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24 The PCB Magazine • March 2017 by John Talbot TRAMONTO CIRCUITS Introduction An electronic heater is created by driving electric current through a resistive element. As the current is drawn through the element, some of the energy is expelled as heat. That heat can then be transferred to other surfaces with positive effects. It is a convenient way to keep components above damaging temperatures or to heat surfaces to a specified temperature and keep them there. Some of the first heaters were simple nickel-chromium wires attached to a power source and wrapped around a mass to transfer heat. This is effective, but not practical in all applications. Heaters that are designed on flexible material can be attached to flat surfaces, equipped with temperature sensing devices, and monitored constantly so that adjustments are possible as the ambient surroundings change. Two types of flexible heater material are com- mon: silicon rubber and polyimide. This article will focus on flexible polyimide heaters. Common Uses Flexible heaters are used to keep compo- nents, typically microprocessors, at a consistent temperature in devices that are exposed to con- ditions that have varying temperatures. They are used to heat surfaces as well. For instance, the seat or steering wheel in your car. Biologi- cal samples are sometimes better analyzed at the typical body temperature for a human or animal. Batteries and electronics in aircraft that must operate normally at 30,000 feet above the earth are kept warm with flexible heaters. Handheld electronics as well as ATMs that must operate accurately in cold climates will use flex- ible heaters to keep critical components in the specified temperature range. The uses are not trivial and one may say critical in many applica- tions. No matter the product or what function it provides, flexible heaters are an important el- ement in the electronics industry. Design Criteria For a flexible heater to be designed accurate- ly, we must first understand several things: 1. The material to be heated 2. The temperature range of the product's surroundings 3. How fast the heat must be transferred to the material Crucial Considerations for Building Flexible Heaters FEATURE Figure 1: ATM.