In a message dated 97-09-08 00:53:58 EDT, you write:

>  We are currently soldering a cap to a cylinder and are creating a closed
> space. We have tried several solder pastes and wires (43Pb 43Sn 14Bi) with
a
> no-clean flux included. These solders came from a variety of companies
> including Alpha Metals, Indium Corp., AIM, and ESP.

Isaak,
Is it necessary to solder to make the hermetic connection?  Could you use an
adhesive of some sort?  What is the end use environment?

>  We found that when the no-clean flux is trapped in a closed space the
> surface insulation resistance at a temperature of 250 deg F drops by
several
> orders of magnitude. We believe this is due to volitile organic content
> contained in the flux.

It may be the VOCs in the flux, but I would suspect it is a combination of
flux residues, such as surfactants, high boiling point solvents, etc.  We
have seen a number of cases on PWAs where no-clean flux under large
footprint, low standoff devices does not reach the minimum activation
temperature (due to thermal shadowing).  Unreacted flux is often electrically
conductive (which can drastically drop SIR) and can be corrosive.  Most
no-clean fluxes are benign, ASSUMING they have reached full activation
temperature and the reaction byproducts evaporate from the board during
preheat and soldering.  Because you are violating this last assumption, by
not allowing the vapors to escape, all bets are off.  I would expect the same
problem regardless of the flux used.

Another responder indicated that you should also consider that there is
something coming out of the cap at 250F which could be affecting the SIR.  I
concur.  You might do some baking studies, taking sensitive weight
measurements of caps before and after various 250F bakes, and see if the caps
are loosing weight.  If they are, you have a material which outgasses, which
could be contributing to the SIR drop.

> We have found that by placing a small hole in the wall
> of our closed space (which acts like a vent) prevents the surface
insulation
> resistance from dropping and we maintain the same insulation resistance
that
> we have at room temp. However, making a hole is not a realistic solution
for
> us, since the device we are making must be hermetically sealed.

Since you have an entrapped air space, and if you determine that the caps do
not outgas, then you need to eliminate volatile materials or unreacted
materials from the equation.  This might mean fluxless soldering and using
some other technology to prepare your surfaces to maintain good
solderability.  ROSA technology, pioneered by Morgan Tench and Dave Hillman,
Rockwell, might be a good application here.  Alternatives might be another
solderable surface on the cap and the cylinder.  You might consider adding a
layer of plated solder on the surface and reflowing under nitrogen.  A
non-metalic seal may be another solution, but some adhesives have detrimental
reaction products, just as fluxes can.

>  Do you know of any type of no-clean flux that can maintain high insulation

> resistance at high temperatures in an enclosed space. (250 deg F)

None that I know of, but I have never tested no-clean fluxes at 250F.  You
might talk with Dave Torp at Kester (847-699-5593) or Dr. Karen Tellefsen at
Alpha Metals (201-434-6778) who might have knowledge of such high temperature
tests.  You might also contact Darrell Drake at Sperry Sun (713-987-4365) who
has worked alot with solders and fluxes for very high temperature
applications.

I hope this has been of some help.

Doug Pauls
CSL