Peter,

No need to apologize, I totally agree with your input. Keep things dry until
needed is the best way to stay clear of all moisture-related problems, for
boards as well as for moisture sensitive devices.

The only caveat is that things can get a little more complicated on a real
production line with any significant amount of changeovers. This is when the
proper management and control can turn into a logistical nightmare for a
normal human being. As you explained so well, moisture diffusion is a
complex phenomenon and moisture-sensitive components that have already been
exposed to ambient conditions are not necessarily protected by simply
placing them back in a dry environment for a while.

Automating the control of materials is only a practical approach to resolve
this issue and to make sure that you mimize the frequency and duration of
any bake cycle. The philosophy is "Make things as simple as possible, but
not more". I could also add "To get high yields and high quality, remove the
human factor from your process control".

Since there is a meeting of the joint IPC/JEDEC committee next week, I can
try to find out if anyone has been thinking about a standard for PWB
moisture control. I'll keep you posted.

Francois Monette
Cogiscan Inc.


Date:    Wed, 16 Jan 2002 12:43:55 +0800
From:    "<Peter George Duncan>" <[log in to unmask]>
Subject: Re: PWB Moisture

This isn't a very useful response, perhaps, but the question is an
interesting one. I offer some additional considerations/mind benders when
contemplating the answer to this:

1) Moisture absorbtion depends on the material(s) the board is made from,
and the overall amount of moisture absorbed will be related to the volume
of the materials used.
2) The material volume depends on the board design and is therefore a bit
variable.
3) Opportunity to absorb moisture depends on humidity, temperature and
time.
4) Tracking moisture exposure depends on having an army of arithmeticians
or alternatively a tracking system such as our friend Francois Monette has
to offer.

What we need here is some way of determining a Moisture Sensitivity
Category for the various materials and their combinations, as used by
component manufacturers and a means of determining the rate of moisture
take-up for any given design - quite a career for somebody.

On the other hand, you can simply pursuade your fab house to pack your
nice, new, dry boards in individual sealed bags with some desiccant that
you don't open until the moment you need them (or reseal properly after
in-coming inspection) on the line. If the boards have been lying around for
a year or so (in their sealed bags), and/or you're in any doubt about how
dry the boards are, you can then bake them out for about 2 hours at 90 deg
C just prior to use. This last has been common practice for many years,
although I object on principle to having to do this - one more thermal
excursion too many for the board.

Call me old-fashioned, but why look for ways to make life so complicated,
when a little management and control can save a lot of unnecessary effort
and cost? (Sorry, Francois!).

Peter Duncan

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