We used to have an assembly with mirrored BGAs. Whenever we needed one
reworked I called the design engineer responsible for this fiasco and
told him to get his sorry butt down to the Airvac rework machine. It
wasn't long before it was completely redesigned.

Here is what you can do.
Use Loctite 3609 to cornerbond the BGA on the opposite side. Cure at 150
for about 10 minutes, no more.
Mask off the adjacent components around the BGA you wish to rework with
a high-temp, ESD-safe, non-polyimid tape such as Tapes II T-32 (this is
actually a 3M product). http://www.tapes2.com/electronics.htm
Flux the BGA you are about to rework, and flux the mirror-image BGA on
the opposite side of the assembly as well. That way if it goes into
reflow it will have the advantage of a flux application.
Allow to cool after removing the BGA. Perform the site prep, replace
with the new BGA and reflow that in place. X-ray at an angle so you can
verify both BGAs are good. If X-ray looks good, then remove the masking
tape, run the board through the wash process, and finally chip off the
cornerbonds using a small exacto knife. Do not cut into the PWB.
If the customer allows it, you can leave the cornerbonds on. 
Qualify this process on your product before you jump in and rework a
number of BGAs this way.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Richard Olsen
Sent: Monday, May 05, 2008 1:29 PM
To: [log in to unmask]
Subject: [TN] BGAs on opposing sides

Hello,

I have a board before me with multiple BGA's on both sides. In some
places they oppose each other.
Does anyone have any manufacturing ideas for soldering the BGA's?
One I have heard is to mask the first side BGA's and heat through the
mask, BGA, board and finally the pads of the second BGA, finally
removing the mask when done. 

Thanks,
Rich


Richard Olsen
Quality Manager/ Sales & Customer Service RNB Enterprises
17816 North 25th Avenue
Phoenix Arizona
85023-2108

Tel - 602-889-3463
Cel - 602-740-2244
Fax - 602-863-2274
Toll Free (800)-270-7585
www.rnbent.com

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