Just for your information, one of our board suppliers recommends baking for
2 hours for every month of storage (up to 24 hours) at a temperature above
boiling. They even suggest a vacuum bake, if available!
Jim Marsico
Senior Engineer
Production Engineering
EDO Electronics Systems Group
[log in to unmask] <mailto:[log in to unmask]>
631-595-5879
-----Original Message-----
From: <Peter George Duncan> [SMTP:[log in to unmask]]
Sent: Tuesday, January 15, 2002 11:44 PM
To: [log in to unmask]
Subject: Re: [TN] 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
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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