Wil,
 I don't know of any public domain documents covering baking of PCB's.
However, baking of moisture affected PCB's is something we have had to contend
with numerous times over the years. We only bake cards when we find we have a
problem with a particular card, such as outgassing. Various things contribute
to this problem. Raw card through hole plating quality (voids in the through
hole plating) can exacerbate the problem. Also if the cards are stored in a
humid environment they will, over time take up considerable moisture. If we
suspect a batch of cards we do a bake of the cards and record the weight loss.
We've found that anywhere over around 2% moisture will likely give problems in
assembly.
( We once had some cards from a vendor in the tropics that had something like
10% moisture, terrible troubles with blowholes, they weren't dry packed and it
was the monsoon season). It is worth noting that moisture is not the only
reason for blowholes. Outgassing can also occur if there is incomplete resin
cure of the raw card or trapped plating solutions.

Baking cards is only a stopgap solution. It needs to be driven back to the raw
card supplier to ensure that cards are supplied in a dry condition (vacuum
sealed packaging with dessicant). If you are storing cards for any length of
time you may need to review your own storage environment
(temperature/humidity).  Some suppliers pack PCB's with dessicant and humidity
indicator cards. This can be useful as an indicator.

Keep in mind too that moisture is not the only factor in length of card
storage. Preservation of surface solderability is also a concern,  the quality
or type of card surface finish (HASL, gold plating, OSP) is a significant
factor. I guess the reason why I haven't seen anyone come out with a definitive
figure for acceptable maximum storage time for PCB's before use is that there
are so many factors affecting it, and therefore a wide variance in the result
depending on these factors.

I hope this is of some value.
Regards Evan

Evan Jones
Manufacturing Engineer
Bluegum Technology
Ph 61 3 5720 2539       Fax 61 3 5720 2412
Email [log in to unmask]

---------------------- Forwarded by Evan Jones/Australia/IBM on 28/04/98 09:01
---------------------------


[log in to unmask] on 28/04/98 02:22:39
Please respond to [log in to unmask]
To: [log in to unmask]
cc:
Subject: [TN] PWB Moisture Baking


     Dear Technical Representative,

     Do you know of any specifications that outline the need to moisture
     bake multilayer (PTH,SMT,or mixed technology) PCB's?  I am trying to
     conduct a literature search on such requirement.  I have concluded, so
     far, that moisture baking boards prior to assembly was a preventive
     method used to minimize yield problems such as blow-holes on boards
     subjected to high humidity for several months.  Which leads to another
     question how is high humidity defined?  How long should a board be
     stored prior to use?


     Wil McCray
     AMETEK Aerospace
     607-763-4745

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