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26 SMT Magazine • August 2014 destined to repeat it. Thus, printed electronics presently seems to be on a third or fourth incar- nation, having, like the phoenix, risen from the ashes of earlier existences. To better appreciate the scope of printed electronics technology's impact, it is helpful to parse it into technology (materials and equipment) and markets, as they are interwoven like threads in a fabric. Printing is obviously the foundation tech- nology for printed electronics. Printing has his- torically been accomplished using one of four basic methods: stencil printing, screen print- ing, flexographic printing and gravure printing. These are illustrated in Figure 2. Fundamental Printing technologies From the simplest stenciling, to the most sophisticated gravure, these printing technolo- gies are still the mainstay of much of today's commercial and industrial printing, and stencil and screen printing are still widely used in elec- tronics manufacturing. Figure 2 illustrates only the last step of the rotary printing processes. For flexographic printing, the previous step meant contact with an ink-coated roller where the ink thickness had been adjusted by means of a doc- tor blade. For gravure printing, the previous step included contact with a doctor blade to remove excess ink from the coated surface. The latter process offers more control over ink by control- ling the depth of the cavities in the print roller. In addition to these fundamental methods, laser printers and inkjet printer technology have also been adapted to the task of manufac- turing printed electronics. These offer the abil- ity to manufacture directly from a data stream and the potential to produce an economical run unit of one, which is one of the "holy grails" of manufacturing. As alluded to earlier, the au- thor was engaged in making the technology work with a laser printer in 1990 (US Patent No. 5,250,758). In addition, newer photolitho- graphic based methods are evolving that show great promise for making additive circuits. One example is a novel technology from the compa- ny eSurface. The technology may be amenable to use with inkjets at some point. Improvements in inkjet printers are making PRINtED ELECtRONICs IN PERsPECtIVE continues FEaTuRE Figure 2: Four basic methods of the printing process.