EPP Europe P2.2023

Source: Indium Corporation
The 0.004“-thick stencils exhibit more than 4% more void
area than 0.005“ stencils
Source: Indium Corporation
There appears to be a
slight reduction of
void area with smaller
particle sizes in the
solder paste from
types 4 to 4.5 to 5.
Only the difference
between type 3 and
the other sizes is
statistically significant
The top X-ray images
show poor area voiding
percentages greater than
30%, while the bottom
photos show voiding of
less than 10%
Source: Indium Corporation
Void percent area as a function of solder pastes. Note the
significant difference between pastes, some as low as 5%
voids and others around 45%
Source: Indium Corporation
Solder pastes
We performed a final experiment in which we
evaluated the effect of different solder paste flux vehicles
on voiding. Three PCBs with 12 QFNs each
were used for each solder paste, for a total of 36
QFNs per paste. The results were striking, as seen in
Figure 11. Some pastes produced about 5% voids,
while others produced as much as 45%. These results
were not only surprising, but also very encouraging.
While the generation of voids is a complicated
process with many variables, selecting the solder
paste alone, with a standard assembly process can
assure voiding area levels of less than 10%.
Conclusions
Although the factors that generate voids are many,
by adopting a standard process as described, several
parameters can be set to minimize voiding. The
strongest determinant in the assembly process is to
have a hotter peak temperature reflow profile. This
factor alone reduced voiding area percent from an
average of about 22% to less than 8%. Although
finer solder particle size reduces solder voiding, the
result was only statistically significant by going from
Type 3 to Type 4 or finer. Type 3 pastes produced
more than 10% more void area than Type 4 or finer.
Using a stencil thickness of 5mils reduced voiding
about 4% on average as compared to 4mil stencils.
The greatest surprise, however, was how significant
solder paste formulations were in reducing voiding.
The better solder pastes significantly reduced voiding
to less than 6% void area, whereas the worst paste
produced greater than 45% voiding.
Productronica, Booth A4.309
www.indium.com
To be continued...
The second part of this paper, in
which Flanagan, Belfield, and
Hotvedt detail the additional
investigations they have undertaken
to identify and refine void reduction
strategies will be published in our
April 2024 issue.
EPP Europe » 11 | 2023 37