Conquering SMT stencil printing challenges with today’s miniature components
Aperture registration
As component pad sizes continue to
decrease, alignment accuracy between the
stencil and PCB is becoming more critical.
The adhesion of the solder paste to the
SMT pad is the sole force involved in pull-
ing the paste from the stencil. Since PCBs
will tend to shrink during the manufactur-
ing process, SMT pad locations tend to be
slightly short of their expected locations.
Long PCBs, of course, will have significant-
ly greater shrinkage than short PCBs.
In addition to PCB shrinkage, the
electroformed stencil process uses Mylar
film to create the stencil image. The
film is dimensionally unstable due to its
susceptibility to temperature and humidity
fluctuations. Without tight temperature
and humidity controls in the manufactur-
ing area, shifts in aperture locations can
occur during plotting of the Mylar film
!
and during its use.
Figure 3. Stencil technology comparison.
Since the electroform process only
produces the electroformed foil, it typically
has to be mounted into a stencil frame.
stencil foil thickness. The result is addi- iature components, resulting in less rework
During the electroform process, no tension
tional solder paste volume for the non-min- and improved solder joint reliability.
is applied to the electroformed foil. When
!
Figure 4. Electroformed (left) and Fine Grain (right) Stencil Aperture Registration Accuracy
2,307 position errors with the electroformed stencil and 27 with the Fine Grain stencil
34 – Global SMT & Packaging – March 2009 www.globalsmt.net
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