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October 2001

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Subject:
From:
Jeff Ferry <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 23 Oct 2001 14:36:11 -0500
Content-Type:
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Bill,

Our experience in reworking thousands of BGA components indicates that
there is a relatively high risk of exposing the intermetallic layer when
using solder wick to clear pads of old solder prior to placing new
components.

Perhaps the small size of the pads relative to the mass of the wick and
soldering iron make them venerable to the exposure of the intermetallic
during the wicking operation.  Our basic instruction to technicians is not
to remove solder from the old pads so that the pads look, crusty or
granular.  Leave a sheen or a slight layer of solder to protect from
intermetallic exposure.

A skilled and experienced 'prep' technician can eliminate this problem.
However, big smoking irons or ultra-light efficient irons can create
problems when wick is ground into the site in an effort to get every last
bit of solder off the pads as quickly as possible.  More and more people
use a teflon tipped vacuum removal device to evacuate solder.  This
prevents intermetallic exposure and limits collateral damage to solder mask
in the area.

Jeff Ferry
CEO
Circuit Technology Center, Inc.

On Tue, 23 Oct 2001 14:18:38 -0400, Bill Raymond <[log in to unmask]>
wrote:

Hello all,
      During the process of replacing PBGA devices, we use solder wick to
remove excess solder from the board pads before stencil printing (with a
mini-stencil) solder paste and then using our Air-Vac system to place and
reflow the BGA.  My question... what risk do we have in exposing the
intermetallic layer while we are cleaning off excess solder from the board
pads with solder wick?  Every once in awhile, we see a solder joint that
just seems not to wet to the board pad... it's as if the solder paste that
was printed on that pad "jumped" up to increase the size of the BGA solder
ball and just didn't wet to the board pad. So now that larger looking ball
is simply resting on the pad.  I am using a small mirror ( .250 inch
square) mounted on a dental pick (for a handle) and a stereo microscope...
to inspect the outer row of BGA balls.
>
>Thanks, Bill
>
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