Hi Chris I 'll do the best I can with the information I have.
(on goes my flame proof suit again)
1. FR-4 0.062" thick boards absorbs between 0.08 to 0.1%Wt of moisture
within a 24 hour period, and 0.2 to 0.23%Wt in 2880 hours (120 days).
These measurements were carried out under environmental conditions of
20deg C (max. variation 1degC) and 50%RH (max. variation 3%),
using specimen sizes of 40mm x40mm, and FR-4 with a 43% resin content.
(Some PCB materials absorb up to about 6 times as much under similar
conditions)
2. Assuming you are using standard FR- 4 and not high Tg stuff the Tg would
be around 130- 140deg C (TMA). At Tg many properties change abruptly,
Young's modulus, shear modulus, specific heat. etc. When heated above the
Tg the boards begin to lose mechanical strength due to softening of the
resin and often exhibit large discontinuous changes in their out-of- plane
CTE. It is these changes which sometimes cause delaminations between the
resin
and glass fibres in the board, or more commonly between the copper traces
and the resin.
3. I think the max. wave soldering temperature in somewhere in the region
of 250 deg C. I am not an expert on this but I don't think the actual board
temperature exceeds 220deg C. I have been reliably informed that the board
temperature exceeds 183 deg C for no more than 90 seconds and is over 140
deg C for about 2.5 minutes.
I have in the past measured delamination time for FR-4 at 260 deg C to be
between 12 and 15minutes. So although the soldering temperatures exceed the
Tg Theoretically it should not be a problem. Factors such as storage
conditions prior to wave soldering could be critical.
General
Below Tg organic substrates are capable of absorbing moisture. although
this moisture is static it can still effectively reduce the Tg and
consequently the maximum working temperature of the substrate. The rate is
highest in the first 24 hrs. (I have not carried out tests over periods
less than this)
Baking at 105deg c for about 2 hours should get rid of this moisture.
The moisture absorption capability is influenced by the degree of cure of
the resin, the resin content, and storage conditions of temperature and
humidity.
Depending on the type of delamination there could also be other causes such
as poor interlaminar adhesion or poor Cu adhesion.
Ramp rate of solder profile.
Additional information can be found in "Circuit world Vol. 12 No 4, 1986 <
Blowholing in PTH solder fillets part 3 moisture and the PCB> By M P Seah
et al.
I hope this helps.
F.K.Frimpong.
PS. Do you also get delamination during the plating process?
Is the delamination characterized by
lifting pads,
measling under copper layers,
glass fiber separation from resin etc.
Chris Elliott <[log in to unmask]> on 11/07/98 00:33:05
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to Chris Elliott <[log in to unmask]>
To: [log in to unmask]
cc: (bcc: Frank Frimpong/HQ/CUK/Celestica)
Subject: [TN] Delamination Problems
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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