Millimeter wave (mmWave) applications have grown steadily over the past few years . This growth has also come from the semiconductor industry, which has been able to mass-produce chips with good mmWave performance. As mmWave applications increase, many aspects of the electronics industry, including the PCB industry, have had to enter a rapid learning process. In general, the quality of circuit boards is much more important for mmWave applications than for low-frequency circuits. Specifically, these issues mainly involve the consistency of circuit graphics, such as conductor line width, shape and spacing, as well as the consistency of substrate thickness, copper foil thickness and surface treatment.

Due to the wavelength, millimeter wave circuits are very sensitive to the accuracy of the processed circuit graphics. The signal wavelength refers to the distance between two adjacent phases that differ by 2π along the propagation direction of the wave. For example, if a microstrip line circuit is made on a laminate with a dielectric constant of 3.0, the phase of the electromagnetic wave changes exactly 360 degrees within a length of 2.3 inches. The length of 2.3 inches is the signal wavelength in the circuit. This phase change is also called the “phase angle”. If a 0.023-inch anomaly is encountered during the propagation of the wave, then the anomaly is equivalent to 1% of the wavelength or about 3.6 degrees. Relative to the wavelength, such a small anomaly has almost no effect on the signal. However, it is different at millimeter wave frequencies. For example, at a frequency of 77 GHz, the wavelength of the signal is about 0.095 inches. If a 0.023-inch anomaly also appears on the path of the signal, then the anomaly is equivalent to 24% of the wavelength or about 87 degrees. This 24% anomaly may affect the propagation of the entire wave, causing waveform distortion and other unnecessary performance effects.

In addition, at millimeter-wave frequencies, the PCB high-frequency materials used in circuit construction are usually thinner. Thin laminates mean that narrower conductor widths need to be designed to achieve a specific impedance such as 50 ohms. For most low-frequency applications, a conductor etching tolerance of ±0.5 mil is usually acceptable and sufficient; however, for millimeter-wave circuits, this tolerance may not be enough to obtain good and consistent RF characteristics. This is because the impedance change caused by a total change of 1 mil on a narrow conductor is larger than that caused by a wide conductor. Some impedance matching networks in the antenna array area of ​​automotive radar sensors have conductor widths of only 5 mils. A 1 mil change from 5 mil to 6 mil will produce an impedance difference of about 6 Ω. However, when using thick substrates in circuits at low frequencies, a 1 mil conductor width difference may only produce an impedance difference of less than 1 Ω. Many aspects of the circuit processing process will change the circuit impedance, and for PCB factories engaged in millimeter-wave circuit processing, being able to achieve excellent etching accuracy control will be an important advantage.

There are many different RF planar transmission structures that can be used in millimeter wave circuits, among which the microstrip line structure is the transmission line whose RF performance is least affected by the normal variations in the circuit processing process. The microstrip line structure is relatively simple, usually on the surface of the RF multilayer board. The second layer is the ground layer. Another commonly used structure is the grounded coplanar waveguide ( GCPW). This structure is also a two-layer circuit, but the top signal has three copper foil areas, which are usually designated as “ground/signal/ground”. This structure is very helpful in minimizing certain unwanted wave characteristics at millimeter wave frequencies, but the normal variations in the circuit processing process have a relatively greater impact on this structure.

The gap between the “ground/signal/ground” conductors in the GCPW structure is very important. If this gap varies greatly, the RF performance of the circuit will also vary greatly. However, the actual situation is much more complicated. On the top signal layer, there is a strong electric field between the adjacent grounds on both sides of the signal conductor. From a cross-sectional view, if the sidewalls of the conductor are vertical, there will be more electric fields in the air. However, if the conductor is trapezoidal, the electric field in the air will decrease and increase in the substrate. The difference in these electric fields in the air in the gap affects the effective dielectric constant of the signal wave . The dielectric constant of air is about 1. When the electric field is more in the air, the effective dielectric constant of the signal propagation will decrease, thus affecting the RF circuit characteristics. This is particularly true at millimeter wave frequencies. In order to make GCPW have more stable RF performance at millimeter wave frequencies, it is important to control the width and spacing of the conductors. At the same time, it is also very important to ensure that the shape of the conductors is consistent.

There are many things to consider when processing circuits to achieve consistency in conductor width, spacing, and shape. For conductor shape consistency, additive development and etching processes are generally better than subtractive processes. However, even additive processes can have some concerns about the trapezoidal shape of conductors. When using subtractive processes, the conductor shape is usually an inverted trapezoid and the same concerns apply.

Another method that helps with conductor shape consistency and etching accuracy control is to use thin copper foil. Thin copper foil can reduce trapezoidal shapes and make conductor width and etching spacing easier to control. Rogers has a variety of high-frequency circuit materials that can be used in the millimeter wave band, all of which provide 1/4 ounce (9 µm) thick copper foil options. Laminates using this ultra-thin copper foil are more conducive to PCB board factories to obtain more consistent conductor shape, width, spacing and other characteristics.

Circuit pattern changes and minor anomalies are usually acceptable for low-frequency circuits, but the performance of millimeter-wave circuits is sensitive to circuit pattern changes and minor circuit anomalies. PCB manufacturers face issues such as the need for tighter control of conductor width and spacing, as well as more consistent conductor shapes. The use of additive methods helps to improve the processing requirements of such millimeter-wave circuits. At the same time, the use of laminates with thin copper foil is also conducive to optimization and improved accuracy.

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