What can be expected if the desiccant pouch comes in direct contact with the PWB surface inside a sealed moisture barrier bag?
Victor,
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, July 01, 2013 10:25 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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