Establishing a precision stencil printing process for miniaturized electronics assembly
!
Figure 6—SMD and NSMD Variability Chart for 12 mil pads using a 5 mil
! stencil (0.60 aperture areas ratio).
Figure 5. VMR Line Chart for NSMD pad designs comparing four paste lots at a
print speed of 100 mm/s.
tally and vertically to the direction of the in the stencil print-
squeegee print stroke. The rectangular ing activity. Figure
apertures are two sizes, 9 x 50 mils, and 8 3 shows transfer
x 50 mils. There is opportunity to observe efficiency results
both solder mask defined (SMD) and non- from six paste print
solder mask defined (NSMD) pad designs. trials. The first
Note that the preponderance of outliers three trials are done
above the mean transfer efficiency is with using the same
rectangular apertures that are oriented ver- paste, but the print
tically to the squeegee stroke, as opposed to speed is varied. It
those oriented horizontally. Also note that can be observed
the NSMD pad designs show the greater that the slower
amount of excessive outliers. print speed for
!
Under a microscope, these outlier the current paste
deposit observations are commonly called product appears
Figure 7—Foil-less Clamps and Regular Board Clamps. This result shows circular
“dog ears”. The print trial is conducted to acceptable. During
10 mil NSMD pads.
identify whether an alternate paste product the trials with
may change the concern about the dog higher print speeds
ears. The current paste product print trial insufficient outliers
includes several timed print activity pauses. occur. It appears
It is observed that outliers below the aver- that a higher print
age occur at the first print after a pause speed can result in
in printing for all rectangular apertures. greater opportu-
The alternate paste shows no significant nity for insufficient
improvement during the initial phase, outliers, especially
and a decision is made to continue use of as the number of
the current paste product. The customer hour-long pauses
should recognize for miniaturized assembly increases. A new
product designs, this set of common experimental paste
observations can become useful by plan- is tried using the
ning new product designs using SMD for same print trial
rectangular pads. procedure, tooling
Square apertures and circular aper- and setup. The new
!
tures will have proportional area ratio for paste product ap-
Figure 8. Three Pastes - Three Months - 25 Aperture Patterns. Note that Paste B is
same-sized square sides and circle diameter. pears to withstand
not always similar.
However, the volume of a square aperture the test of timed
is greater so while the transfer efficiency pauses, and may
tends to be similar, more paste gets deposit- even perform as printer housekeeping, incorrect squeegee
ed using a square aperture. A common size well at the higher print speed as the cur- length, worn blades, improper support sys-
square aperture and pad design pattern for rent product performs at the slower print tem, or a host of other process setup condi-
both 0.4 and 0.5 mm pitch components is speed. tions. In a pristine stencil printing process
an 11 mil square. The small space between Squeegee print pressure has been a dif- setup, minimal squeegee pressure is re-
pads frequently results with a NSMD pad ficult challenge in manufacturing settings. quired. Experimental results indicate that
design. This situation presents a stencil Often print pressure is increased to mask the interaction factor of squeegee pressure
printing challenge when there are pauses an unnoticed dilemma caused by general in a good setup is nearly nonexistent. How-
14 – Global SMT & Packaging – August 2009 www.globalsmt.net
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