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July 2013

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From:
"David D. Hillman" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Mon, 1 Jul 2013 13:02:30 -0500
Content-Type:
text/plain
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Hi team - Ya, save thing for Rockwell Collins - amazing how physics can be 
so reproducible : - )  .

Dave Hillman
Rockwell Collins
[log in to unmask]



From:   "Fox, Ian" <[log in to unmask]>
To:     <[log in to unmask]>
Date:   07/01/2013 10:54 AM
Subject:        Re: [TN] Moisture in PCB
Sent by:        TechNet <[log in to unmask]>



Hi Ed, that pretty much duplicates what we found with regard to loss and 
take-up of moisture.

Regards
Ian

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ed Popielarski
Sent: 01 July 2013 16:46
To: [log in to unmask]
Subject: Re: [TN] Moisture in PCB

At a previous employer, we had a terrible problem with blowholes at wave. 
Baking the boards @ 125C for 4 hrs did relieve the symptom, but we still 
had problems with it popping up from time to time inexplicably.

I did a study (but do not have a copy) on moisture content by weighing the 
assembly (many of them) before baking with an enclosed precision lab 
balance (+/- .001g), then after bake (measured hour by hour) until 
flatline (occurred around 3 hours), then resting at ambient (25 C +/- 3, 
60% rH +0/-10) measure hour by hour until returning to original.

What I learned with this exercise was they returned back to very near the 
original weight (within 5%) in only 4 hours! The histogram plot was 
somewhat parabolic for the first 3 hours, so we made the rule 1 hr. out of 
the oven max, i.e., take only what you can wave within 1 hour.

It worked like a charm! We worked that facility 3 shifts, so there wasn't 
a problem with "over-baking" causing oxidation/solderability issues since 
the process would roll continuously.

Ed Popielarski
Engineering Manager


                               970 NE 21st Ct.
                              Oak Harbor, Wa. 98277

                              Ph: 360-675-1322
                              Fx: 206-624-0965
                              Cl: 949-581-6601

https://maps.google.com/maps/myplaces?hl=en&ll=48.315753,-122.643578&spn=0.011188,0.033023&ctz=420&t=m&z=16&iwloc=A


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, July 01, 2013 8:25 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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Registered office at Moor Lane, PO Box 31, Derby, DE24 8BJ. Company number 
6686268, UK VAT number GB 939 396 075

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Aero Engine Controls is part of the Rolls-Royce Group, whose legal entity 
is Rolls-Royce Engine Control Systems Limited which is registered in 
England.
Registered office at Moor Lane, PO Box 31, Derby, DE24 8BJ. Company number 
6686268, UK VAT number GB 939 396 075

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