Conquering SMT stencil printing challenges with today’s miniature components
and contains smaller and fewer voids release, leads to more repeatable and ac- results of the current technologies were
in the material. With smaller and fewer curate solder paste release. as expected—of those, electroformed had
voids, the solder paste does not adhere as The new materials are a stainless steel the best solder paste release. How did the
easily to the stencil walls. This is primarily composition and are rolled so thickness new stencil laser and material technology
due to the micro size of the voids (in some tolerances are extremely tight. They also compare to current stencil technologies?
cases smaller than the particle sizes in the have improved thermal conductivity as
solder paste) that makes it more difficult well as similar mechanical and corrosion New Fine Grain performance
for the solder paste particles to get a grip resistant properties when compared to The surface area ratio limit for elec-
on the stencil walls. When the solder standard 300 series stainless steel. Stencil troformed technology is 0.5. While a
paste is pulled from the stencil as the life and durability are similar to standard significant improvement over standard
PCB drops, release is easier and less paste 300 series stainless steel stencils. laser-cut stencils, materials that offer
residue is retained in the stencil. The easier improved performance at area ratios of 0.5,
release allows for the printing of smaller stencil laser and material technology and below are going to be a requirement as
stencil apertures, without reductions in foil advancements: performance component and PCB technology continues
thickness, and the reduction in paste resi- Technological developments in component to advance.
due allows for an increase in the number and PCB design are beginning to outpace
of prints before having to clean the stencil. current stencil technology. Do these sig- Solder paste volume
In addition to improved paste release nificant advancements in stencil laser and The results above illustrate the print
for smaller apertures and much cleaner material technologies provide the current performance of the various stencil tech-
paste release throughout the entire stencil, and future solutions the electronics assem- nologies over the entire range of solder
the finer grain structure of these new mate- bly industry requires? That question is best paste types tested. All were laser-cut on
rials also produces a more defined aperture answered through a design of experiments the new LPKF Multicut fiber laser, except
edge when cut with a properly tuned laser (DOE) comparing the new laser and mate- the electroformed stencil, which utilized
beam. As the aperture size decreases, the rial technologies with the standard stencil traditional electroform technology. The
importance of repeatable and accurate technologies available today. electroformed stencil was the performance
solder paste release rises. With miniature baseline with acceptable paste volume %
components, small fluctuations in solder
Doe: The viability of new stencil
at a surface area ratio of 0.5. The laser-cut
volume have a much larger impact on
laser and material technology
electroformed nickel sheet had acceptable
solder joint reliability due to the minimal
The objective is to determine the viabil-
paste volume % down to 0.45, but its
solder volume required. A more defined
ity of this new stencil laser and material
print performance quickly flattened out
aperture edge, along with improved paste
technology and its impact on the current,
compared to the Fine Grain and electro-
and future, demands
formed stencils. The Fine Grain stencil
of the electronics
had acceptable paste volume % at 0.45 and
Test Items assembly industry. its print performance continued to out-
The objective of
perform electroformed as the surface area
Lasers:
the DOE is to de-
ratio increased. The performance increase
• LPKF LPKF MultiCut (high-power Nd:YLF fiber laser,
termine the viability
new technology)
down to a surface area ratio of 0.45 allows
• LPKF SL 600 (lamp-pumped Nd:YAG laser,
of the new stencil
the printing of even smaller components
current technology)
laser and material
without a corresponding reduction in the
technologies. The
Materials:
• New Fine Grain (UltraSlic™ FG) material
• Slic™ material
Test Board
• Electroformed
• Electroformed nickel sheet with laser-cut apertures
Finish: Electroless nickel/immersion gold (ENIG)
• Rolled nickel sheet with laser-cut apertures
Surface are ratios: 0.17 to 15
• SS 300 series
Pad count:
!
4,188
Solder Pastes:
• Water Soluble—WS150 Type 3 and Type 5, WS157 Type 3
and Type 5
• No-Clean—NC650 Type 3 and Type 5 Lead-Free—Sn100C
Type 3 and Type 5, SAC 305 Type 5
Test Equipment and Parameters
Printer: DEK 265GSX
Blades: DEK
Print Speeds: 50.8 mm/sec and 127 mm/sec
Separation: 0.3 mm/sec and 7 mm/sec
Print Gap: 0 (on contact)
Stencil Clean: Every print
Inspection: Koh Young KY-3030 3D
32 – Global SMT & Packaging – March 2009
www.globalsmt.net
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