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60 The PCB Magazine • March 2014 sistive film and good tolerance results were obtained with relatively small resistors. Figure 2 shows the results of this study for the PTFE- based laminate with resistive film, with values for resistance tolerance plotted as a function of resistor width. The resistor tolerance values shown in Fig- ure 2 are considered quite good. Surface-mount resistors commonly used in the industry typi- cally have a resistor tolerance of ±10% while higher-quality discrete resistors typically have a resistor tolerance of ±5%. Insertion loss is often a concern with high- frequency circuit laminates, in particular for laminates with resistive films and planar re- sistors. Such laminates with resistive films are often used in fabricating RF/microwave power dividers/combiners and such circuits must minimize insertion loss, especially when called upon to handle high signal power levels. Exces- sive insertion loss in high-frequency circuits that handle high power levels will result in un- wanted temperature rises within the power di- vider/combiner circuit, which can be a destruc- tive mechanism for the circuit. To better understand the impact of planar resistor technology on the insertion-loss per- formance of high-frequency circuit materials, insertion-loss testing was recently performed on a standard PTFE-based high-frequency cir- cuit laminate with and without the resistive film and the planar resistors. The materials were evaluated by means of a simple microstrip transmission-line circuit pattern using the dif- ferential length test method. Figure 3 shows the results, using laminates with the same copper conductor material and with and without the resistive film and the planar resistors. As the plots of loss versus frequency show, the inser- tion-loss characteristics are quite similar for the 10-mil circuit laminates, whether or not they include the resistive film and the planar resis- tors, leading to the conclusion that the resistive layer does not have a significant impact on the circuit material's insertion loss. Low-loss, high-frequency circuit materials with planar resistor technology have at times been plagued by differences in insertion-loss performance for parts within a circuit build, with some parts showing significantly higher insertion loss than others. Because this did not occur on a regular basis, the cause for deviations in insertion-loss performance was not found for some time. One theory proposed that if a circuit panel was relatively thin, any mishandling of the cir- cuit laminate could cause microfractures in the laminate's resistive layer and copper, but this was never proven. In another case, an event revealed some circuits with elevated insertion loss within a build of other circuits with normal insertion loss. After an investigation, it was dis- covered that poor etching quality could result in a significant difference in insertion loss. When viewed as a cross-section, a copper conductor typically has a trapezoidal shape, due to the standard types of copper etching pro- cesses used as part of PCB fabrication. For most high-frequency designs, this trapezoidal shape typically has minimal impact on insertion-loss performance. However, for a circuit material with resistive foil, the trapezoidal shape may result in higher insertion loss than expected. To explore this concern, evaluations were per- formed by fabricating microstrip transmission lines on high-frequency laminates and pur- MAKE THE MOST OF HIGH-FREqUENCY LAMINATES WITH RESISTIVE FOIL continues Figure 4: cross-sectional views of microstrip circuits show where the etching is ideal (left) and where an underetched condition leads to severity in the trapezoidal shape (right).