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60 The PCB Magazine • March 2017 Low-cost printed circuits can be formed by printing and drying conductive paste on flex- ible films via roll-to-roll processing. Screen printing is the predominant paste application process. These silver filled pastes are commonly referred to as polymer thick film (PTF) pastes because they have a significant polymeric com- ponent (commonly polyester) and are generally thicker than copper foils or vacuum deposited thin films. Like-wise, the most popular sub- strate is heat-treated polyester film. (Polyester is a common name for the semi-crystalline ther- moplastic polymer PET.) This method of addi- tive manufacturing enabled the development of literally thousands of low cost electronic circuits from membrane switches used in com- puter keyboards to disposable electro-chromic battery testers. While these types of flexible circuits are cost- effective and easy to produce, they also have performance limitations relative to those made with copper-clad thermoset substrates. One is- sue is increased conductor resistance. Typical silver-based PTFs are approximately two orders of magnitude less conductive than solid cop- per (1.0E-06 ohm-m versus 1.8E-08 ohm-m). Therefore, circuit traces of similar dimensions have much higher resistance. Additionally, the thermoplastic resins typically used for this type of flex circuit construction have a much lower temperature tolerance which can limit their compatibility with conventional surface mount assembly processes and their applicability for harsher end-use environments. PEN and PEEK films are sometimes used as substrates for PET in flexible circuits that require incremental ad- ditional heat tolerance is required. New Electronics—New Performance Requirements New generations of electronic devices are pushing the limits of current circuit manufac- turing technologies. Evolving product designs need PCBs which are beyond flexible; they re- quire circuits that are pliable, conformable, and resilient. In short, they need stretchable circuit- ry. Stretchiness can be defined as the ability of a material to resume its normal shape after being deformed. The terms stretchable and elastic are often used synonymously. In addition to withstanding occasional stretching, bending, compression, twisting and other deformation forces, product designs require stretchable circuits that are durable. Depending on the end-use, they may need to withstand hundreds of wash/dry cycles, UV exposure, water immersion, chemical contact, thermal excursions, shock, vibration and me- chanical abrasion. While flexible, nether of the conventional flex circuit material sets (copper-clad polyimide or silver PTF on PET) meet the requirements for truly stretchable circuitry. The basic materials like copper, polyimide and PET just aren't ca- pable of being elongated to a significant degree without imparting permanent deformation. Product design engineers are seeking alterna- tive approaches to provide circuitry that is both stretchable and durable. First Generation Stretchable Circuit Materials—Thermoplastic Films and Silver Pastes Screen printed circuits formed on thermo- plastic elastomers have emerged as the go-to material set for the first generation of stretch- able electronics. Elastomeric resins are the principle polymer system for creating both the stretchable substrates and conductive pastes on the market today. These materials have found their way into high volume production in sev- eral markets including the wearable device and health monitoring sectors. While polydimethelsiloxane (PDMS) and several other types of elastic thermoplastic films STRETCHING BEYOND FLEX " While these types of flexible circuits are cost-effective and easy to produce, they also have perfor- mance limitations relative to those made with copper-clad thermoset substrates. "