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March 2014 • The PCB Magazine 61 MAKE THE MOST OF HIGH-FREqUENCY LAMINATES WITH RESISTIVE FOIL continues posely underetching the conductors to cause different levels of severity in the trapezoidal shape, as shown in Figure 4. These evaluations of the trapezoidal shapes involved a 10-mil-thick substrate, a high-fre- quency PTFE-based circuit material with a 25- ohm planar resistive layer. Circuits were pro- cessed under a number of different conditions, with the intent to create trapezoidal shapes with varying severity. Unfortunately, the differ- ences in trapezoidal shapes were not large, and some circuits were found to be near ideal while others had trapezoidal shapes oversized by 0.4, 0.6, and 0.7 mils at the foot of the conductor on each side. As Figure 5 shows, these differences in trapezoidal shapes resulted in only small dif- ferences in insertion loss, notably when com- pared to the near-ideal circuits. Although the trend in Figure 5 corresponds with the theory that a more severe trapezoi- dal shape would result in higher insertion loss, the differences in the trapezoidal shape were not significant and the insertion loss differ- ences were also considered minor. In general, the study warns of the need for concern with etching quality when using laminate materials with resistive foils and planar resistors, but it is not clear how severe the etching quality must be affected before it impacts the insertion-loss performance of a circuit laminate with planar resistors. PCB Figure 5: microstrip insertion loss comparisons of circuits made on the same ptFe-based high-frequency materials with planar resistors and with varying etching quality. John coonrod is a market de- velopment engineer for rogers corporation, advanced circuit materials division and a regular columnist for the pcB design magazine. to contact coonrod, or read past columns, click here.