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February 2010

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Subject:
From:
John Goulet <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Sun, 7 Feb 2010 17:33:34 +0000
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Since your only baking out the moisture we bake our boards overnight at 125C. 

The other detail you need to know are: 

1. The oven has to be the vented type to get rid of the moisture. 

2. You need to place spacers like break-way rail strips evenly across a bundle 

   of PCB no more than 2-3 inches thick to allow the moisture to escape 

   and keep the boards straight. 

3. Put a heavy piece of steel plate, that has been drilled with lots of holes, on top of a bundle of very thin boards to keep them straight and to allow the moisture to leave the boards. 

- Caution make sure the inner base of the oven is flat or you may need a straight piece of steel in the bottom of the oven. 



I've seen the stock rooms try to bake an 18" thick stack of boards and then SMT would wonder why the boards bowed after baking. The devil was in the details. 

 John Goulet Sr Process/MFG Engineer 

  
----- Original Message ----- 
From: "Lee Whiteman" <[log in to unmask]> 
To: [log in to unmask] 
Sent: Monday, February 1, 2010 3:47:56 PM GMT -05:00 US/Canada Eastern 
Subject: Re: [TN] IPC-7721 LESSONS LEARNED 

First, thanks for all the inputs. 

Second, Russell made an excellent point concerning the board material. I 
just found out that the boards in question are polyimide, that has a Tg 
~ 250C. I doubt that going to 125C would make a difference - am I wrong 
on this assumption? 

When I get my copy of IPC-7721, I'll read it. I am curious if you have 
to bake a polyimide board above the Tg (~ 250C), what temperature do you 
go to and for how long? What is the heating rate and cooling rate? Or is 
this a case of On The Job Learning (OTJL) better known as trial and 
error? 

Lee Whiteman, PMP 
Senior Member Engineering Staff 
L-3 Communications East 
Telephone: (856) 338-3508 
FAX: (856) 338-2906 
E-Mail: [log in to unmask] 
  

-----Original Message----- 
From: TechNet [mailto:[log in to unmask]] On Behalf Of Nowland, Russell 
Howard (Russell) 
Sent: Monday, February 01, 2010 3:41 PM 
To: [log in to unmask] 
Subject: Re: [TN] IPC-7721 LESSONS LEARNED 

After reading the original question I had to pull the spec and read it. 
I could see doing this for bare boards but the spec eludes that you can 
also do this on a populated board.  Since you are forcing a board flat 
with components on it is anyone concerned about cracking components or 
solder joints here? 

The spec also states the bake temperature is 125 C but what if you are 
using material with a Tg of 170C? 

I have never tried this and I am a little curious. 

Russell Nowland 
Alcatel-Lucent 
Advanced Manufacturing Engineer 
Address: 14000 Quail Spring Parkway, Suite 300 
Oklahoma City, OK 73134 
email: [log in to unmask] 
Desk: 405-302-1660 
Cell: 405-203-0034 
Fax: 405-302-1622 
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