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36 The PCB Magazine • June 2014 BASIC PRINCIPLES oF PoLyMER THICK FILM FLExIBLE CIRCUITS continues final values for the conductivity and/or resis- tivity of the ink used when cured. In addition, there are also the limits of the screen printing materials and the processes used. Traditional screen printing methods are typically limited to the production of relatively course features (e.g., 250 µm). However, very fine-featured screen printed circuit technology has been re- ported down to ranges closer to 125 µm. For the foreseeable future, screen printing will re- main an important technology for the manu- facture of PTF circuits, both rigid and flexible. That said, inkjet printing is well positioned to augment time honored screen print methods. It also opens the door to realizing the possibil- ity of producing circuits economically at a run unit of one, which has been one of the ideals of manufacturing. With respect to PTF inks, the particle size of the included conductor material (historically silver but with other conductor materials com- ing on line) and the polymer carrier, its rheol- ogy and flow characteristics will establish the limits of screen printing. Emerging nanotech- nologies being applied to conductive particle production appear poised to greatly boost con- ductivity, possibly opening the door for much broader use of polymer thick film technology in areas once reserved for copper circuits. While PTF circuits are not generally considered suit- able for dynamic applications, they can actu- ally perform quite well in certain types of dy- namic applications. Some experimenters have actually reported increases in conductivity with cycling. PTF membrane switches also provided historical witnesses to the efficacy of PTF as a flexing technology in the contact areas of dome switches. Basic Circuit Design Considerations with PTF Among the most basic measures of any cir- cuit manufacturing technology are feature siz- es. These include circuit feature such as mini- mum hole sizes, conductor line and space limi- tations, and the base material's performance limitations. As mentioned above, minimum conductor width and spacing is considered to be in the range of 250–375 µm (0.010"–0.015"). It is possible to produce finer lines and spaces using PTF inks as was also mentioned, however, circuit conductivity can become more of a de- sign performance concern. In that regard, ap- preciation of ink conductivity is a key concern in designing PTF circuits. The circuits are typi- cally low-power with maximum current ratings of around 1/10 of an amp. Current generation silver-based polymer thick film inks, under normal conditions can be expected to carry approximately 25% of the current of copper circuits for equivalent line widths and using a nominal PTF ink thickness. However improvements in materials are be- ing made on a regular basis and there has been some suggestion that conductivities approach- ing that of copper metal are on the horizon. From a conductivity standpoint, values pres- ently are nominally in the range of 5–20 mil- Figure 1: the list of materials used in the manufacture of PtF circuits is a short one and a primary factor in their being one of the lowest cost of current flexible circuit solutions.