[time-nuts] Helium leaks

[Brooke Clarke]

Hi Jim:

When I was working in Semiconductors we used a fluorocarbon liquid in a heated fish tank first.  If bubbles come out you 
have a "gross" leak.  After that we put a whole bunch of parts in a 5 gallon paint pot and pressurized it with He and 
let them soak.  Then a small number of parts were put into the Veeco He leak tester and it was pumped down to a vacuum 
and we looked for leaks.

The problems are:
1. The first fluorocarbon test is very important since a gross (very large hole) will pass the He leak test.  Think of a 
small can with no lid at all.
2. A leak may be small enough to pass the fluorocarbon test yet be big enough so that by the time it gets put into the 
leak tester and the pump has had time to pull a vacuum, it's been emptied.  i.e. it passes both tests but is really a 
very leaky part.

If your CSAC parts were leak tested I expect they used the above procedure, but with some added details such as the 
exact fluorocarbon, it's temperature, time observed for bubbles, pressure in the paint pot, soak time, maximum time 
between depressurizing the paint pot and active He leak detection is working.  You might ask for the leak test procedure 
they used.

AFAICR all of the parts I worked with were sealed in the clean room atmosphere.  I don't remember sealing under Nitrogen 
or some other atmosphere.  So the pressure after sealing will be near the clear room pressure at the time of seal.  
Maybe more different if the sealing process heats the part.

It would be interesting to try testing a CASC in a pressurized He environment.  But the result for a hermetic unit would 
be no effect. For a "Gross" leak or one of the above very leaky parts you would see something if this He problem is 
real.  To check that you might be able to get a leak test reject that's leaky.

-- 
Have Fun,

Brooke Clarke
https://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
axioms:
1. The extent to which you can fix or improve something will be limited by how well you understand how it works.
2. Everybody, with no exceptions, holds false beliefs.

-------- Original Message --------
> There was discussion last October about the very tiny MEMS oscillators failing from He leaking in.  By now, there's 
> several videos online showing folks taking that MEMS oscillator and putting it in a chamber and demonstrating the 
> failure.
>
> So here's something interesting - There are a remarkable number of places with high atmospheric helium concentrations. 
> And it causes a variety of mystery failures.
>
> The one of interest here is that helium can leak into a physics package with Rb or Cs, and that changes a) the total 
> pressure (which causes line broadening, apparently) and b) the thermal conductivity of the vacuum.
>
> The latter is of some concern to me, since I have some CSAC devices in space that may have been exposed to a helium 
> rich environment during storage. The failure mode on a CSAC is that more heater power is needed to keep the vapor 
> pressure up, due to increased heat loss through the "not as good a vacuum" in the physics package.
>
> Has anyone on the list ever had a CSAC in a He-rich environment? Noticed any performance differences?
>
> I've asked Microsemi - their first response was that it's not something they tested for and that the leak rate is very 
> low.  I suppose it's probably in a hermetic can (which are *tested* with Helium leak tests)
>
> Jim
>
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