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SCREEN: PRINTING IN ELECTRONICS
should be used with a particular ink. If you
want to print special materials make sure
that the mesh opening is at least three times
the largest pigment particle size, otherwise
the mesh can separate out the larger
particles, which remain on the squeegee side
of the stencil. Others may block the mesh
openings.
Steel yourself
Steel mesh is generally preferred for printing
in the electronics industry. This both a
historical choice and an operational choice. It
provides the dimensional stability required
and is currently available in mesh down to an
18 micron, compared to 27 micron for
polyester meshes. Finer threads offer less
interference with ink flow and lower ink
deposits. As with all mesh, effective use
depends on accurate process control and
consistent set up. Control of off contact,
squeegee pressure and flood coater profile is
also essential when working with finer
threads.
Dek International is a global player in
screen printing for the electronics industry.
This UK firm has been involved in the industry
for 40 years and its printing machines work to
produce image size and positional tolerances
in the micron range. They can be used for a
host of applications, from applying adhesives
Precise deposition of adhesive pads can be used to mount components onto a printed circuit board
for surface mounting on circuit boards to the
accurate placement of enzymes on bio-
required, with whatever ink system, the quality of screen printing comes to the medical sensors. One of the techniques used is Dek's ProFlow DirEKt Imaging,
fore. which provides an alternative to a squeegee. ProFlow is an enclosed print head
Getting ink to transfer through a stencil onto a substrate using the differences technology, capable of metering a range of fluids and materials onto a substrate.
in surface energy/tension, known as wetting, is a reliable process. The First developed ten years ago, this technology has been licensed to machinery
interference of the mesh is not unlike the individual nozzles of an inkjet head manufacturers throughout the world. In its latest guise, the ProFlow TX is used to
and in both cases a film is formed when the ink flows to form a continuous deposit enzymes. Here its ability to maintain the condition of water based
layer. chemistries is a tremendous advantage over a conventional squeegee. With a
normal squeegee, the enzyme solution has a large surface area open to the
A fine mesh atmosphere, resulting in evaporation and changes in concentration. The enclosed
Mesh manufacturers constantly strive to use finer meshes. G Bopp Mesh, for print head removes this variable.
example has developed the BOPP-SDS stainless steel wire cloth with threads The system can be used with both conventional mesh based stencils and laser
down to 18 micron and some meshes in the range featuring an open area of up cut stencils. With laser cut stencils there is no mesh and the mesh/emulsion
to 72 per cent. The use and careful selection of raw materials, using special alloys combination is replaced with a solid steel foil with the required image laser cut
and advanced weaving techniques is said to make the mesh strong, with close through it. Alternatively, the image can be created by electroformed nickel,
and guaranteed mesh thickness tolerances. This makes it possible to achieve high which gives close control of aperture wall definition. The aim is to achieve
tensioning values and allows a minimal off-contact distance, with increased consistent lay down and image edge definition.
registration precision for extreme fine line, high density printing. One further alternative is to use an acrylic foil, which can be easily machined
A key element of this mesh is the production of the thread itself, carried out and can even be machined on the print side to accommodate small but
by G Bopp at its Swiss plant. Wire drawing determines not only the diameter, but consistent irregularities in the surface to be printed such as tracks or small
also the tensile characteristics of the thread that help govern mesh performance components on a circuit board.
during the printing process. Although some of the techniques adopted by Dek appear to be distant from
Another point to note is how steel meshes are designated compared to graphics printing, there is much to learn from the company's precision
polyester. Steel mesh is identified by the mesh opening and the wire diameter, engineering approach. The process is the same, but the culture of rigid process
while polyester mesh is identified by threads per centimetre (mesh count) and measurement and control is often lacking.
thread thickness. When deciding on a mesh, refer to these technical
specifications and carry out trials. www.boppmesh.co.uk
With standard printing inks there are recommendations as to what mesh www.dek.com
| 8 | Screen Process & Digital Imaging | July 09 Visit us online at www.spdi-online.com
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