With
all of the stenciling options available to those assembling printed circuit
boards, how does one choose the correct stencil for the job at hand?
For
the prototype assembler, whether it be at the first prototype or at the beta
prototype level, there are numerous stencil options to consider using in lieu
of hand soldering. Stencil printing is a faster more consistent assembly
operation when there are SMT components as part of your assembly. For very
small prototype assembly requirements “foil only” or “prototype” SMT stencil
options are appropriate. “Foil only” stenciling options, as the name implies,
are those stencils containing only the aperture patterns. These stencils are
simply taped off to the printing surface as the underlying PCB patterns are aligned
with the corresponding apertures. For large pitches or large printing patterns, these
types of stencils are available in Kapton™, Mylar and stainless steel. The
plastic film stencils require more finesse and experience in terms of the
printing performance whereas the stainless-steel stencil printing process is
more forgiving. When too fine of features are used in the plastic film stencils
the squeegee pressure can cause smearing underneath the stencil and onto the
PCB surface. The stainless-steel stencils are also available in a greater
variety of thicknesses. The prototype SMT assembly process presents a variety
of options for stencil printing.
For
the medium volume producer both “frameless” stencils as well as “framed”
stencil options are the most often used stencil types. “Framed” stencils are
typically driven by the kind of printer being used to apply solder paste to the
PCB. An automatic or semi-automatic stencil printer is configured to accept a
given stencil frame size. The outline dimension is what the stencil fabricator
needs to know in order to provide the proper SMT framed stencil. “Frameless”
stencils come in fewer available sizes but provide the end user with a lower
cost alternative which is easier to store and retrieve. “Frameless” SMT
stencils also require some setup time compared to their “framed” stencil
counterparts. Both of these types of stencils are used in machines that can
control many of the input variables to the SMT printing process including
squeegee pressure, squeegee angle, stencil separation speed, PCB support, and
print stroke takt time. In most cases, the medium volume SMT assembly operation
has both a machine to control the printing parameters as well as providing a
rigid structure for the PCB to supported and printed repeatably.
For
higher run rate PCB assemblies, the framed stencil option is the most common
choice amongst stencil types. It provides the choices in sizes; it maintains
the rigidity of the structure as well as numerous color and frame material
choices. The most rigid frames are made from cast aluminum with lower weight
extruded aluminum being an option.
Another
item to consider when choosing an SMT stencil supplier is the type of material
being used. Tight print tolerances, micro BGAs and other fine-pitched
components require the use of high-grade stainless steel. The microstructure of
these higher-end stainless foils allows for a smoother release of the solder
paste from the walls of the aperture. In addition, higher-grade materials are
not as geometrically impacted by the heat produced in the laser cutting
process.
Not
only should all of the SMT stencil options be considered when choosing the type
of stencil to be ordered but having the right experience at your stencil the manufacturer is critical. While the IPC 7525 provides guidance on aperture
geometries and standard deductions, a knowledgeable and experienced stencil
engineering team will also be an attribute to get a consistent print quality.
Choose a supplier who has years of experience will be important in finding
“gotchas” in the stencil design.
