Millimeter-waves frequencies, in-vehicle radars, and soon in 5G cellular networks are being used more frequently. But before such frequencies can become common, it is important to build low-loss circuits at frequencies. Such as 60 and 77 GHz and suitable circuit materials will be needed to produce such circuits. It will rely on understanding which circuit and material parameters have the most effects on performance. And finding materials with a favorable set of characteristics for millimeter-wave frequencies to select circuit materials at such high frequencies.
The previous installment of the ROG Blog investigated how at millimeter-wave frequencies different transmission lines behaved; now, it is time to investigate which parameters of circuit material make the most difference at millimeter-waves frequencies. For RF/microwave applications, the ROG Blog series has regularly stressed the significance of certain material parameters, such as dielectric constant (Ds) and dissipation. As wavelengths and circuit dimensions decrease at millimeter-wave frequencies.
Since circuit dimensions shrink at higher Ds values for given impedance, millimeter-wave circuit designers generally opt for circuit materials with lower Ds values, usually about 2 to 4 or around three on average.
Circuit Material’s Ds
A circuit material’s Ds can specify the line’s width for a given impedance at a particular frequency for microstrip transmission lines. However, the Ds’ accuracy is maybe even more important at millimeter-wave frequencies than the value of the DK. For the microstrip and other transmission line technologies, such as strapline and grounded coplanar waveguide, Ds variations can result in impedance variations (GCPW).
As experienced by a high-frequency circuit, several things can contribute to the Ds. And these numerous factors can all affect the output of a circuit at millimeter-wave frequencies. While a circuit substrate material exhibits a certain Ds value. When tested with a specific test tool, the actual Ds that defines the output of a circuit on that material, referred to by Rogers Corp.
As “Design Ds,” is a combination of the air around the circuit, the material of the circuit, the copper conductors’ roughness, and the circuit characteristics. Ds Design in which circuit substrate Ds is one part, is more reliable for predicting circuit output than circuit material Ds (as in computer simulation software) because it reflects the Ds more accurately as shown by the actual circuit.
Design Ds variations cause impedance variations, which at higher frequencies may be more important. For example, Design D variations can cause unexpected variations in phase angles for millimeter-wave automotive radar systems, resulting in errors in radar detection. Any differences relating to a PCB should be minimized for millimeter-wave applications. Whether they involve Ds, circuit thickness, or conductor width. These variations can result from a PCB material’s composition and the material’s behavior under various environmental conditions, such as temperature, humidity, or humidity.
Absorption of Water
Its thermal coefficient of dielectric constant (Tacks) parameter defines how the Ds of a circuit material varies with temperature. Its moisture absorption parameter defines the way a circuit material’s Ds changes through the absorption of water. The accumulation of water also results from the high-humidity state of circuits.
Close attention should be paid to a material composition for millimeter-wave circuit materials. Different material compositions result in substantially different Tacks and how the Ds can change with temperature. Some circuit materials, such as FR-4 and pure PTFE, may experience significant Ds variations with temperature changes and lack the consistency required at millimeter-wave frequencies to maintain constant impedance. However, materials designed for stable temperature Ds (low Tacks). Such as PTFE materials with certain forms of ceramic fillers, retain almost constant temperature Ds. And have proven their utility in demanding environments, such as automotive sensor applications with millimeter waves.
Through Thick or Thin
Variations in substrate thickness and conductor width may affect PCB efficiency for the small wavelengths of millimeter-wave circuits. Substratum thickness plays a major role in deciding the impedance of transmission lines on that circuit material, along with circuit material Dk. For microstrip circuits, circuit thickness differences have the greatest effect on impedance variations of the different circuit material parameters. Conductor width is next on the list of circuit material parameters that can influence impedance variations, followed by the copper thickness and, finally, material Ds variations.
The copper thickness for a PCB is the total copper thickness of the circuit, a mixture of the laminate copper and the plated copper added during PCB fabrication. Copper thickness variations may have more to do with impedance variations than material Ds variations. The minimization of variations in copper thickness. And material Ds should be at the top of a circuit designer’s troubleshooting list to preserve high efficiency. And reliable impedance in millimeter-wave circuits, accompanied by variations in conductor width and substrate thickness.
All of these material parameters can contribute to determining circuit impedance at millimeter-wave frequencies, in varying quantities. At millimeter-wave frequencies, defining circuit materials and manufacturing procedures. That results in limited variations in these material parameters can lead to more stable, higher-performance circuits.
Ideally, circuit materials should also have low moisture absorption for millimeter-wave applications. Because water absorption impacts the Ds and high-frequency output of material. For millimeter-wave circuits. And their limited wavelengths and circuit characteristics. Circuit materials with low Ds values (around 3) are favored. And retaining low Ds can be difficult if a material absorbs even a small percentage of moisture.
Ds of 70
Water has a Ds of 70; however, the value varies with frequency: very literally. The more moisture a substance absorbs, such as in high humidity conditions, the more its Ds increase. And induces differences in the impedance of the circuit and changes in the angle of phase.
No one material may have all the features required to certify it as the optimum material for the millimeter-wave circuit. But Rogers Corp.’s RO3003TM circuit material has many of the characteristics required to make. It an excellent base for millimeter-wave circuits. It has a low Ds of 3.00 ± 0.04 that is highly stable with frequency.
Low Tacks to maintain temperature-consistent Ds, low dissipation factor. And 0.04 percent low moisture absorption. In 77-GHz automotive radar systems, it has proven its reliability. And has the electrical characteristics that enable circuit features necessary for high-performance millimeter-waves circuits to be manufactured.
This great article is written by the Eravant, one of the best millimeter wave components manufacturers such as low noise amplifiers, DC blocks, Conical Horn Antennas or Power Dividers etc.