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64 The PCB Magazine • November 2015 The advantages of digital circuitization tech- niques 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 sub- stantial drawbacks, such as Orbotech's DP100, which used an argon ion laser with limited radi- ation power, high power usage, and high cool- ing requirements. For years, laser direct imaging (LDI) was syn- onymous with digital imaging. While most ear- ly, commercially successful digital processes in- volved the use of lasers, other more recent pro- cesses use non-laser light sources such as LEDs (light emitting diodes), or various types of mer- cury lamps, making use of more than one wave- length. 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 pat- tern. 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. Or the switch might be micro-mirrors on a chip, such as Texas Instruments' Digital Micromirror Device™ (DMD), whereby tiny by Karl Dietz Karl DIetz conSultIng llc karl's tech talk Digital Imaging revisited ColuMn

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