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February 2010

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From:
John Goulet <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Tue, 2 Feb 2010 14:48:43 +0000
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We always bake our boards if the seal on the bag from the vendor has been broken. We can't afford to have warped boards which result after having reflowed the bottomside. The board can be so warped that accurate placement for the top side becomes a problem before you even get to the reflow problems. 
----- Original Message ----- 
From: "Paul Edwards" <[log in to unmask]> 
To: [log in to unmask] 
Sent: Monday, February 1, 2010 5:04:20 PM GMT -05:00 US/Canada Eastern 
Subject: Re: [TN] IPC-7721 LESSONS LEARNED 

I don't use a low temp profile with a populated PCB.  However if you take the PCBA above the solder reflow temp then I the SJs would be no problem... The only problem is that now you probably have the same conditions that generated the warped PCBA in the first place... However if you are attentive to the thermal profile you might be able to eliminate some of the PCBA warp initial induced by the wrong profile by the use of a little different thermal zone biasing... 

Paul Edwards 

Surface Art Engineering 


-----Original Message----- 
From: TechNet [mailto:[log in to unmask]] On Behalf Of Nowland, Russell Howard (Russell) 
Sent: Monday, February 01, 2010 12:41 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 
-----Original Message----- 
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick 
Sent: Monday, February 01, 2010 2:03 PM 
To: [log in to unmask] 
Subject: Re: [TN] IPC-7721 LESSONS LEARNED 

Steve, 

From an outsiders [microelectronics oriented] point of view it would seem 
that the layer to layer copper coverage variance would have the most to do 
with warpage.  Like you say, you could force it flat for P&P, but the sucker 
is most certain to warp again during reflow. 

I bet that is when you beat on the layout guys! 

Glad you are feeling a bit better! 

Steve C 

-----Original Message----- 
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory 
Sent: Monday, February 01, 2010 2:50 PM 
To: [log in to unmask] 
Subject: Re: [TN] IPC-7721 LESSONS LEARNED 

Lee, 

In my experience of straightening warped/twisted bare board, I've done it by 
putting solder bars on top of the board and then baking them like the -7721 
says, then cooling them with the bars remaining on top. 

The board will be flat for printing and pick and place, but it's been my 
experience that most of the time the board will re-warp during reflow. 

Steve 

-----Original Message----- 
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee Whiteman 
Sent: Monday, February 01, 2010 2:20 PM 
To: [log in to unmask] 
Subject: Re: [TN] IPC-7721 LESSONS LEARNED 

No - These are bare unpopulated boards. 

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 Paul Edwards 
Sent: Monday, February 01, 2010 2:19 PM 
To: [log in to unmask] 
Subject: Re: [TN] IPC-7721 LESSONS LEARNED 

Lee, 

Are these populated PCBs? 

Paul 

Paul Edwards 
Surface Art Engineering 


-----Original Message----- 
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee Whiteman 
Sent: Monday, February 01, 2010 7:07 AM 
To: [log in to unmask] 
Subject: [TN] IPC-7721 LESSONS LEARNED 

We got some boards that are warped (bent & twisted that exceeded the 
flatness requirements per the J-STD-001). IPC-7721 has a repair 
procedure for warped boards. I'm getting a copy of IPC-7721 so I can 
familiarize myself with the process. In the meantime, does anyone have 
any "lessons learned" from using this procedure? 

THANKS. 

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] 


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