Resolution of solder voids in pin-in-hole product
Figure 7: Solder void with remnants of copper nodule.
to be more ideal for PTH diameter is even more significant.
adhesion or wetting Reducing the finished PTH diameter while
of solder. Therefore, and using the original OSP pre-treatment
two sets of PCBs resulted in the dppm being lowered to 417.
were treated with The experimental results indicated that
these two differ- the reduction of finished PTH diameter
Figure 6: FIB cut into solder void.
ent micro-etchants had a much more significant impact than
respectively. the OSP modification in suppressing the
Variables
It was hypoth- frequency of solder void occurrence. By
The experimental design was based upon
esized that baking before wave soldering combining the two factors, the dppm was
five variables with each variable having
would remove moisture or volatiles that further decreased to 307.
two levels for study. The original size of
may be the source of out-gassing. There- The results from the first test implied
the PTHs under test was 29 mils nominal
fore, prior to soldering, a set of finished that the solder voids were more likely to be
finished hole diameter. In order to study
PCBs was baked at 105oC for six hours thermally induced. Therefore, the aim of
the effect that finished hole diameter has
in order to study the effect of baking on the experiment in the second section was
on the wave soldering process, a set of
solder void formation. to study the effect of solder wave contact
boards was fabricated with the finished
Wave soldering, contact time and, time on solder void occurrence. Two dif-
hole diameter decreased by 2 mils. In this
therefore, thermal exposure were also ferent conveyor speeds were evaluated and
case, 2 mils was the maximum that the
of concern and could be shortened by two finished PTH diameters were used
finished hole diameter could be decreased
increasing the conveyor speed. Two sets again to reconfirm the effect of finished
in size and still remain within the specified
of PCBs were soldered with two different PTH diameter on solder void formation.
tolerance limits.
conveyor speeds: the original speed of 95 The results shown in Table 3 proved that
As stated previously, it was sus-
cm/min and a faster speed of 120 cm/min, by increasing the conveyor speed from 95
pected that the seeds of the nodules were
producing a wave contact time of 5 and 4 cm/min to 120 cm/min, the dppm can be
deposited on the hole walls during the
seconds respectively. reduced by 50% for both of the original
permanganate desmear process. Therefore,
and the reduced PTH diameter.
two sets of PCBs were processed through
After verifying the effect of conveyor
permanganate desmear processes from two experimental results
speed on solder void formation, an ex-
factories of the same PCB supplier. The There were five variables of interest, each
periment was done to determine if baking
primary difference between the two per- variable having two levels, so there are 32
prior to wave soldering could lower the fre-
manganate desmear processes is that while possible combinations for investigation.
quency of solder void occurrence. One set
both processes recirculated the permangan- Instead of evaluating all 32 combinations,
of PCBs was baked before wave soldering
ate desmear bath, only one incorporated the experiment was broken down into
while the other set of PCBs went directly
a filtration system to remove the residue several sub-experiments, and no more than
to wave soldering without baking. The
generated by the process. two variables were studied at a time.
results show that a significant enough bake
The original OSP pre-treatment for In the first evaluation, the effects of
can reduce the number of solder voids.
production uses a hydrogen peroxide finished PTH diameter and OSP pre-
Nodules produced as a result of the
(H2O2) based micro-etchant that provides treatment were studied. The results of
permanganate desmear process were one
a relatively mild etching effect as compared the four combinations are shown in Table
of the major areas of concern in this study
to persulphate based micro-etchants. 2. The boards that were fabricated with
since nodules could be found in the defec-
This resulted in the persulphate-based the original settings are the benchmark
tive PTHs. Therefore, two sets of PCBs
micro-etchants producing a copper surface which resulted in a PTH dppm of 2,292.
were fabricated in permanganate desmear
topography which is generally considered The effect of the reduction in finished
processes from two factories of the same
24 – Global SMT & Packaging – February 2009 www.globalsmt.net
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