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28 The PCB Magazine • January 2016 MEDICAL RESEARCH IS GOLDEN catalytic ink. Precursor catalysts that are depos- ited in thin atomic layers have unique properties like so many other nano materials. Additionally, a catalyst that is deposited via a liquid or "ink" can fill in many areas, nooks and crannies that would not be touched by line-of-sight methods like sputtering. This provides a basis for electro- less plating that will fill vias of all types with more thorough coverage than conventional methods. Their semi-additive process works by apply- ing a very thin electroless metal to the base layer, followed by photo resist and imaging allowing the plating of a thicker electrolytic metal when required. As with a traditional semi-additive process, the resist is then stripped and the un- needed metal is etched away forming the trace pattern. In the neural probe example, when working with gold rather than copper, a very thin layer of conductive palladium is applied electrolessly followed by the gold plating. Gold is a difficult metal to etch; the palladium is eas- ily removed without impacting the gold plating. The key difference with the technology is the ability to work with thinner metallization than the traditional semi-additive process. Without the technology barriers associated with the traditional subtractive etch process, the additive process enables both fine lines and spaces (less than 1 mil) and very thin metalliza - tion (less than 5 microns). Medical applications using this technology are often single- or double-sided configurations that have been designed with fine lines and spaces. The ability to design features less than .001" adds a new flexibility to maximize break- outs and eliminate, or minimize, multilayer blind via constructions. When this is coupled with the ability to plate pure gold without nick- el, chrome, or exposed copper layers, a unique offering emerges for applications where the cir- cuits may need to be exposed in end use. This same technology has applications in other medical applications as well. Conductive layers are often used for shielding. In some cas- es, minimal thickness is required for bulk and flexibility. Utilizing the technology for a semi- additive base layer, as noted above, enables a very thin, yet very conductive metallization on flexible substrates as well as insulation on wires and cables for coaxial type shielding. This thin metalized layer can be cut to a specific size and installed around critical components in the fi - nal assembly. Many shielding applications re- quire copper, but both gold and palladium can be used as required by the application. Metallization of fabric is also an emerg- ing market need. Using this additive technol- ogy, electrodes and other conductive paths can be formed by coating individual fibers in fabric, down to two microns in diameter, with thin metal layers. This has been demonstrated in gold, palladium and copper. The metalized surface can provide electrical, mechanical and chemical benefits. Selecting the most interesting flex applica - tion related to medical field is not an easy task. There are just so many interesting applications and design developments to choose from. This field is moving at a rapid pace. At least for today, the neural probe technology, requiring both the traces to be metallized with gold instead of cop- per, and requiring those traces to be at or below .001", gets my vote. Not only does it push out- side of standard technology in one way, but in two ways, simultaneously. PCB Figure 3: metal-coated fibers (Velcro). Tara Dunn is the president of omni pcB. to read past columns, or to contact the author, click here. Flex talk