Mythology Of Screen Printing Technology
Electronic engineers use the terms
screen printing, screen printing legend and silk screen to denote writing on a
printed circuit board. Screen printing is far more convenient and versatile than
other traditional printing methods. For one thing China Prototype PCB, the surface does
not have to be printed under pressure, unlike etching or lithography, and it
does not have to be planar. Screen printing inks can be used to work on a
variety of surfaces - such as textiles, ceramics, metal, wood, paper, glass, and
plastic. As a result, screen printing is used in many different industries, from
clothing to product labels to PCB printing.
The screen printing
technology is rather simple and does not call for high skills. A screen is made
out of a piece of porous, finely woven fabric - originally made of silk, but now
they are made of polyester or nylon - stretched over a wooden or aluminum frame.
Areas of the screen are blocked off with a non permeable stencil which is a
negative of the image to be printed. In other words, the open spaces are where
the ink will appear. The screen is placed on top of a piece of dry paper or
fabric. Ink is placed on top of the screen, and a squeegee or rubber blade is
used to push the ink evenly into the screen openings and onto the substrate. The
ink passes through the open spaces in the screen onto the paper or fabric placed
underneath and then the screen is taken away. The screen can be re-used after
cleaning.
The electronics Printed Circuit Board industry is evolving at
a fairly rapid pace as new cost-effective production technologies are become
available. Direct digital printing is usurping the traditional photolithography
and screen printing processes that have ruled the PCB industry for the last two
decades. Direct digital printing is used to individualize PCBs just prior to
their assembly. The obvious aim is to print a unique mark on each board that can
be later used to identify the board. The mark must be permanent and inerasable
and will usually contain 1D or 2D machine readable codes along with alpha
numeric characters.
Large scale manufacturers of PCB say 1,000,000
boards and more per year use screen printing with thermally cured ink to print
the legends that cover each side of the board. In this manner, digital printing
can not print the whole side of a board at a price that will be competitive with
screen printing costs. Several other options for individualizing are however
available including Preprinted Label Application Rigid flex PCB, Solvent Based
Continuous Ink Jet Digital Printing, Laser Ablated Ink Patches
etc.
Preprinted Label Application: People who manufacture low volumes of
PCBs start with labels printed on office equipment and hand applied to each PCB
either prior to or after assembly. But if the product volumes shoot up, the
reliability and cost of hand applying labels becomes a problem. Automated
systems that print and apply labels are also available. The handicap is these
systems must place labels very accurately due to PCB space constraints.
Solvent Based Continuous Ink Jet Digital Printing: This technology has
been adapted to mark on a variety of industrial products including PCBs. With
these systems a continuous stream of ink droplets is applied to the desired
location using electromagnetic fields. Although solvent based inks produce
permanent marks on porous surfaces, it may not be permanent on non-porous
surfaces.
Many industries are today using lasers to engrave marks onto a
variety of industrial parts. Lasers have been used to engrave marks onto PC
boards with limited success due to the lack of contrast of the resulting mark.
Some further experiments are taking place to overcome and one of them is to have
the circuit boards printed with ink patches.
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