In a message dated 18/05/99  06:29:51AM, you write:

> 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

-----
Hi Ingemar

When one responds to an open forum it is necessary to be circumspect, and 
sometimes give answers which can only hint at what is meant. In a private 
communication it is possible to be a little more direct, so for oyu here is 
my more direct answer.

In a legal court we are obliged to tell the truth, the whole truth and 
nothing but the truth - and be prepared to support our story with evidence 
under cross examination. In data sheets this rigorous environment does not 
apply.

My experience of adhesive suppliers is that they are sometimes .... how shall 
we say ..... "optimistic" about their products. Some are very optimistic 
indeed bordering on the creative in fact. [I am sure that you have 
experienced this particularly on cure schedules and so on.]

So in my open response I suggested that there were different conditions of 
test one of which might  be less onerous to pass and so on. This was a very 
polite way of suggesting that people might like to check up on the actual 
test mathods Epotek used and maybe ask for some documentation  of the the 
test results and so on. 
If you did this I think you will find that what they are reporting is the 
result of a simple evaluation done by a customer where they found the number 
quoted. We are now looking at this number in isolation and putting it in our 
own minds into the context of the general tests methods for MIL specs and so 
on and as a result wondering how it can be true.

Be aware that I write as a former supplier for Epotek and Ablestik here in 
UK. You can be sure that I recognised the significance of the the claim and 
looked for corroboration immediately I saw it when representing Epotek, and 
later checked it out with Ablestik when I moved to the company selling them. 
My advise is not to believe it means that you can generally use H77 instead 
of metal or ceramic packaging/sealing and meet MIL spec test requirements. 
The test they did was no indicator for long term reliability, it simply 
produced the number quoted.

BUT But, having said all that, I also think that the whole MIL/Space etc  
requirements scenario  should be looked at with a certain amount of detatched 
cynicism. Specifications are after all a great marketing weapon, for 
promoting yourself or keeping the other people out, for creating a whole new 
industry for writing testing having meetings and revisions and .... so on. 
MAybe Epotek are right to calim that you can be hermetic with their product 
if by hermetic you mean it can function as well as a metal sealed part in 
real life. 

Here is an example of what I mean:

I did some work here with a company that make military spec computers, 
basically these were standard IBM PC types that had to work in the 
battlefield. So all the processors were in metal or ceramic packages and so 
on and the whole assembly was put in sealed dark green metal enclosures and 
so on. This was very expensive as you can imagine and when peace broke out 
they were told that the cost of their PCbased equipment had to go from 
something like £10K to £5K. So I was asked to see how to help them with some 
conformal coatings and special thermal management products and so on.
My advise  was like this:
1) You are already buying the most efficent way you can,
2) it might be posible to shave another few pounds off here and there.
3) It will not be possible to shave half the cost away. You need a 
fundamental change.
3) Why not go down to the local computer shop and buy a standard PC fpr £1K, 
and then look at spending £3K on it to make it conform to MIL requirements. 
THis will be much easier.
4) Keep the other £1k for a better profit.

So the point of the story is that by careful application of some UV cure 
material to the plastic components and by conformal coating the whole 
assembly it was possible to get the same reliability performacne for the 
whole assembly as when individual components were made from hermetically 
sealed metal/ceramic components. However the individually treated components 
were not "hermetic" when tested individually to the same standards as their 
ceramic/metal packed counterparts..


Best regards 

Mike

ps: the effective additional cost of the coatings and process costs was less 
than £50. 

pps: I prefer not to imagine all the money that has been spent on military 
hardware, my tax bill and ...

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