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56 The PCB Magazine • March 2014 F e a t u r e by John Coonrod rogers corporation, adVanced circuit materials diVision Make The Most of High-Frequency Laminates with Resistive Foil Resistive foils have been part of PCB lami- nates for some time and for a wide range of applications. They allow significant savings in space on a PCB in contrast to the use of discrete resistors, even compared to tiny SMT resistors. Some applications even use resistive foils to minimize or eliminate the inductive reactance of an SMT resistive device. Resistive foils can reduce the discrete component device count, free up real estate on a PCB, and even improve circuit board assembly processes. Many high- frequency circuit applications rely on resistive foils as termination resistors for transmission lines or matching resistors for power dividers. Regardless of the application, planar resistor technology has been well defined and estab - lished over the years and offers many advan- tages compared to alternative resistor technol- ogies. Consistency of resistance values was often an issue during the early days of planar or bur- ied resistors based on resistive foils. Some of the inconsistencies stemmed from how the re- sistive foils were incorporated into the circuit laminate materials and some issues resulted from how certain circuit fabrication processes impacted the properties of the resistive mate- rial. Fortunately, as resistive foil technology has matured, present-day circuit laminate materials with planar resistors achieve consistent resistor values, with minimal changes in those values when subjected to laminate and circuit fabrica- tion processes. Resistive foils have long been characterized in terms of their nominal surface resistance (R S ) values, or the amount of resistance exhibited by a nominal surface area of the material, such as ohm/square. Common R S values for resistive foils include 25, 50, and 100 ohm/square, and, in some cases, 10 and 250 ohm/square. When designing circuits with these resistive foils, a simple relationship can be applied to determine the design resistance value of a planar resistor: the length is divided by the width and then multiplied by the surface resistance of the re- sistive foil material, where the length and the

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