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Karl's Tech Talk
By Karl Dietz
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Digital Imaging Revisited
The advantages of digital circuitization techniques have been described in detail by suppliers of equipment and photoresist. Since phototool generation and conditioning are omitted, there is the advantage of shorter lead time. Small lots can be customized at no extra cost (e.g., with added date and lot number information). There may be an advantage in fine-line imaging of surfaces with poor co-planarity because of the depth of focus of the laser beam. But the biggest advantage may be the ability to “scale” (i.e., to change the dimension of each individual exposure for best fit to reference points on an underlying pattern of a multilayer structure). However, early digital imaging systems had substantial drawbacks, such as Orbotech’s DP100, which used an argon ion laser with limited radiation power, high power usage, and high cooling requirements.
For years, laser direct imaging (LDI) was synonymous with digital imaging. While most early, commercially successful digital processes involved the use of lasers, other more recent processes use non-laser light sources such as LEDs (light emitting diodes), or various types of mercury lamps, making use of more than one wavelength. Others use inkjet technology to build image patterns such as legend print, soldermask or etch resist. They all have in common the building of a pattern, pixel by pixel, and they employ digital on/off switches to form the pattern. The switch might be an optical modulator or an array of LCPs (liquid crystal polymer cells) that can be rendered translucent or opaque by addressing it with an electrical pulse.
Editor's Note: This article originally appeared in the November 2015 issue of The PCB Magazine.
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