemblies and parts.
>
> If you can share any information concerning this type box build and "lid"
> sealing, please advise and ask all the questions you wish.
>
> MoonMan
>
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