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Precision printing drives the digital revolution
It is unfortunate that screen printing is sometimes overlooked when in fact most advanced technologies are dependent on the process. This dependency is at the core of advances in electronics. Printed circuit boards (PCBs) are largely produced using screen printing technology and as circuits have become smaller, and the range of applications increased, so screen printing has strengthened its position in electronics production.
A digital printing machine, for example, would not issue a single pulse of ink, were it not for the rigid and flexible circuits contained within its control system, all of which have been screen printed.
Furthermore, the application of screen printing in its various forms is growing. Where there is a need to apply specialist materials to a substrate, the ability to apply a range of "inks" on the same printer is valuable. When high volumes are required, with whatever ink system, the quality of screen printing comes to the fore.
Getting ink to transfer through a stencil onto a substrate using the differences in surface energy/tension, known as wetting, is a reliable process. The interference of the mesh is not unlike the individual nozzles of an inkjet head and in both cases a film is formed when the ink flows to form a continuous layer.
A fine mesh
Mesh manufacturers constantly strive to use finer meshes. G Bopp Mesh, for example has developed the BOPP-SDS stainless steel wire cloth with threads down to 18 micron and some meshes in the range featuring an open area of up to 72 per cent. The use and careful selection of raw materials, using special alloys and advanced weaving techniques is said to make the mesh strong, with close and guaranteed mesh thickness tolerances. This makes it possible to achieve high tensioning values and allows a minimal off-contact distance, with increased registration precision for extreme fine line, high density printing.
A key element of this mesh is the production of the thread itself, carried out by G Bopp at its Swiss plant. Wire drawing determines not only the diameter, but also the tensile characteristics of the thread that help govern mesh performance during the printing process.
Another point to note is how steel meshes are designated compared to polyester. Steel mesh is identified by the mesh opening and the wire diameter, while polyester mesh is identified by threads per centimetre (mesh count) and thread thickness. When deciding on a mesh, refer to these technical specifications and carry out trials.
With standard printing inks there are recommendations as to what mesh 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 for surface mounting on circuit boards to the accurate placement of enzymes on bio-medical sensors. One of the techniques used is Dek's ProFlow DirEKt Imaging, which provides an alternative to a squeegee. ProFlow is an enclosed print head technology, capable of metering a range of fluids and materials onto a substrate.
First developed ten years ago, this technology has been licensed to machinery manufacturers throughout the world. In its latest guise, the ProFlow TX is used to deposit enzymes. Here its ability to maintain the condition of water based chemistries is a tremendous advantage over a conventional squeegee. With a normal squeegee, the enzyme solution has a large surface area open to the atmosphere, resulting in evaporation and changes in concentration. The enclosed print head removes this variable.
The system can be used with both conventional mesh based stencils and laser cut stencils. With laser cut stencils there is no mesh and the mesh/emulsion combination is replaced with a solid steel foil with the required image laser cut through it. Alternatively, the image can be created by electroformed nickel, which gives close control of aperture wall definition. The aim is to achieve consistent lay down and image edge definition.
One further alternative is to use an acrylic foil, which can be easily machined and can even be machined on the print side to accommodate small but consistent irregularities in the surface to be printed such as tracks or small components on a circuit board.
Although some of the techniques adopted by Dek appear to be distant from graphics printing, there is much to learn from the company's precision engineering approach. The process is the same, but the culture of rigid process measurement and control is often lacking.
