Selecting Optimal Microwave Substrates

Understanding some of the key microwave substrate materials parameters can help when trying to weigh the cost and performance of various choices for different frequencies and applications.

Microwave substrates are often compared to building materials. They are available at many levels of quality and cost, but cutting corners can set limits on the overall performance possible for a circuit. Most dielectric materials' data sheets list the key parameters to consider when choosing a printed-circuit-board (PCB) material for a microwave application. These include dielectric constant, dissipation factor, coefficient of thermal expansion (CTE), thermal conductivity, and coefficient of dielectric constant. A substance's dielectric constant, for example, characterizes that substance as an insulator, using a vacuum, with a dielectric constant of unity or 1, as a reference. The relative dielectric constant or permittivity (εr or Dk) of practical insulators, including microwave PCB materials, is always greater than 1.

The relative dielectric constant typically quoted by manufacturers is a dimensionless value measured in the z-axis (thickness) of the material and at some reference frequency (most manufacturers also provide values in the x and y directions). The value of the permittivity will change with the thickness of the material and reference frequency, so it is important to normalize these values when comparing materials.

Why use materials with different dielectric constants? It has an impact on the dimension of microstrip conductors fabricated on the material. Since the dimensions of microstrip and other high-frequency circuit technologies are determined by the size of the fractional wavelengths (such as quarter wavelengths) of the signal frequencies to be carried by those circuit conductors, the dimensions decrease with increasing frequencies. But the dielectric constant also affects the dimensions of microstrip conductors, shrinking their dimensions with increasing value of dielectric constant. This can play a role in the "manufacturability" of a circuit and its conductor widths at higher frequencies. Circuit dimensions and manufacturing tolerances can become challenging at millimeter-wave frequencies (30 GHz and above). Prudent design choices usually dictate the use of a PCB material with a lower dielectric constant at millimeter-wave frequencies, to avoid fabricating circuits with dimensions that result in reduced production yields.

Dielectric materials, such as those based on polytetrafluoroethylene (PTFE), with low dielectric constants (around 2.2 in the z-axis), are usually thought of as low-loss materials, with materials having higher dielectric constants associated with higher loss values, although it is more precise to refer to a material's dissipation factor to gauge the actual loss of that material.

PCB materials are available in a variety of standard panel sizes, including 9 x 9 in. and 12 x 18 in., with various thicknesses and copper weights. At higher frequencies, the use of thinner dielectric materials results in finer conductor line widths for a controlled-impedance line, such as the 50-O lines commonly used in microwave circuits. Although thin dielectric materials may be attractive for making low-profile, light-weight PCBs, a thin microwave PCB with a high dielectric constant can result in extremely fine microstrip circuit dimensions. Designers should usually weigh the tradeoffs of circuit-board thickness with dielectric constant when calculating the expected dimensions for a microwave or millimeter-circuit. Some microwave materials suppliers, such as Rogers Corporation, offer guidance in the form of application notes and even an on-line calculator for computing circuit line widths for different impedances, PCB thicknesses, and dielectric constants.

PCBs use dielectric materials typically based on a resin, such as PTFE, with or without filler material that modifies the dielectric constant of the base material and can also add structural integrity to softer resin materials, including hydrocarbons and liquid-crystal-polymer (LCP) materials. The fillers are often some form of random or woven glass or ceramic material. They can strengthen a dielectric material, but also increase the dissipation loss of the material.

Microwave materials are often called "laminates" because the dielectric material is bonded to a copper conductor layer on one or both sides of the dielectric. In addition to choosing a material for its dielectric thickness, it can be selected for the amount of copper weight or thickness, which may be as fine as 1/8 or 1/4 oz. per square foot of dielectric material or as thick as 2 oz. of copper per square foot of dielectric material. Obviously, the thicker copper laminates add weight to the PCB, but also aid in improving the thermal properties of the laminate, especially for applications such as microwave power amplifiers where heat must be dissipated.

When sizing up laminates for largesignal applications, such as power amplifiers, materials parameters such as thermal conductivity and CTE are worth comparing. The thermal conductivity is a measure of the amount of heat that passes through a unit area of a laminate of unit thickness (in W/m/C). Higher values indicate better capability of handling higher power levels. The CTE describes the physical changes that take place to a laminate with changes in temperature (in ppm/C). The temperature coefficient of dielectric constant describes the effects of temperature changes on a laminate's dielectric constant. As with the CTE, it is presented on data sheets as a measure of ppm/C for a specified temperature range.

In terms of trends, the quality of laminate materials has improved in recent years, in performance and consistency, with more choices for designers. Materials suppliers such as Rogers offer materials based on different resins and fillers, to provide a choice of dielectric constants and dissipation factors. The firm has even developed materials optimized for particular applications, such as antennas and amplifiers. The firm's RO3200 series materials, for example, have been engineered for antenna applications. They are ceramic-filled laminates reinforced with woven fiberglass and are available with dielectric constants of 3.02, 6.15, and 10.2 to accommodate antenna designs through 40 GHz. For microwave designers in need of highperformance PCB materials, additional materials suppliers include Arlon Materials for Electronics Division, Polyflon Company, Taconics Plastics, and W. L. Gore.

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