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58 The PCB Magazine • March 2014 width are the dimensions of the planar resis- tor used in a design. Other details concerning these resistive foils and how they translate into resistors can be found by visiting the website for suppliers of resistive foils, such as Ohmega Technologies and Ticer Technologies. Ticer of- fers a close look at its embedded resistor-con- ductor material and how the reliability and consistency of such materials have been ap- plied to many critical electronic applications, including in many medical electronic devices, with good results. Using PCB materials with resistive foils to fabricate circuits with planar resistors is fairly straightforward. A circuit pattern is first imaged and etched on the PCB material. In areas where conductive copper has been etched away from the PCB, resistive foil material will be exposed at the surface. This exposed resistive material is then chemically removed. Next, a photo- resist is applied to protect the circuit pattern and to have selective openings imaged in the photoresist to define resistors. The copper is then etched in the selectively open areas of the photoresist to expose the resistive material in just those areas. The photoresist layer is then removed so that planar resistors remain formed between copper conductors, as shown by the il- lustration in Figure 1. The nominal resistive values of these resistive foils tend to shift somewhat predictably during the process of manufacturing a PCB laminate, and designers should be aware that nominal val- ues for the material may be somewhat different than the actual design values. For example, for a common RF/microwave circuit material with resistive foil that is based on polytetrafluoro- ethylene (PTFE), the nominal values of the foil change from 25, 50, and 100 ohm/square to 27, 60, and 157 ohm/square, respectively. A circuit designer should consult their material supplier to ensure they are aware of the effective resistive values to use for a given circuit material with planar resistors. Depending upon how a lami - nate is made, there may be very little difference between the nominal and the design resistive values, although the safe design strategy is to check with the material supplier. Resistance tolerance for resistors formed from these resistive films can be well con- trolled, although there are some dependencies in achieving tight tolerance. For example, the physical size of the resistor will have an im- pact on resistance tolerance, with larger resis- tors formed from resistive films typically being capable of attaining much tighter resistance tolerance than smaller resistors formed from resistive films. In a study evaluating planar re- sistors of different sizes based on the commer- cial PTFE-based circuit material noted earlier, extremely good tolerance results were achieved with medium to large planar resistors from re- MAKE THE MOST OF HIGH-FREqUENCY LAMINATES WITH RESISTIVE FOIL continues Figure 1: a portion of a pcB shows how planar resistor technology is used to form a resistor (the grey material) between copper conductors.