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July 2010

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From:
"Kane, Joseph E (US SSA)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Kane, Joseph E (US SSA)
Date:
Wed, 14 Jul 2010 11:05:09 -0400
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If I have the right paper, it looks like the alternatives are
coulometric analysis (evolved vapor) or microwave resonance - can 
either of those be used on printed boards or PB samples?  Or is 
there another way?

-Joe 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Wednesday, July 14, 2010 10:35 AM
To: [log in to unmask]
Subject: Re: [TN] BAKING OF PWB QUESTION

Chris,
Yes, I have read the NPL paper a couple of times. I think it has enough merit that we are going to try it. 
Bev
RIM

----- Original Message -----
From: Chris Mahanna [mailto:[log in to unmask]]
Sent: Wednesday, July 14, 2010 10:29 AM
To: [log in to unmask] <[log in to unmask]>
Subject: Re: [TN] BAKING OF PWB QUESTION

One item of caution:
From our perspective, at the moment, the most notorious wet boards are those that have deep moisture.  This problem is not necessarily well characterized by weigh-bake-weigh / re-uptake experiments.
This issue has kept us from implementing a slick weigh-bake-weigh system, as other metrology may very well be the best answer.
NPL has published a study on the pros and cons of the different metrology.  Has anyone digested that info and come to conclusions?
Chris

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Joseph E (US SSA)
Sent: Wednesday, July 14, 2010 9:38 AM
To: [log in to unmask]
Subject: Re: [TN] BAKING OF PWB QUESTION

Two separate questions: (1) how long and what temperature to bake, and (2) how long after bake to process the boards?  In other words, how fast does moisture leave during bake, and how fast does it crawl back in under ambient conditions?

Both questions depend on the factors that Bev and Dewey mentioned, and you can get answers easily enough if you have an analytical balance and can weigh some samples before, during, and after bake.  There's a new IPC-TM-650 test method coming out very soon, but the details are fairly obvious.

If you don't feel like weighing, there may be guidance available from your laminate supplier, and there are a few published papers out there, including one that I did for IPC Midwest last fall.  

Rough rule of thumb, we bake flex after one hour of exposure at ambient, but rigid boards can go much longer.  A one hour limit after bake seems like overkill for most rigid boards; think how those same boards are handled at the fabricator, or what you'd do with the same boards if you received them dry and opened the dry pack to start processing them.

As for bake, we follow Paul's prescription exactly for most boards.  I don't necessarily agree that longer bakes will be detrimental to the dielectric properties, our designs can withstand continuous service above 105C, and I suspect so can many laminates.  But solderability degradation is a very valid concern for some surface finishes like immersion silver, OSP over copper, or immersion tin.

-Joe
  

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Reid
Sent: Tuesday, July 13, 2010 4:55 PM
To: [log in to unmask]
Subject: Re: [TN] BAKING OF PWB QUESTION

My rule of thumb has been 105°C for 4 hours max.

Consider
- Time and temperature degrades the dielectric.
- Time and temperature degrades the surface finish.
- Lead/free applications have a greater need for baking as compared to tin/lead applications.


Paul Reid 

Program Coordinator
PWB Interconnect Solutions Inc. 
235 Stafford Rd., West, Unit 103
Nepean, Ontario
Canada, K2H 9C1
613 596 4244 ext. 229
Skype paul_reid_pwb
[log in to unmask] <mailto:[log in to unmask]> 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Roberts, Jon (SA-1)
Sent: July 13, 2010 11:35 AM
To: [log in to unmask]
Subject: [TN] BAKING OF PWB QUESTION

Is there a rule of thumb or any best manufacturing practices of how long
after baking does the PWB have to be processed?   Jon


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