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62 The PCB Magazine • March 2017 have been evaluated as viable high volume sub- strates for stretchable circuitry, thermoplastic polyurethane (TPU) based films have emerged as the current market leaders. Although urethanes are generally associated with the thermoset- ting class of resins, a thermoplastic version is the substrate of choice for many stretchable cir- cuit applications. TPUs are cost-effective, wide- ly available and have good elastic properties. However, they exhibit some degree of hyster- esis (permanent deformation) after stretching: approximately 3–15% after one cycle of 100% elongation. The hysteresis performance can be improved by modifying the resin, but generally, there exists a trade-off between the elastic prop- erties and heat resistance for TPU films. If the formulation is modified to increase elasticity (i.e., reduce hysteresis), the temperature resis- tance is lowered. Conductive PTF pastes are applied in the desired pattern on the TPU film. These paste formulations generally combine surface treated metal particles (usually silver flake), thermo- plastic resin, solvent and rheological modifiers. The amount of metal relative to the amount of polymer is a critical factor in determining elec- trical and other properties of the paste. Ideally, the amount of conductive particles should be sufficient to achieve the percolation thresh- old—the minimum filler content in the poly- mer matrix after which there is no significant change in the electrical properties of the dried composite. Screen printing and rotary gravure are the most established paste application meth- ods. However, inkjet, aerosol jet and other addi- tive PTF circuit patterning processes are also in development. After application, the pastes are dried at relatively low temperatures to evapo- rate solvents and other VOCs without melting or damaging the paste or the substrate. (Occa- sionally, this process is mistakenly referred to as the curing step, but as noted earlier, thermo- plastic polymers don't crosslink, so drying is the more appropriate term.) Typically, TPUs have lower temperature re- sistance than the PET materials used for creat- ing non-stretchable PTF circuitry via the same process. Because of temperature limitations, subsequent assembly of stretchable thermoplas- tic circuits can be challenging. Neither the con- ductive traces formed with thermoplastic PTF silver pastes nor the TPU films can withstand the elevated temperature regimes required for typical reflow soldering processes or bonding with epoxy based thermosetting adhesives. In- terconnection and component attachment are usually accomplished using thermoplastic ad- hesives or crimp type mechanical connectors. Many electronic material suppliers, EMS companies and OEMs are actively developing alternatives to thermoplastic film and PTF con- structions. They are seeking approaches to man- ufacture stretchable circuitry that overcomes the inherent limitations of the thermoplastic based materials. Stretchable Thermoset Films Numerous companies are actively develop- ing thermosetting elastomeric films as an alter- native to TPU films as a stretchable circuit sub- strate. These new materials hold the promise of stretchability combined with superior tem- perature tolerance, better chemical resistance, higher surface energy and increased durability. Thin versions of addition-cured silicone rubber films have been introduced to overcome some of the challenges associated with TPUs for electronic applications. These films feature attractive properties like stretchability, high di- electric strength, chemical inertness, low glass transition temperatures and good heat resis- tance. Silicone films tend to have very low sur- face energy which creates adhesion challenges. Surface treatments like plasma cleaning and primers are used to enhance the interfacial ad- hesion between the silicone film and conduc- tive paste. Panasonic Electronic Materials has devel- oped a proprietary non-silicone thermosetting STRETCHING BEYOND FLEX " TPUs are cost-effective, widely available and have good elastic properties. "