A new angle on printing
! !
Figure 7. Main effects plot for all area ratios. Figure 8. Interaction plot for all area ratios.
! !
Figure 9. Interval plot of TE for all. Figure 10. Main effects plot for 0.49 area ratio apertures.
discarding the data with less than 0.15 that the lowest contact angle and lowest for the main effects of pressure and contact
mm spacing, as this was prone to bridg- pressure give the greatest transfer effi- angle, shown in Figure 14.
ing, discerned by visual examination. ciency, and that contact angle has a greater Figures 15 and 16 respectively show an
This left seven sets of 100 data points for effect on transfer efficiency than pressure. example of the best and the worst transfer
the 0.2 mm up to the 0.5 mm devices. A We noticed no significant interactions efficiencies.
total of 176,400 solder paste deposits were between contact angle and pressure on the
measured. transfer efficiency (Figure 8). conclusion
The data were evaluated for transfer The slope of the TE vs. contact angle The tests led to four major conclusions:
efficiency (TE), or the percentage of the curve decreases with increasing pressure
1. Greater transfer efficiencies
theoretical maximum volume for the and decreasing area ratio (Figure 9). In
are obtained for the same area
aperture in question. Our results are sum- other words, there is a smaller benefit in
ratio by reducing the blade
marized in the charts. decreasing the contact angle at higher print
contact angle.
Figure 5 shows that area ratio has the pressures. Figures 10 to 12 show details of
2. Increasing print pressure
greatest impact on transfer efficiency. the 0.49 area ratio.
decreases the attack angle, but
Figure 6 shows no significant interac- We performed a visual inspection of
has a negative effect on transfer
tions between the contact angle, pressure pictures taken of all these patterns for
efficiency.
and area ratio. the 0.15 mm pads with 0.15 mm spacing.
3. The best results were found
Board designers have a restricted These were labeled by run number only,
at the lower limit of the DoE,
amount of spacing around fine feature in order to minimize analysis bias. Each
although further testing is
components. Once area ratio has been unique combination of two boards was
needed to determine if this is
maximized under these limitations, further compared for transfer efficiency. The board
the true optimum. Experience
improvements to transfer efficiency can with the greater transfer efficiency was
shows that too low a pressure
be made via contact angle and pressure given a score of +1, and the other a score
and too low an angle will
optimization (Figure 7). of -1. These results were then added for all
cause solder paste to remain
Figure 7 shows the main effects plot for the combinations to achieve a final score
on the stencil, resulting in
transfer efficiency with contact angle and for each board (Figure 13).
inconsistent and thicker prints.
pressure being the factors. It can be seen Data were then decoded and analyzed
38 – Global SMT & Packaging – February 2009 www.globalsmt.net
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