Miniaturization changes the rules for stencil manufacture.
The days of setting up a stencil production facility with mesh, a
roll of stainless steel and a laser are over. When the smallest
components were 0402s, the average aperture count was 1200, and the
average pitch was 0.65 mm, a basic stencil operation with typical
lasers and a manual check was usually satisfactory. But as we move into
the era of mainstream 0201 production and the early stages of 01005s,
manufacturing a stencil with such small apertures and tight tolerances
(approximately 5 µm) will require not only the most sophisticated
production equipment, but an advanced set of checks and balances as
well.
Traditionally, stencils are cut from Gerber data supplied by the
circuit board manufacturer; from this data, stencil apertures are
laser-cut in the stencil material, be it stainless steel or nickel.
Electroformed stencils are produced by means of an additive process,
but still rely on the same Gerber data for manufacture. Provided that
all the Gerber data are correct and the proper tolerances defined, this
process is fairly straightforward for standard SMT stencils. Once the
stencil is produced, the product usually goes through a manual
five-point inspection to check aperture presence and dimensions. During
this process, the technician identifies five points of particular
interest on the stencil, visually inspects to ensure the presence of
the aperture, and then evaluates the dimensions of each to confirm the
integrity of the data and laser. In most cases, this level of process
control is adequate for basic SMT production.
As I have said before, with nearly 60% of manufacturing defects
directly related to errors in printing, ensuring precision in the print
operation is essential for maximum line yield. Obviously, a large part
of that optimization is accurate stencil production, precise
board-to-stencil alignment and error-free paste on pad deposition –
especially in this age of miniaturization. It is safe to say that a
simple, manual five-point visual inspection will not be sufficient to
assess the integrity of more than 4,000 aperture stencils with
dimensions as small as 150 x 200 µm with 0.4 mm pitches (and 0.3 mm
pitches on the horizon)! The production and verification processes must
be much more robust.
So, what exactly does that mean? When manufacturing stencils with
such fine features, there is absolutely no substitute for good Gerber
data. But it is also possible that the laser being used to cut the
stencil is out of calibration, and therefore could cut an aperture the
wrong size or in the incorrect position. Because of this condition,
when dealing with next-generation stencils, automated inspection will
now be tantamount to ensuring the integrity of the fabrication process.
With such small apertures at high-density pitch, verification of
aperture position is equally as important as dimensional checks. As you
can imagine, on a stencil with 4,000 apertures with 150 x 200 µm
dimensions, verification – particularly positional verification – is
not something that can be done well with a manual five-point check.
High-powered stencil inspection systems that can quickly assess
aperture size, position and absence/presence will be an absolute
necessity.
The other critical aspect of stencil manufacture is Pb-free
production. Not only will stencil material selection play a major role
in efficient paste release, but alignment and print accuracy must be
absolute because of the non-self-aligning characteristics of current
Pb-free solder paste formulations. Stencil-to-board alignment must be
precise, with little room for error.
The final, notable consideration is the possibility that, even
though the board is manufactured to spec, is within its tolerances, and
the stencil is manufactured according to the Gerber data supplied,
board-to-stencil misalignment may still occur. With such fine features,
the board could be at the lower end of its tolerance and the stencil at
the upper end (or vice versa) and could result in misalignment and thus
manufacturing defects. Though not yet a common problem, in this
situation it may be an option to use scanner tools to create data based
on the manufactured PCB, in essence, generating the stencil from the
physical board instead of the Gerber data to account for any slight
misalignment that may cause production errors.
While stencil manufacture has often been viewed – often unfairly –
as a rather rudimentary and less sophisticated part of electronics
assembly, that mindset will quickly change as the era of
miniaturization takes hold. High-level manufacturing and sophisticated
inspection capabilities will be a must for next-generation stencil
production.
Clive Ashmore is global applied process engineering manager at DEK (dek.com); cashmore@dek.com. His column appears bimonthly. |
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