QUESTION:

I have no experience dealing with RF/microwave laminates, why do people need Rogers’ laminates for these applications?

ANSWER:

Circuit board materials were developed as a way of supporting conductors to replace cable connections as a means for conducting electrical current. The combination of conductor/support material had the primary purpose of providing a reliable connection between two points. This approach came about as a way of reducing the cost and size of electrical circuits. The requirements for these materials were simple, good adhesion of the conductor to the base material, base material to be rigid and low cost. This new technology brought about a whole new way of producing circuits, which in turn added to the list of requirements for a circuit board material, such as the capability of handling high temperatures during processing (reflow and bonding), allowing for drilling and the ability to plate metals onto its surface. Now conventional circuit board materials have many complex properties, many of them mechanical, that allow for high levels of integration by allowing multilayer board constructions and extremely high conductor densities while maintaining a focus on low cost.

While these developments were going on, another means of transmitting signals was being developed. Time varying electromagnetic fields were being used to induce currents on metal surfaces such as waveguides. Again, as technology progressed, the need to replace a large, heavy and expensive medium brought about the development of circuit board substrates for these applications. The board material was much more than just support for the conductor, it also formed part of the circuit by dictating the length, width and spacing of the traces as well as the way which the circuit behaved. It is important to understand that at frequencies above 500 MHz, a signal trace becomes an element itself of the circuit with distributed resistance, capacitance and inductance. At these frequencies, the dimensions of the circuit play a vital role in its function. Changing dimensions of the traces and spaces to obtain better utilization of a board can drastically alter the performance.

There are two very important factors that differentiate RF/microwave materials from standard printed circuit board laminates, besides cost. These two factors are known by several names depending on one’s background (academia, RF/microwave designers, digital designers, printed circuit board manufacturers), but they all mean the same;

The difference between materials lies not only on the values for dielectric constant and dissipation factor themselves, but just as important, also in the tolerance and control of these properties. Table I provides information regarding the various properties of conventional Epoxy/glass material and two of Rogers RO4000® family of  high frequency materials, RO4003™  and RO4350™  laminates, used for commercial applications.

Dielectric Constant

This property determines two very important things about the circuit, 1) the size of the board and 2) the speed of
the signal. A circuit is laid out based on the value and tolerance of this property. Patch antennas and filters are some of the circuits where er variation lot to lot must be kept to a minimum. Using a filter at 2.5 GHz with bandwidth of 5% (62.5 MHz), typical frequency being used for RF ID tags, we can evaluate the effect of varying the dielectric constant tolerance on circuit performance. The comparison was done with 0.020” material with a dielectric constant of 3.38± .05 for RO4003 and 4.2 ± 0.2 for FR4 (the dissipation factor of FR4 was set equal to that of the RO4003). In the case of FR4, there is no specified tolerance, so 0.2 was taken as a conservative estimate. Figure I and II display the effect on the shifting passband for these filters (as modeled with HP-EEsof Touchstone software).

Figure I. RO4003 Band Pass Filter

Figure II. FR4 Band Pass Filter

From these graphs, we can observe that a variation of ±.2 can cause enough of a shift in a filter, that half of the passband lies outside the selected frequencies of operation (the variation in K' shift the bell shaped curves up and down from the center). If FR4 where chosen for this circuit, it would require testing of all circuits and tuning those as needed to compensate for the varying K' . This operation is time consuming and costly, in many cases selection of the lower priced FR4 is outweighed by the increased cost of testing and tuning . By using a material with a tight tolerance on dielectric constant , like the RO4003 material, these types of operations can be done on a sampling basis and maintain the cost of the finished circuit at a minimum, especially as the complexity of the circuit increases.

Loss Tangent

The second property that differentiates materials labeled for RF/microwave applications from those used in DC and high speed digital is the lossiness of the material. Losses in a circuit are caused by resistance in the conductors and attenuation of the electromagnetic fields in the substrate. As frequency imcreases, so do the overall losses. This factor is very important when working with circuits like low noise block down-converters (LNB’s) for satellite communications (DBS, DSS, VSAT). Here one is working with a signal that is very weak, could be in the picowatt range, and must be separated from the noise before being amplified and brought down in frequency. Selecting a material with minimal loss is essential to a successful design. In order to quantify the difference between a dissipation factor of 0.002 for RO4003, 0.004 for RO4350 and 0.02 for FR4 material, insertion loss test was performed on all these materials for a microstrip 50W line on 0.020” material and plotted in Figure III.

Figure III. Microstrip Insertion Loss on 0.020” Material

There is a definite difference between the RO4000 materials and the FR4 laminate. The level of loss allowable is application dependent since it is a function of the power levels used, the overall circuit size and the level of signal processing needed. As a general rule of thumb, materials with dissipation factor less than 0.005 are desired for RF/microwave applications.

Conclusion

High frequency applications, above 500 MHz, require materials with many special properties. Electrical characteristics not critical for DC and high speed digital circuitry are essential when selecting materials for RF/microwave use. Dielectric constant control, low dissipation factor and controlled thicknesses differentiate Rogers materials from those typically used in the high volume printed circuit board world like FR4 and BT/epoxy. Ensuring lot to lot consistency of electrical properties by selecting appropriate raw materials and implementing a controlled manufacturing process differentiate Rogers’ RF/microwave materials from conventional printed circuit boards.

RO4000, RO4003, RO4350 are licensed trademarks of Rogers Corporation.