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The Rog Blog is contributed by John Coonrod and various other experts from Rogers Corporation, providing technical advice and information about RF/microwave materials.

Matching Materials To Millimeter-Wave Circuits

March 5, 2012

Millimeter-wave frequencies offer great potential for transferring wide-bandwidth, high-data-rate signals. But handling signals at these frequencies with minimal distortion requires the right printed-circuit-board (PCB) material, along with an understanding of how to apply that material to the requirements of circuits in the millimeter-wave frequency range. Processing signals from 30 to 300 GHz—the classic millimeter-wave frequency range—presents a unique set of challenges, and choosing the right PCB material can go a long way towards helping to meet those challenges.

Signals from about 30 to 300 GHz have traditionally been referred to as being in the millimeter-wave range or millimeter-wave band because of their wavelengths, which shrink from about 1000 mm to only 1 mm from the low end to the high end of that frequency range. Signals in this frequency range are strongly affected by atmospheric absorption, and millimeter-wave signals are typically designed into shorter-range applications as a result. But the small wavelengths allow the use of extremely small and directional antennas. And the enormous bandwidths available in this frequency range allow for such applications as high-data-rate communications and radar imaging. Millimeter-wave signals have long been used in military applications and have more recently been adopted for a growing number of commercial uses, including in automotive adaptive cruise control (ACC) and collision-avoidance systems as well as for short-range, high-data-rate communications. The benefits of using millimeter-wave signals in one of the many unlicensed bands made available by the FCC in the United States are enormous, but require an understanding of how signals with such small wavelengths are handled by PCB materials.

Signal losses generally increase rapidly with increasing frequency, implying a need for a PCB laminate with low relative dielectric constant and low loss tangent. But because of the small wavelengths of these signals, many other PCB material parameters come into play, including the thickness of the dielectric material, the roughness of the copper conductor, and even the consistency of the dielectric constant across the circuit board. Thinner laminates, for example, are desirable at higher frequencies to avoid unwanted modes of propagation by signals with such small wavelengths. The effects are frequency dependent, with a general trend of thinner dielectrics supporting higher-frequency signals. At millimeter-wave frequencies, laminates with thickness of 10 mils or less are usually selected to avoid moding effects. The use of a thinner PCB laminate with a given relative dielectric constant will result in finer circuit features for a given frequency than a thicker material, which can aid electrical performance in terms of mode suppression but provide additional challenge in terms of manufacturing tolerances and yields.

As signal frequencies increase, circuit features shrink accordingly and transmission line width dimensions narrow for a desired impedance, which is most often 50 Ω. Choosing a PCB material with a low relative dielectric constant can be an advantage since it can help maintain manageable circuit dimensions: as the relative dielectric constant increases, it has the effect of shrinking the circuit dimensions for a conductor with a given impedance at a given frequency. Since thinner PCB laminates, which will shrink circuit dimensions, are desirable to minimize the moding effects at millimeter-wave frequencies, the use of a laminate with low relative dielectric constant can help offset this circuit “dimensional miniaturization” that can impact the design of millimeter-wave circuits.

For example, if a 50-Ω microstrip transmission line is fabricated on a 5-mil-thick laminate with 0.5-oz. electro-deposited (ED) copper conductor layer and relative dielectric constant of 2.2, the transmission line will be 14.8 mils wide. If the same 50-Ω microstrip transmission line is fabricated on a microwave laminate that is almost identical, with 0.5-oz copper conductor and 5-mil thickness, but has a relative dielectric constant of 4.5, the microstrip transmission line will be only 8.9 mils wide. An increase in dielectric constant can mean a dramatic reduction in circuit dimensions, which can significantly impact manufacturing yields for millimeter-wave circuits with their fine dimensions.

In addition to choosing a laminate with low dielectric constant for millimeter-wave circuits, it is important to look for a PCB material with good dielectric constant consistency. High-frequency circuit laminates are constructed from a variety of different dielectric materials and with different “filler” materials, sometimes including woven glass and ceramic particles. These laminates are formulated for tight control of dielectric constant across a PCB panel, as well as from panel to panel, to support multilayer and high-yield millimeter-wave applications. 

A PCB material’s copper conductor roughness can also play a role in millimeter-wave performance. A rougher copper conductor surface translates to greater conductor area and a longer path for a wave to propagate. This effect on millimeter-wave signal propagation through a PCB becomes greater as frequency increases and/or the thickness of the dielectric material decreases. Fortunately, circuit laminates are available with low-profile copper conductor layers to minimize the effects of copper conductor roughness on millimeter-wave propagation.

To compare some materials, RT/duroid® 5880 laminate from Rogers Corp. features PTFE dielectric with micro-fiber glass filler. Its low dielectric constant of 2.20 at 10 GHz in the z-direction is well suited for millimeter-wave circuits, and its tight ±0.02 tolerance of the dielectric constant means that the value remains consistent across the PCB material. Its loss tangent of 0.0009 translates into low loss for circuits with small wavelengths and fine features. In addition, the material is available in thicknesses down to 3.5 mils in support of high-yield millimeter-wave circuits. For even thinner circuit materials, Rogers

ULTRALAM® 3850 is a liquid crystal polymer (LCP) dielectric material that can be made about 1 to 4 mils in thickness, with a low dielectric constant of 2.9 at 10 GHz in the z-direction, and low loss tangent of 0.0025 at 10 GHz. The material’s thermal coefficient of dielectric constant of +24 ppm/°C from -50 to +150°C means that the dielectric constant remains stable with temperature, for minimal effects on millimeter-wave propagation.

In short, there are always some tradeoffs when selecting a PCB laminate. At millimeter-wave frequencies, the most basic tradeoff is between achieving required electrical performance and producing circuits with adequate manufacturing yield. The choice of laminate thickness, dielectric constant, and other material parameters can greatly impact expected electrical performance and consistency as well as manufacturing reproducibility, so that performance and yield requirements should be carefully weighed against mechanical needs (such as circuit height), cost, and other factors.

Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.   

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