In a message dated 11/05/99  09:16:19AM, you write:

> To:   [log in to unmask]
>
>  Hellouu.
>  EPO-TEK H77 is hermetic. At least according to their databook:
>  1.0x10exp-8 cc/sec Helium. Excellent for sealing MCMs and other
>  packages, they say. Now, why do we laserweld, seamweld or solder metal
>  cans if you can just use H77? We are old byuers of EPO-TEK and we like
>  them, but this question has to be ventilated: can a "plastic" really be
>  that tight against Helium? Or worse...against Hydrogene? A teacher at
>  one of our universities (chemistry professor) said that 'an organic seal
>  can never,never and never be hermetic'. Time for revolution? If the MIL
>  people really accept e.g. H77 as a sealant, I think it's kind of smash!
>  We can creep out of the cave and drop the club!/
>  Ingemar Hernefjord
>  Ericsson Microwave Systems

Hermetic is one of those words which can mean what you want it to mean. As
used in top whack space or MIL spec connotations conventional wisodm is that
organic compounds can not be Hermetic with a capital H, because their
molecular density is too low and anyway there is always an interface which
acts as a conduit for moisture. H77 has been around a long time and its
almost as long since I looked at it and the claim in detail. So far as I
recall the xplanation for the claim for hermeticity is explained something
like this: there are/were several test procedures in MIL601 of differing
severity and the one which most people think of is the hardest, overlooking
the less severe ramps etc in other test methods under the spec. H77 meets a
less severe case. This is not to say that H77 is without merit, it all
depends on what you mean by Hermetic and what you actually need....

Mike Fenner

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