Hi,
I did some experiments with simple clear plastic bags and desiccant
pouches. You easily get several weeks of storage ay < 5% RH as the desiccant
does its job. The nice moisture bags have fairly heavy layers of aluminum
that render them non transparent. Kind of nice at time being able to see
what is inside a bag.
So for temporary holding the clear bags are nice.
Also the low cost desiccant pouches I played with (clay I believe) bake out
quickly at 100C and are reusable. Very Green.
Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, July 01, 2013 11:25 AM
To: [log in to unmask]
Subject: Re: [TN] Moisture in PCB
Thanks, Ian. Good post.
One more tip:
If you do not have a drybox, you can dry-pak the baked PWBs after baking,
but this is not a preferred method. Place the baked PWBs inside of a
moisture-barrier bag and seal them with desiccant pouch and moisture
indicator card inside. Keep the desiccant pouch from coming in direct
contact with the PWBs (wrap desiccant in layer of cheesecloth). Or purge the
MBB with nitrogen and seal. I would not depend on this method to keep the
PWBs dry for more than 10 days, tops.
-----Original Message-----
From: Fox, Ian [mailto:[log in to unmask]]
Sent: Monday, July 01, 2013 10:15 AM
To: TechNet E-Mail Forum; Stadem, Richard D.
Subject: RE: Moisture in PCB
Selim, as Richard points out, the board surface layers will absorb moisture
fairly quickly if stored in air following bake-out. From your description,
it would appear to be the surface layer in contact with the wave exhibiting
the delamination (as would be expected).
All PCBs are not created equal. Lamination integrity is key to providing
delamination resistance during soldering and a solder float test at 288degC
will answer that question. Materials absorb moisture at different rates,
polyimide tends to be worse from a moisture absorption perspective than FR4
for example. You also don't say what the wave solder alloy and processing
temperature is. Assuming it is a Pb-free process (and I'm assuming the
solder bath temp will be above that typical of a SnPb process, then you
really can't take liberties with bake-out and post bake storage of the PCB.
Moisture loss measurements I've carried out show that at least 4 hours at at
least 105degC is required to reach a stable PCB weight i.e. as much moisture
as is capable of being baked out has been. If you do this and desiccate
properly then provided the board integrity is good, delamination during wave
soldering shouldn't occur
Regards
Ian Fox
Lead Materials and Process Eng
Aero engine Controls
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: 01 July 2013 15:49
To: [log in to unmask]
Subject: Re: [TN] Moisture in PCB
Some PWBs can re-absorb full moisture content within a few hours after
removal from the bake oven. This is dependent on the type of board, the
design, and the relative humidity. Circuit boards actually absorb moisture
from humidity in air much faster than they do from direct immersion in
water.
As a general rule, after baking the boards move them to a drybox
(desiccator) if they will not be processed through reflow within 8 hours. If
you run batches of the same PWB at a time, and you do not have a drybox,
then leave the boards in the oven without any heat applied after the bake is
completed. This will help keep them dry a little longer, but only for a day
or so at the most.
Refer to IPC-1601 "Printed Circuit Board Storage and Handling Guidelines".
http://www.dr-storage.com/index.asp?lang=2
http://www.seikausa.com/mcdry
http://www.terrauniversal.com/desiccators-dry-boxes/desiccator-storage.php
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Avni Selim Özçukurlu
Sent: Monday, July 01, 2013 6:36 AM
To: [log in to unmask]
Subject: [TN] Moisture in PCB
Hello All,
After a wave solder process of a PCB with lots of through-hole components on
it, I have seen that there are popcorn part near via on that PCB. The
flatness of the first layer of the PCB is gone after that process. I think
that moisture remaind inside PCB gets outsite during the wave solder process
and cause this problem. I didn't put the PCB in a vacuum oven before wave
solder. Is it possible that the problem is related of this. Is there any
alternative solution about that problem?
Actually I applied the heat up and vacuum to empty PCB before all process of
manufacturing. After that vacuum oven process, it has passed about 10 days
to start the wave solder process. During that 10 days, did PCB get moisture
inside?
Thank you advance.
Selim.
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