Gregory Berube | Director of Innovations Americas
For decades, polymer thick-film (PTF) systems have provided a low-cost option for screen-printing simple electronic circuits, especially on temperature sensitive substrates. The ability to apply PTF pastes on a wide variety of substrates has facilitated numerous applications, for instance membrane touch-switch keypads, buss bars for touch screens, various types of sensors, and flexible circuitry. PTF is also commonly used in the rapidly emerging Printed Electronic market, where flexible, stretchable, durable materials are paramount to the success of these technologies. These pastes include silver pastes for conductors, carbon pastes for resistive applications, shielding and biosensors; silver-silver chloride pastes for glucose and other biosensors, and dielectric pastes. Carbon based PTF resistors are a low cost and easily scalable option for low temperature polymer-based heaters, especially since they may be printed on any type of surface. However, PTF heaters normally require complex control circuitry that add to the cost of the heater. Without these controls, the heater may overheat, resulting in injury or a fire. These types of risks are especially unacceptable in automotive applications, where occupant safety is paramount. However, it is possible to design a carbon paste that will print a resistor with the ability to self-regulate by dramatically increasing in resistance at and above the target operating temperature. This effect is called “positive temperature coefficient (of resistance),” or the PTC effect. Elimination of the control circuitry in PTC heaters results in lower cost and fewer potential failure modes. In our presentation, we will demonstrate a new line of self-regulating carbon resistor pastes that have target operating temperatures at approximately 60, 80, and 100oC. We will describe how the pastes are formulated, printed, and cured into a resistive trace. We will demonstrate how the sheet resistance can be tailored with blending allowing for more circuit design flexibility. Finally, we present data that will show the PTC effect and that the heater is self-regulating at the desired temperature while being robust against possible runaway conditions. These properties make the PTC heaters especially appropriate for in-cabin passenger comfort and other automotive low temperature heating applications.
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