Practical BGA rework—the simple way forward with POP paste
Step 1
Obtain a sample of dip paste from your
supplier. Some suppliers will refer to this
as POP paste, and in many cases it was
probably developed from dispense grade
materials for the current mainstream
process. Many of the well-known suppliers
have products available for sale. In the past
12 months, I have been using two commer-
cial products on all my hands-on rework
workshops.
Figures 4 & 5. Examples of BGAs after placement with paste of different volumes.
Step 2
Take a flat surface for the paste. I normally
rework. or board printing. Up until now, printing
use a glass slide and make up metering
balls was the best method for an area array
height control strips of Kapton tape on
Step 5 rework process.
both edges of the slide. The tape separa-
Using the camera system to monitor, you
tion gap is dependent on the width of the
will be able to determine the correct time/ Bob Willis is a process engineer working in
ball array on the package. With the tape
temperature profile as you will see reflow the electronics industry, providing training,
in place, the paste can be placed on the
and when the paste coalesces with the balls consultancy and product failure analysis. Bob
surface of the glass using a metal squeegee
or, in the case of high temperature balls, also offers on site workshops on conventional
blade across the surface of the glass. The
wetting occurs on the ball or column. and lead-free for customers. He will be running
blade must be slightly larger than the gap
I understand that my good friend workshops at ITRI during October-November
between the tape strips. The tape controls
Dave Hillman of Rockwell Collins will be and at IPC MidWest and SMTA International
the thickness of the paste when the blade is
presenting a reliability paper on the use in September details at www.bobwillis.co.uk
dragged across the surface of the glass.
of dip paste for rework at SMTA Interna- Bob sets up and optimises production lines for
tional. Dave Bernard from DAGE and I users and also provides conferences and work-
Step 3
will also be presenting at the conference on shops worldwide.
Place the BGA component into the surface
the results of POP inspection procedures
of the paste and on the glass surface. In
during the SMTA’s 25th Anniversary
this way, the amount of paste transferred
This process sounds simple, and it
to the balls is consistent, and the side
is. Although you will find the dip paste
surface of the balls will have paste in place.
product is more expensive, it has a long
Remove the BGA from the surface of the
shelf life in a sealed container, and you
glass, inspecting the surface of the paste
have eliminated the use of a stencil for ball
for dips to indicate paste pickup and the
BGA balls for paste coating. The ball
impressions in the paste can be very easily
inspected.
Trials can be conducted on the weight
change on the BGA for paste pick up
compared with the paste volume from a
stencil print. In turn this can be compared
with the soldering results and stand off
height of the package from the board after
assembly inspection or reliability tests.
After use, place the glass slide with
paste in a sealed container as the paste on
the surface will last for some days in this
state for reuse.
Step 4
Place the BGA onto the surface of the
board assembly and reflow in place as
normal. If your rework system has a camera
systems to monitor reflow, you will see the
paste slump down the balls, guaranteeing
a wet contact with the pad surface. When
reflow of the paste takes place, you are
guaranteed a solder connection unless
significant warping of the BGA or board is
Figure 6. A high-resolution x-ray showing, in this case, a POP board assembly with a two-up BGA package. Here
experienced. This is one of the significant
the paste on the board and on the second level BGA can clearly be seen. The difference in the paste sphere size can
also be noted. Image taken on a DAGE system during in-process evaluation of assembly process.
advantages of paste compared to flux only
6 – Global SMT & Packaging – June 2009 www.globalsmt.net
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