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Conquering SMT stencil printing challenges with today’s miniature components
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Figure 1. SEM photos of SS 300 series (left) and new material (right)
Figure 2. Grain size comparison: SS 300 series (left) and new material (right).
aperature walls.
This manufacturing process produces an the surface area ratio formula is the most The new fiber lasers produce shorter pulse
exceptionally smooth stencil aperture wall accurate when determining which stencil widths and higher frequencies and have
compared to chemically etched and tradi- technology to utilize. a fully programmable pulse/pause ratio.
tional laser-cut stencils. The smoothness of To successfully print small components In addition, they produce a smaller laser
the aperture wall is a vital component of a without reducing the stencil foil thickness, beam diameter of 19 microns with a cor-
high performance stencil allowing printing the stencil must be capable of producing responding 4X increase in energy density.
of smaller apertures without reducing the acceptable solder paste release at as low a The 4X increase in energy density signifi-
foil thickness. surface area ratio as possible. The historical cantly increases the laser beam’s ability to
For many years, electroformed stencils limit of chemically etched and traditional cut through the metal and the result is a
have been the premier solution for these laser-cut stencils has been a surface area much smoother aperture wall (Figure 1).
new, challenging assemblies. However, ratio of 0.66. In the case of electroformed
the assembly industry as a whole is being stencils, the limit has been improved to
driven to turn product faster and cheaper. 0.5. The lower surface area ratio limit of New developments in
Electroformed stencil prices typically are electroformed stencils is the reason for stencil material technology
3X-4X higher than traditional laser-cut selecting this technology when facing chal- Along with advancements in laser technol-
stencils, and it takes longer to produce an lenging assemblies. ogy, there also have been advancements
electroformed stencil. In most cases, the With all of the advancements in in stencil material technology. For many
turnaround time is three to four days and component and PCB design, has advance- years, laser-cut stencils used either 300
this technology is limited to only a few ment in the stencil industry remained series stainless steel or a higher nickel alloy
shops that have the knowledge and exper- stagnant, or has technological improve- (Invar Alloy 36, Alloy 42) for the stencil
tise in plating very thin nickel foils. Many ment benefited this industry as well? Is the foil material. These are good solutions for
OEMs and CMs need stencils produced stencil industry now in a better position to the majority of assemblies, but their paste
and shipped the day the order is placed in provide solutions for printing miniature release performance reduces considerably
order to meet their schedule. The higher components while meeting customers’ when printing apertures with surface area
cost and longer lead-time of electroformed tighter delivery and cost requirements? ratios below 0.66. As a result, one would
stencils make it more difficult to meet have to either increase, or ‘overprint,’ the
the schedule and cost demands of today’s New developments in aperture sizes when selecting a thicker foil
assemblies. How does one determine if stencil laser technology or reduce the foil thickness for acceptable
a traditional laser-cut or electroformed Stencil laser technology has seen continu- prints.
stencil is required? ous advancement over the past ten years. Overprinting miniature components,
There are two formulas used to deter- The majority of advancement has been in however, is not always a guaranteed solu-
mine whether or not the smallest aperture linear motor technology, leading to im- tion since the crucial surface area in the
on a stencil will have acceptable solder provements in the cutting speed of stencil surface area ratio formula is the common
paste release with a given stencil technol- lasers. Until recently, the source of the laser metallic surface area between the SMT
ogy. These are surface area ratio and aspect beam has remained the same, with reliance pad and the stencil aperture. If a PCB has
ratio. The surface area ratio can be used on lamp pumped technology. The lamp a CSP component with a 0.010” diameter
for any stencil aperture shape and is the pumped technology is comprised of flash pad and the stencil overprints with a
contact area between the paste and PCB lamps, YAG rods, mirrors and focal lenses. 0.012” diameter aperture, the common
pad (L x W) divided by the contact area With this technology, the smallest diameter metallic surface is still limited to the 0.010”
between the paste and stencil ((2 x L x T) laser beam possible was approximately 40 diameter of the SMT pad. The additional
+ (2 x W X T)). The aspect ratio is limited µm. While this diameter beam is fine for paste beyond the 0.010” limit of the SMT
to rectangular, square and round stencil the majority of stencil designs, the energy pad is not in contact with the metallic
apertures and is the smallest dimension of density with a 40 µm beam diameter is not surface and therefore does not contribute
an aperture (width (W) for rectangles and high enough to produce the smoothest ap- to pulling the paste from the stencil.
squares, diameter for circles) divided by the erture walls when cutting stencil apertures Advancements in stencil material tech-
thickness (T) of the stencil foil (historically for miniature components. nology include new stencil materials spe-
the aspect ratio has been limited to 1.5 for In the past two years, there has been cifically designed for stencil laser-cutting.
rectangles and 2.5 for squares and circles). a major leap forward in laser technology. The Fine Grain material (distributed by Ed
Since the majority of stencils have a mix- The most significant development is the Fagan, Inc.) has a much finer grain struc-
ture of aperture shapes, including custom introduction of the single mode CW Yt- ture (Figure 2) when compared to standard
shapes (homeplates) for 2-pin components, terbium fiber laser (“fiber laser” for short). 300 series stainless steel and alloys
www.globalsmt.net Global SMT & Packaging – March 2009 – 31
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