Just one possible clue for you. Plastics hold on to moisture quite well   
below Tg. Only above or plausibly close to Tg is moisture free to move   
about or even out. 100 C is the boiling point of liquid water. Absorbed   
water is not in liquid form. 100C is irrelevent.
David Heywood.

 -----Original Message-----
From: [log in to unmask]
Sent: 11 July 1998 00:43
To: [log in to unmask]; Dave Heywood
Subject: [TN] Delamination Problems

 << File: ENVELOPE.TXT >>
 --------------------------------------------------------------------------  
 --
We have been having a reoccurring problem with PCB delamination.  At
first we were thinking that it was a moisture problem. The boards were
being stored in our store room open to the ambient air. Once, after
two boards in a run delaminated, the assembly house flagged the entire
run, and I actually personally took the rest of the boards home to my
electric oven to bake. I then re-packed them (warm) in some ammo cans
that I have with a desiccant pack in each. None of the baked boards
delaminated. We thought that we would solve our problems if we
specified for the assembly house to bake the boards and handle them
post-bake per IPC-PE-740, (as per a similar discussion here that I
found in the archives). All this was with boards from two PCB
manufacturers (call them PCHouseA and PCHouseB).

Well just recently we got some boards in from a third manufacturer
(call them PCHouseC) that we have never used before. (The board is for
a product line that we bought from another company so we are still
using their resources for purchasing.) We received the boards from
them, opened them enough to make a quick visual inspection and
immediately sent the boards to our assembly house. Unfortunately, due
to communications lags, the board baking didn't happen for that run
and we had delamination problems with those boards. Our QC inspector,
(thinking that moisture may not be the problem because of the lack of
storage time in our stock-room), talked to PCHouseC about this and
their thoughts were about wave solder temperature and FR-4 temperature
maximums.

My questions are:
 1. About how long does it take for FR-4 (0.062" thick) boards to
    absorb moisture? (Any one got (or can point me towards) graphs for
    this? If so, please use private E-Mail to MIME attach the
    message.)

 2. How would approaching (meeting, or exceeding) the Tg affect the
    board at the solder wave in such a way as to produce delamination?
    Is there another critical temperature that I should be aware of
    here?

 3. What wave solder temperature profile is considered 'standard' or
    'average'?


Many apologizes if any of the above questions are too vague for a
concise answer, but I'm learning this stuff as I go... (definite
trialby-fire here...)

TIA for any help.


Pax
 --
     Member: Team AMIGA                       --} WatchDog
Fingerprint: 2C 8A 03 3C D6 D3 32 7F         (Chris Elliott)
             66 0F 9B 9F 03 77 1D 85      PGP Key ID: A6A79259
<tsb>
Experience is directly proportional to the value of equipment destroyed.
 -- Carolyn Scheppner

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