Dr Baber,

    Are you sure that it is flux residue or volatiles and not something else in

the package outgassing at 250 deg F that is causing the insulation resistance
problem? If it is the flux then there could be a number of ways round it. You
haven't given details of the product, it's use conditions or construction, so I

can only offer a couple of alternative sealing suggestions.

    If the container and it's contents can stand a temperature excursion of
around 330 deg C for a few minutes in an inert atmosphere then using a 80/20
gold tin solder will work well without solder on surfaces that have at least 1.5

micron of gold plate. This process is used extensively with ceramic packages to

give hermetic sealing. Nitrogen atmosphered belt furnaces, or inert atmosphere
seam sealers are typically used to carry it out. Indium, amongst others, make
solder preforms of this sort.

    Again depending on package construction there are some glass frits that give

good hermetic sealing at lower temperatures than were available in the past. If

processed with a bit of care these don't outgas enough to cause problems.

    Alternatively there are a couple of fluxless tin/lead soldering processes
that have been developed. Two I have read about are MCNC of North Carolina US
who have a process called PADS that converts the oxides on surfaces to a
solderable condition. There is also a UK company I have details on who were
developing a plasma oxide reduction and soldering system. I can give you
contacts for either of these and there may be others. The UK company had what
looked like a commercial machine a couple of years ago, but I've not heard much

since. I believe MCNC were collaborating with someone to build a commercial
version of their process. These would require lower temperatures than the gold/
tin process above and may be suitable for your sensor. Let me know if you want
contacts.

    In a previous job I did quite a bit of hermetic sealing to a variety of
package types. The materials of the package and contents, as well as prior
processing have a huge influence on their reliability. It is quite easy to
hermetically seal a package and make a less reliable device that it would have
been if it had been packaged non hermetically.


Hope this helps,
Jeremy Drake
Design To Distribution Ltd
Stoke on Trent, England

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Name:     Isaak Baber
Company:    Oceana Sensor Technologies
E-Mail:    FLG8RZ.aol.com

Comments:

 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.
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. 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.
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)
Do you have any experience dealing with soldering practices in enclosed spaces?

We would appreciate any suggestions or help that you may be able to supply us
with. You can contact me. Dr. Isaak Baber at 757-426-3678.


Thank you very much,
Dr. Isaak Baber



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