Hi Sam:

It's late, I'm tired, and I just got home from work.  Please forgive me if I ramble.

My first job out of school was in the food industry.  That company manufactured powdered drink
mixes.  The sugar in the powdered mix is quite hydroscopic (if the dew point in the packaging
room exceeded 52F, the sugar turned to rocks).  During my employment there I learned a few things
about the ability of packaging to resist moisture:

1. Polymer films slow the rate of moisture transmission into a dry material but do not stop it.
2. Sophisticated multilayer films transmit moisture slower than polymers, but do not stop it.  An
example of a powdered lemonade pouch at that time was: Polyethylene, aluminum foil, Surlyn,
mylar, kraft paper, ink, and lacer (and you thought pwb's were complicated).
3. Glass or rigid metal with good seals will resist moisture transmission.

As you might guess, my opinion is that a polymer film will slow the absorption of moisture, but
definitely not stop it.

For the other part of your question, my recollection of the reason for avoiding sealed polymer
containers is a concern that heat sealing will result in decomposition of the polymer.  In the
case of polyethylene this could result in short chain polyethylene and ethylene molecules with
free radicals which might attack the finish on the boards.  I am not aware of any research to
support this concern.

I understand that some pwb manufacturers use a blister package (polymer blister and cardboard
back), but I do not have knowledge of their experiences.  Given that FR-4 absorbs something on
the order of 0.4% moisture (if my memory is working - it might be 0.04%) and that many printed
wiring assemblies have been successfully built without resorting to sophisticated packaging, I
question the benefits of a vacuum package.  My experience at the assembly level has been that a
properly operated soldering operation (wave IR or convection) using quality boards (decent
quality drilling with 0.9 mils or more plating in the holes) will produce solder joints that
consistently meet IPC standards.

The hermetic package sounds nice, but it has a limited shelf life (polymer type, thickness, etc.
affect it).  The addition of a desiccant pack will increase the shelf life.  An experiment to
evaluate the improvement in solder quality usinng a vacuum package would certainly be
interesting.

Hope I didn't wander too far off the path.

Don Vischulis

sam mccorkel wrote:

>         We have a customer that wants us to begin using vaccuum sealed
> wrapping to ship their boards in. They want to reduce moisture absorbtion in
> the board.
>
>         It's been awhile, but I remember reading that this is a "No-No". I
> don't remember WHY but I think it had to do with trapped moisture.
>
>         Could anyone let me know if this is or isn't recommended and also why?
>
>         Thanks for your input!
>

snip the rest

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