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
MAURY HARRIS
<MAURY_HARRIS To: [log in to unmask]
@EFTC.COM> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
Sent by: Aero/ST Group)
TechNet Subject: [TN] PWB Moisture
<[log in to unmask]
ORG>
01/16/02
04:05 AM
Please
respond to
"TechNet
E-Mail
Forum.";
Please
respond to
MAURY HARRIS
Anyone,
What is the max absorbed moisture a PWB may have prior to reflow soldering?
Also, what IPC or Mil-Spec addresses PWB moisture?
Thank you,
Maury Harris
EFTC-SCO Engineering
602 282-5179
[log in to unmask]
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