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Integral Planar Resistors Save Circuit Board Space

Planar resistors fabricated as part of a printed-circuit-board (PCB) laminate material provide circuit designers the means of saving board space and assembly costs at the same time. The resistors are formed on special resistive layers that are bound to a PCB dielectric material, such as polytetrafluoroethylene (PTFE). The PCB dielectric is also laminated with electro-deposited copper for forming transmission lines and other circuit features in stripline and microstrip technologies, in addition to the embedded resistors. Rogers Corp., for example, offers a number of PCB laminates with thin-film resistive foils, including its RT/duroid® laminate and its ceramic-filled thermoset-based RO4000® series laminates.

Designing a circuit using integral planar resistors can save the cost of assembling chip or surface-mount resistors to a PCB as well as the cost of the resistors themselves. By minimizing chip resistor attachment and solder joints, it is also possible to improve the reliability of a high frequency circuit by using planar resistors that are integral to the PCB.

Rogers RT/duroid 6202PR™ laminate is a high-performance glass-reinforced PTFE material with dielectric constant (Dk) of about 3.00 measured in the z-axis at 10 GHz that supports high frequency applications well beyond 40 GHz. With a dissipation factor of only 0.0020 at 10 GHz, signal loss from the material is extremely low, making it ideal for such circuits as filters, couplers and power dividers as well as active circuits. The laminates are available fabricated with 0.5 and 1 oz. electro-deposited copper as the base material (formed on the dielectric) with an added resistive layer for forming integral planar resistors.

The resistive layers are available in different thicknesses that yield different resistivities, including 10, 25, 50, 100 and 250 Ω/square, for forming resistors of various sizes and resistance values. The nature of these resistive foils is that resistor tolerances are dependent upon the thickness. The lowest resistivity value (10 Ω/square) is produced by the thickest resistive foil, while the thinnest foil yields the highest resistivity value (250 Ω/square). The typical variations in the resistance value for a 0.5 × 0.5 in. resistor formed from these films are ±3 percent for a film with resistivity of 10 Ω/square and ±10 percent for a film with 250 Ω/square resistivity.

Factors to consider when designing circuits with integral planar resistors include possible changes in resistance value due to environmental conditions, such as humidity and changing temperature. However, when RT/duroid laminates with resistive layers were subjected to an environment with 95 percent humidity at +40°C, the maximum change in resistance value for any circuit tested was 2 percent or less after 240 hours exposure. As with dielectric materials, these resistive foils can be characterized by their temperature coefficient of resistance (TCR), which is a measure of the change in resistance versus temperature. For the 10 Ω/square resistive foils with RT/duroid materials, the maximum TCR was -20 ppm/°C. For 250 Ω/square resisitive foils used with RT/duroid materials, the maximum TCR was +100 ppm/°C. In an analysis of aging effects, the change in resistance for all of the different values of resistive foils was 1 percent or less after 1,000 hours at +70°C.

These resistive foils are typically formed of such materials as nickel-phosphorous (NiP) and chromium silicon monoxide (CrSiO2), which can be laser trimmed or chemically etched to form resistors of a desired value. With laser trimming, resistor values as tight as ±1 percent can be realized.

In contrast to PTFE-based RT/duroid laminates, cost-effective RO4003C™ and RO4350B™ laminates are woven-glass-reinforced, ceramic-filled thermoset materials designed to be processed with the standard fabrication processes used for FR-4 and other low-cost PCB materials. They feature excellent dimensional stability during the fabrication process, lending themselves to reliable manufacturing models. RO4003C laminates feature a Dk of 3.38 ± 0.05 at 10 GHz, with dissipation factor of 0.0027 at 10 GHz and 0.0021 at 2.5 GHz. RO4350B laminates have a Dk of 3.48 ± 0.05 at 10 GHz with dissipation factor of 0.0037 at 10 GHz and 0.0031 at 2.5 GHz. Both materials offer excellent electrical performance in high frequency circuits, with the outstanding z-axis stability needed for forming reliable multilayer circuits.

Unfortunately, the resistive foils that provided excellent results with RT/duroid laminates did not show the same strong adhesion to the company's newer lines of RO4003C and RO4350B laminate materials. By experimenting with different resistive foils, however, Rogers' engineers were able to locate a thin-film resistive foil compatible with the RO4003C and RO4350B thermoset materials.

The RoHS-compliant resistive foil that is used with the RO4003C and RO4350B laminates is a true thin film, with thicknesses ranging from 0.01 to 0.1 mm and sheet resistance values of 25 through 1000 Ω/square. The material's sheet resistance exhibits strong isotropic character, with variations of less than ±5 percent for most resistance values. Its low TCR minimizes changes in resistance during the material lamination process as well as during normal PCB processing, with excellent thermal stability even for high-power active circuits, such as amplifiers.

Rogers Corp.,
Advanced Circuit Materials Division,
Chandler, AZ
(480) 961-1382,
RS No. 300

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