What Type of Circuit Material Works Best for Millimeter Waves?
Millimeter-wave frequencies are being used more often, in automotive radars and soon in 5G wireless networks. But before such frequencies can become widespread, low-loss circuits must be designed at frequencies such as 60 and 77 GHz, and fabricating such circuits will require suitable circuit materials. Selecting circuit materials at such high frequencies will depend upon knowing which circuit and material parameters have the most effects on performance, and finding materials with a favorable set of characteristics for millimeter-wave frequencies.
The preceding installment of the ROG Blog explored how different transmission lines behaved at millimeter-wave frequencies; now, it is time to investigate which circuit material parameters make the most difference at millimeter-wave frequencies. The ROG Blog series has regularly stressed the importance of certain material parameters, such as dielectric constant (Dk) and dissipation factor, for RF/microwave applications. As wavelengths and circuit dimensions shrink at millimeter-wave frequencies, tighter tolerances are needed in various circuit material parameters. Since circuit dimensions for a given impedance shrink at higher Dk values, millimeter-wave circuit designers usually opt for circuit materials with lower Dk values, typically around 2 to 4 or an average of about 3.
For microstrip transmission lines, a circuit material’s Dk will determine the width of the line for a particular impedance at a particular frequency. At millimeter-wave frequencies, however, perhaps even more important than the value of the Dk is the consistency of the Dk. Variations in Dk will result in variations in impedance, for microstrip as well as for other transmission-line technologies, such as stripline and grounded coplanar waveguide (GCPW).
Many things can contribute to the Dk as perceived by a high-frequency circuit, and these different factors can all influence a circuit’s performance at millimeter-wave frequencies. While a circuit substrate material exhibits a particular Dk value when measured with a particular test method, the actual Dk that determines the performance of a circuit on that material—which Rogers Corp. refers to as “Design Dk”—is a combination of the air around the circuit, the circuit material, the roughness of the copper conductors, and the features of the circuit. Design Dk, in which circuit substrate Dk is one component, is more reliable than circuit material Dk for predicting circuit performance (as in computer simulation software) since it more accurately represents the Dk as seen by the actual circuit.
Variations in Design Dk cause variations in impedance, which can be more significant at higher frequencies. For millimeter-wave automotive radar systems, for example, variations in Design Dk can cause unexpected variations in phase angles, resulting in errors in radar detection. For millimeter-wave applications, any variations related to a PCB should be minimized, whether they concern Dk, circuit thickness, even conductor width. Variations in Design Dk can result from the composition of a PCB material and from the behavior of the material under different environmental conditions, such with temperature or with moisture or humidity.
The way a circuit material’s Dk changes with temperature is described by its thermal coefficient of dielectric constant (TCDk) parameter. The way that a circuit material’s Dk changes by absorbing water is characterized by its moisture absorption parameter. Water absorption often results from circuits being in a high-humidity environment.
For millimeter-wave circuit materials, close attention should be paid to material composition, since different material formulations result in considerably different TCDk and how a material’s Dk will change with temperature. Some circuit materials, such as FR-4 and pure PTFE, can undergo large variations in Dk with changes in temperature and they lack the consistency needed for maintaining constant impedance at millimeter-wave frequencies. But materials engineered for stable Dk with temperature (low TCDk), such as PTFE materials with certain types of ceramic fillers, maintain nearly constant Dk with temperature and have proven their worth in challenging environments, such as for millimeter-wave automotive sensor applications.
Through Thick or Thin?
For the small wavelengths of millimeter-wave circuits, variations in substrate thickness and in conductor width can have significant effects on PCB performance. Along with circuit material Dk, substrate thickness plays a major role in determining the impedance of transmission lines on that circuit material. For microstrip circuits, variations in circuit thickness have the greatest impact of the various circuit material parameters on impedance variations. Next on the list of circuit material parameters that can affect variations in impedance is conductor width, followed by copper thickness and, finally, variations in material Dk.
The copper thickness for a PCB is the total copper thickness of the circuit, which is a combination of the laminate copper and the plated copper added during PCB fabrication. Variations in copper thickness can have more to do with variations in impedance than variations in material Dk. For maintaining high performance and consistent impedance in millimeter-wave circuits, minimizing variations in copper thickness and material Dk should be at the top of a circuit designer’s troubleshooting list, followed by variations in conductor width and substrate thickness. All of these material parameters will contribute, in varying amounts, to determining circuit impedance at millimeter-wave frequencies. Specifying circuit materials and fabrication processes that result in minimal variations in these material parameters can contribute to more consistent, higher-performance circuits at millimeter-wave frequencies.
Ideally, circuit materials for millimeter-wave applications should also have low moisture absorption, since absorption of water affects a material’s Dk and high-frequency performance. Circuit materials with low values of Dk (around 3) are preferred for millimeter-wave circuits and their small wavelengths and circuit features, and maintaining low Dk can be difficult if a material absorbs even a small percentage of moisture. Water has a Dk of 70; however, that value does change with frequency: quite simply, the more moisture a material absorbs, such as in conditions of high humidity, the more its Dk rises and causes variations in circuit impedance and changes in phase angle.
No one material may have all of the characteristics needed to qualify it as the optimum millimeter-wave circuit material. But RO3003™ circuit material from Rogers Corp. has many of the traits needed to make it an excellent foundation for millimeter-wave circuits. It has a low Dk of 3.00 ± 0.04 that is extremely stable with frequency, low TCDk for maintaining consistent Dk with temperature, low dissipation factor, and low moisture absorption of 0.04%. It has proven its reliability in 77-GHz automotive radar systems and it has the electrical characteristics that enable the fabrication of circuit features needed for high-performance millimeter-wave circuits.
Note: This ROG Blog is based on a presentation by the author, “Understanding Circuit Material Performance Concerns for PCBs at Millimeter-Wave Frequencies,” scheduled for Tuesday, February 14, 2017, at the IPC APEX EXPO 2017 in the San Diego Convention Center (San Diego, CA). The presentation includes detailed analyses of circuit materials how variations in materials and manufacturing processes can affect performance at millimeter-wave frequencies.
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