Joe,
I agree with Jason.  I have measured standoffs of first side BGAs on double sided boards and they are indeed a bit bigger, but this in fact is a good thing.  You now have longer, more "leggy", more compliant joints. The only reason you would want to tack the corners is if they were literally falling off.  Loctite makes an adhesive just for this problem.  It has enough give in the cured state that you will not be adding more problems than you are solving.
regards,
Bev Christian
Research in Motion

-----Original Message-----
From: Jason W. Gregory [mailto:[log in to unmask]]
Sent: July 1, 2003 2:24 PM
To: [log in to unmask]
Subject: Re: [TN] double sided reflows and BGAs


I'm confused about the reliability being jeopardized. Underfill
encapsulation is a common technique. I wouldn't see a problem with
"dotting" the corners with adhesive as long as you don't go overboard and
hinder future, possible rework.



Jason Gregory
SMT Production Supervisor/
Integrated Product Team Leader
LaBarge Inc.
(918)459-2367
(918)459-2221 fax
[log in to unmask]
http://www.labarge.com




                      "Macko, Joe @
                      IEC" <joe.macko          To:       [log in to unmask]
                      Sent by: TechNet         cc:
                      <TechNet@listserv        Subject:  [TN] double sided reflows and BGAs
                      .ipc.org>


                      07/01/2003 01:08
                      PM
                      Please respond to
                      "TechNet E-Mail
                      Forum."; Please
                      respond to
                      "Macko, Joe @
                      IEC"






Hello Tech Netters,

New board designs are incorporating BGAs on both sides of the board.
During
the 2nd side reflow, it appears that some of the larger BGAs installed on
the 1st side reflowed which are now upside down are dropping slightly I
suspect due to gravity.  Dropping in the wrong direction, away from the
pwb.

Who can shed some info on how to either stop the BGAs from dropping, or
apply some type of adhesive to the corners after 1st side reflow to secure
the BGA in place?  I am worried that securing the corners with an adhesive
my later cause a reliability problem since you are now securing the corners
to the pwb.

thanks
joe

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