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July 2010

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Subject:
From:
Paul Reid <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Paul Reid <[log in to unmask]>
Date:
Wed, 14 Jul 2010 10:57:13 -0400
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Hi Joe, 

You are correct. Time and temperature to get the water out; time, temperature and relative humidity to reabsorb water back in. It appears water leaves faster than it reabsorbs.

I met a person, (whose name I do not remember) at the fall IPC meeting two years ago who made an interesting observation. He connected the out flow of the oven to some sort of spectroscopy. He found when baking boards (at 125°C I believe), that up to 4 hours water was being driven off. After four hours water and CO2 were being emitted.  Where does the carbon come from? Epoxy! Degrading epoxy should liberate water and CO2. At any rate, when baking gets silly, like two weeks at 150°C, and the sample is still losing weight, it may not be just water being liberated.

I agree with you that most materials can handle a 105°C bake. Dielectric materials, however, degrade over time, increased temperatures accelerates the degradation. Some low Tg materials may be aged by long bakes even at 105°C. 

It is reasonable to assume that reliability testing can enunciate the degree of degradation, or performance improvement, based on baking reflected as changes thermal cycles to failure. If baking is playing a significant influence in reliability we would observe the change in cycles to failure. In practice baking rarely makes a difference in reliability results. I mean by that it does not usually extend nor does it reduce the cycles to failure. And by failure I mean failure of copper interconnect or material degradation. Baking is generally benign in reliability testing. Coupons that delaminate in testing will still delaminate with our without the bake. 


I understand that aged material may reabsorb more water than it liberated. It appears as the material ages the strength of cross links increase until they break. As the material ages small voids and pores expand and there is more intermolecular volume which allows more water to be absorbed than before. These internal voids expand until the material fails cohesively. That explains why, on TMA, testing T260 or T288, we see the material shrink in size before it delaminates catastrophically.



Sincerely, 
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 Kane, Joseph E (US SSA)
Sent: 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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