Legends of screen printing

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, 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 A screen is made out of a piece of porous, finely woven fabric - originally made of silk, but now they are made of polyester or nylon - stretched over a wooden or aluminum frame.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, 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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