Imagine being confronted by this challenge?
just been given a circuit board with a shorted BGA site. If management decides
the board is worth salvaging, there's only one thing to do; remove the BGA
component, clean the site, paste the site, and replace the BGA.
done this many times before, but still, as anyone who has attempted BGA removal
and replacement knows, BGA rework at any level can be finicky and challenging.
it's complicated enough, but in this case the challenge is magnified as this
BGA component has two thousand, six hundred solder balls at 1 millimeter pitch.
no quitter so you plow ahead. Removal of the component is delicate but routine
for an experienced pro.
You get that done with a little cautious foresight and
plenty of monitoring thermocouples under and around the site.
you find that clearing the site of excess solder is a little more delicate and
you decide to use your BGA rework machine's installed vacuum solder removal
far so good, but now you get hung up on the application of solder paste at this
site. Even after you've chosen solder paste with the best possible slump
characteristics you begin to confront some frustrating problems.
metal stencil you had fabricated for this site just isn't cutting it. You find
that the old reliable metal stencil is causing a depositional nightmare on a
site of this size.
application of solder paste, an acquired skill, even for low pin counts, is
crucial for successful BGA rework, and the more pins there are, the more
chances there are for uneven paste deposition.
a metal stencil, you get one pass, if you try for a second or third, you risk
pumping excess paste between the stencil apertures. This can cause paste
bridging and, ultimately, a solder short, which is what you were trying to correct
in the first place.
in the effort to prevent shorts, you under-paste several pads on this very
large BGA site, it's not inconceivable that you could induce opens under the
component. Something as simple as whether or not there is even enough room to
securely tape the stencil to the board has become an issue.
doing a little research you finally decide to reduce your paste deposition risk
factor by using a special adhesive-backed stencil for the rework.
This type of
stencils is made of plastic polymer, which allows the stencil to conform to the
surface of the circuit board, eliminating co-planarity issues.
these stencils have a special adhesive backing, if inadequate paste is
deposited in stencil apertures on the first pass, you can try another, or even
adhesive effectively isolates each pad, preventing paste bridging and
dramatically reducing the risk of shorts and opens. Lastly, the adhesive
backing means no tape to hold down the edge of the stencil, so this type of stencils
works well in a tight spot.
is no board available to complete a proper profile so you have to work your
magic with an existing profile. On top of that you decide to be extra careful
and place five digital thermocouples under and around the component so you can
properly track the critical temperatures during the process.
you place Kapton spacers under the four corners of the component to prevent
shorting due to uneven column collapse or "potato-chipping" at the
you can't hold your breath for an entire BGA reflow cycle, but neither can you
breathe easy until the component is properly reflowed and passes x-ray
inspection. But your hard work, preparation and care pay off in a successfully
reworked site. Congratulations!