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24 The PCB Magazine • June 2017 Embedded Resistor/Heater Material The thin film embedded resistor material is constructed by plating a thin layer of a nickel- phosphorous resistive alloy onto copper foil [1] . The resistive conductive material is subtrac- tively processed (print/etch) to form the thin film embedded resistor elements that can be used as heaters. Using the resistive conductive material in a design does not require adding an- other layer to the circuit board as the formed re- sistors are typically added to an existing layer of a printed circuit board. Designing Embedded Heaters in PCBs using Resistive Conductive Material The first two bits of information needed to start the design of the heater are the power re- quired and the area to heat. Once the required power and target area has been specified, the embedded heater can start to be designed. The PCB substrate which acts as the con- duit for the embedded heater needs to be con- sidered. The main property is the substrate glass transition temperature commonly referred to as Tg. The surrounding substrate materials must be selected to handle the steady state operating temperature of the heater. If the heater needs to continuously operate at 200°C for example, the base material needs to be selected to handle this temperature. High Tg FR-4 materials are roughly rated to 170°C while polyimides and polyimide glass materials are roughly rated to 250°C [2] . Ta- ble 1 compares the Tg for a few different sub- strate materials. The PCB fabricator and lamina- tors are a good resource for more information on the best dielectrics to use for a given appli- cation. The next step to consider after suitable sub- strate material selection is the PCB stack-up. Lo- cating the heater under the target on the sur- face of the board or on an innerlayer directly below ensures more of the heat energy is de- livered and not lost in the surrounding materi- als. Inner or surface layers, substrate thickness- es, and copper planes/pours acting as heat sinks will have an impact on heater performance. For complex PCB stack-ups or heater systems with multiple material interfaces optimization may be best achieved using tools designed to model thermal dynamics. The amount of power specified will deter- mine the heater circuit size. The heaters need to be sized to one; handle the power required and two; fit in the space required. The NiP resistive Figure 2: Illustration showing construction of resistive conductive material laminate. THIN FILM NiP EMBEDDED RESISTORS IN HEATER APPLICATIONS Table 1: Comparison table showing Tg for various substrates, excerpted from IPC-4101B reference chart [2] .