Hi Mike,
in response to your mail, I still am wondering how EPO-TEK can insist
that helium leakage rate of 1x10exp-8. Such low rates use to be obtained
in metal housings only. If, I say, if the rate is such low, then you
meet the military specs, and accordingly, you can seal with H77. It's a
serious temptation, isn't it? /Ingemar Hernefjord


----------------------------old messages------------------------------
> In a message dated 11/05/99  09:16:19AM, you write:
>
> > To:   <a href="/scripts/wa.exe?LOGON=A3%3Dind9905%26L%3DTECHNET%26E%3D7bit%26P%3D2065522%26B%3D--%26T%3Dtext%252Fplain%3B%2520charset%3DUS-ASCII%3B%2520name%3D%2522Re%3ARe%3A%2522%26N%3DRe%3ARe%3A" target="_parent" >[log in to unmask]</a>
> >
> >  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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