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34 The PCB Magazine • June 2014 by Joe Fjelstad Verdant electronics F e a t u r e Polymer thick film (PTF) technologies have been used to manufacture printed circuits for decades. In fact, some of the very first circuits ever produced in volume were made by print- ing conductive inks patterned on an insulating substrate using a stencil. While the versatile nature of the materials used and the simplicity of the manufacturing process have made pos- sible its continued use for more than six de- cades. In more recent decades, the technology has seen extensive use in the manufacture of inexpensive, polyester film-based membrane switches for electronic devices of every imag- inable type, from hand-held calculators and computer keyboards to household appliances including microwave ovens and washers and dryers and beyond. An example of such a pro- spective construction is shown in the cross sec- tion in Figure 1. Polymer thick film circuits are similar in many ways to their rigid and flexible counter- parts; however, because of the materials and processes, they have their own very specific de- sign rules. Because most PTF processing is most commonly based on screen printing technolo- gy, the limits of design are intrinsically linked to the printed ink's processing considerations and limitations. For example, there are the unique processing properties for conductive and resis- tive as well as insulating inks in terms of their thixotropy, which influence their printability. There is also the important consideration of the Basic Principles of Polymer Thick Film Flexible Circuits

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