PoP: An EMS perspective on assembly, rework and reliability
chamber. Two racks of test vehicles were
placed in the chamber to accommodate all
of the test vehicles.
All of the test vehicles are fully in-situ
monitored by dataloggers. A failure is
defined as five consecutive readings show-
ing a 20% increase in resistance over the
maximum resistance reading recorded
during the first thermal cycle. The tests will
be stopped periodically to allow the failures
to be verified using a multimeter. The total
test length is scheduled for 2000 cycles.
Interim results
Figure 14. Time zero cross sections—primary attach (left) and reworked (right) upper joints.
At the time of writing, over 425 cycles were
complete. To date, there are no confirmed
failures reported on any of the cells. Previ-
ous studies have shown a characteristic life
for this package with no underfill under
similar ATC test conditions to be in excess
of 2800 cycles
9
, so the results to date are
expected. Testing will continue until at
least 2000 cycles, or until all test cells show
at least 63.2% confirmed failures.
conclusions
Work to date has demonstrated success-
ful primary attach processes for package
on package using three different dippable
Figure 15. Time Zero cross sections—primary attach (left) and reworked (right) lower joints.
fluxes and three different dippable solder
pastes. All six materials gave excellent yield.
A successful rework process using a manual
all time zero cross section samples. Three reworked components, some difference in
version of the dippable solder paste process
joints were measured on each layer—one height is not surprising. The dipping pro-
was also demonstrated, again with excel-
at the right side of the package, one at the cess generally transfers less paste than the
lent yield. No yield improvements were
left side of the package and one in the mid- screen printing process for these packages,
possible using the dippable solder pastes,
dle. The results are summarized in Table 4. which would result in a lower standoff.
as the yields obtained using dip flux in this
All values in the table are averages of the
experiment were excellent. Larger sample
three measurements on the layer and are accelerated thermal cycling
sizes may, however, show differences in
measured from copper to copper. Test set up
yield between the two processes more
No significant variation in the standoff Thermal cycling is being conducted in
clearly.
height of the upper joints was observed—as accordance with IPC-9701A
8
using the
Accelerated thermal cycling is currently
all the upper joints were made using dip -40°C-125°C profile, with 10-minute dwells
underway, with no early failures to report.
paste, this is not surprising. Some variation at both temperature extremes. Custom-
Though no differences have been identi-
in the height of the lower joints was ob- ized fixtures were created to hold the test
fied to date, thermal cycling will continue
served. Generally, the reworked joints had vehicles in place and to allow the airflow
in order to determine whether any signifi-
lower standoffs than the primary attach to circulate freely around the cards. Figure
cant differences in reliability exist between
joints. As the paste for the lower packages 16 shows a rack of
was applied using a stencil for the primary package on package
attach components, and by dipping for the test vehicles in the
upper Joint lower Joint
component assembly standoff standoff
Ball alloy Process (mils) (mils)
SAC405 Primary 15.00 9.12
SAC305 Primary 14.93 8.87
SAC305 Rework 14.60 7.67
SAC105 Primary 14.86 7.87
SAC105 Rework 14.78 7.48
Table 4: Standoff measurements, time zero cross sections. Figure 16. Rack of package on package test vehicles in ATC chamber.
22 – Global SMT & Packaging – March 2009 www.globalsmt.net
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