Conquering SMT stencil printing challenges with today’s miniature components
Minimum Aperature
Vapor phase vs. convection reflow in
surface registration
RoHS-compliant assembly, continued
Technology area ratio Cost Material accuracy
from page 22
Chemical etch 0.66 Low SS, Alloy Moderate
accelerated aging, the average absolute
change in resistance on measured daisy
Traditional laser-cut 0.66 Low SS, Alloy Very High chains is summarized in Table 1. The differ-
Traditional laser-cut 0.55 Low Slic™ Very High
ence between the performance of VP sol-
dered and convection soldered test boards
Electroformed 0.5 High Electroformed High is insignificant considering the limited data
Nickel
set. The resistance change for each sample
Advanced laser-cut 0.45 Medium Fine Grain Very High
is reported as the average of absolute values
of the changes in resistance for a set of sam-
Table 1: Stencil technology summary.
ples. No special preparation or seasoning of
samples was performed. Resistance values
were recorded ‘blind.’ A small amount of
mounted into a stencil frame, tension The answer to these important ques-
ohmmeter drift was experienced at the low
is applied to electroformed foil by the sten- tions is in our view an unequivocal ‘yes.’
resistances measured.
cil frame’s polyester mesh. This tension Stencil laser and material technologies
The shear force required to cause SOIC
pulls on the foil causing slight shifts in the have advanced to the point where laser-cut
joint failure was measured and found to
locations of the stencil apertures. In most stencil performance is beyond that of be the same for convection and vapor
cases, the electroformed stencil aperture current electroformed technology. Using phase reflowed test boards. Shear force was
locations will be long, or further away the new LPKF high-power short-pulse fiber measured on an SOIC16 (U25 and U26)
from their expected locations. If the PCB laser technology and the new Fine Grain exerting a combination shear and tensile
has SMT pad locations that are short of material, stencil performance is significant-
force that pushed the component parallel
expected locations and the electroformed ly improved over electroformed, especially
to the plane while lifting the component
stencil has aperture locations further away when printing miniature components.
by means of a 30˚ wedge. These measure-
than expected, there can be a significant Improvements in stencil laser and mate-
ments did not deteriorate after shock and
shift, or misalignment, between the stencil rial technologies have lead to significant
accelerated age. While thermal shock re-
apertures and PCB pads. improvements in solder paste release down
sults were measured at 500 and 1000 shock
A shift between the stencil aperture to a surface area ratio of 0.45 as well as
cycles, only those from the 2000 cycle test
and PCB pad reduces the amount of solder improved aperture registration accuracy.
are reported here. Results are summarized
in Table 2.
paste in contact with the surface of the These improvements are critical to meet-
PCB pad. This lowers the adhesive force ing future requirements when printing
conclusion
between the solder paste and PCB pad, miniature components like 01005s. The
VP key benefits include:
effectively reducing the ability of the board technology summary is as follows:
• Lower peak reflow temperature
to pull the paste from the stencil. Minia- At a cost savings of 30-50 percent
• Inert environment without
ture components already have very low compared to electroformed, the ability to
nitrogen
surface area ratios. The lower the surface produce multi-thickness (step) stencils,
• Improved solder wetting and flow
area ratio, the more critical the alignment and the option of same day turn times,
• Reduction in profiling time.
between the stencil aperture and PCB Fine Grain stencils, cut with the new
pad. The Fine Grain stencil in this DOE fiber lasers, are a marked improvement As the data above indicates, thermal
was cut in the frame on the new LPKF compared to the high-performance stencil profiles using vapor phase soldering equip-
high-power short-pulse fiber laser. The solutions available today. OEMs and CMs ment are controllable with the maximum
intent was to minimize stencil aperture can get the performance they need while temperature dictated by the specific ther-
registration errors, thereby increasing the reducing costs and meeting critical delivery
mal transfer fluid employed. The tin-lead
alignment accuracy between the stencil and schedules. The new stencil laser and mate-
and lead-free solder joints created using
PCB. The results (27 position errors for rial technologies available today give stencil
vapor phase technology have equivalent
the Fine Grain stencil and 2,307 position manufacturers the tools and materials
performance to those created using convec-
errors for the electroformed stencil) below needed to supply an ever-changing industry
tion equipment, while offering a uniform
show a marked improvement in aperture for many years to come.
fixed maximum temperature of controlled
registration when compared to an electro-
duration. Vapor phase solder joint creation
formed stencil. Acknowledgement
offers a viable alternative to convection
The authors would like to thank Stephan
reflow. Convection reflow has less uniform
conclusion
maximum temperatures over complex
Schmidt and Sebastian Gerberding of
circuit board surfaces.
As advancements continue in component LPKF Laser Electronics (www.lpkfusa.com)
and PCB technologies, will the stencil tech- for their contribution to this article.
references
nology of today provide current and future
1. Munroe, C., “Beating the RoHS Heat,”
solutions to the challenging assembly issues
Circuits Assembly Magazine,” March
faced by OEMs and CMs? Is electroformed Robert F. Dervaes is V.P. technology and engi-
2005, pp. 38-47.
technology the right solution or have new neering for Fine Line Stencil, Inc. Jeff Poulos,
2. Fraser, S. and Munroe, C, “Lead-Free
developments in stencil laser and material is V.P. of manufacturing and sales, Alternative
Using Vapor Phase Reflow in Lead-
technologies caught up with and surpassed Solutions, Inc. Scott Williams is product/ac-
free Processing,” SMT Magazine, April
the electroformed technology of today? count manager with Ed Fagan, Inc. 2005, pp. 48-49.
36 – Global SMT & Packaging – March 2009
www.globalsmt.net
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