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August 2008

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Thu, 28 Aug 2008 12:33:53 -0500
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Within the new IPC spec for PWB handling there will be instructions for
how to determine the optimum bake time/temperature for any given PWB.
This procedure will allow you to scientifically determine the optimum
bake time to get to a moisture level less than 20%. It will also tell
you how to determine just how long any given assembly or circuit board
can remain out, unprotected from humidity, before it should be re-baked,
and just how to determine the bake time based on the time exposed. For
most substrates, a moisture level less than 20% of its saturated
moisture weight is usually sufficient to prevent damage during reflow,
wave solder, or hot gas rework. Of course, this is a guideline. There
could be some substrate materials out there where a moisture content
above 20% of saturated weight can lead to damage during thermal
excursions, but I cannot find any information or data suggesting that.
So there will be a caveat within the procedure that a sample should be
tested to qualify the process for "unusual substrate materials".
Following this procedure will allow you to determine the shortest/lowest
acceptable time/temperature such that the amount of oxidation is
minimized, and the amount of baking that could possibly be harmful to
the PWB or the CCA is minimized.  

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joe Macko
Sent: Thursday, August 28, 2008 11:46 AM
To: [log in to unmask]
Subject: Re: [TN] Moisture in PCB Assemblies

Denny,

We have always treated FR4 PCBs as moistures sensitive and baked them
out before reflow to be safe. Everyone has there own idea about bake out
time and temperature and most CMs do consider the PCB moisture
sensitive. We bake for 5-6 hrs at 110 C. Boards are generally used
within 2-weeks of bake out.

Best Regards,
-joe



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From: TechNet [mailto:[log in to unmask]] On Behalf Of Dennis Fritz
Sent: Wednesday, August 27, 2008 7:10 AM
To: [log in to unmask]
Subject: Re: [TN] Moisture in PCB Assemblies

Bill, 

There has been some speculation that with the higher assembly
temperatures of lead-free, that PCBs should have a Moisture Sensitivity
Level - just like the components being mounted on them.? However, I
don't think JEDEC will step up to the plate to adopt anything to do with
PCBs.? So, it would be the job of the IPC to do that.? 

With the upcoming IPC meeting in late September, Schaumburg, Illionois,
now would be a good time to debate the need for circuit board MSL
levels. As pointed out, the group working on Storage and Handling will
go a long way, but moisture absorbsion propensity is also a property of
the resin/glass composition of the board.? 

How about it TechNetters?

Denny Fritz
MacDermid, Inc. 


-----Original Message-----
From: [log in to unmask]
To: [log in to unmask]
Sent: Tue, 26 Aug 2008 7:19 pm
Subject: [TN] Moisture in PCB Assemblies



Fellow TechNetters:? I was asked a question by a valued customer if
there is any other references made to moisture in raw PCBs and populated
assemblies other than the J-STD-033 and also the IPC rework standards.?
I've looked in IPC Standards I have on file however I could not find any
references/guidlelines at all.? Can anyone out there shed some light on
this???

I thank you folks in advance for your efforts on my behalf.

Bill Black
Medco
Q.A. Manager
Tel:? 631-667-9699

E-Mail:? [log in to unmask]

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-----Original Message-----

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