PCB Tech - GCPW circuit applied to millimeter wave frequency

GCPW circuit applied to millimeter wave frequency

With the rapid development of modern communication technology, spectrum resources in low frequency and microwave frequency bands are increasingly depleted, and more and more wireless applications are expanding to higher millimeter wave (mmWave) frequencies. For example, applications such as fifth-generation (5G) wireless cellular mobile communications and advanced driver assistance systems (ADAS) all use frequency bands above 24 GHz. However, the power of a signal usually decreases as the frequency increases. Therefore, millimeter wave circuit technology must make full use of the existing signal power while minimizing signal loss. Maintaining signal power in millimeter wave circuits not only depends on the printed circuit board(PCB) material, but also on the choice of transmission line technology. If the influencing factors in the circuit design and manufacturing process are fully considered, then the grounded coplanar waveguide (GCPW) transmission line at millimeter wave frequency and the use of low-loss PCB materials can achieve excellent circuit performance.

Compared with other high frequency transmission line technologies (such as: strip line, microstrip line), GCPW circuit technology has natural advantages, especially at millimeter wave frequencies. The structure of GCPW is simple and clear: the top-level transmission line adopts a "ground-signal-ground (GSG)" structure, the middle layer is a single-layer dielectric layer, the bottom layer is a ground layer, and the top and bottom ground layers are interconnected by plated through holes (PTH) . Although GCPW does not conform to the simple structure of a microstrip line, GCPW is much simpler than a stripline (with a dielectric layer on the top and bottom). Compared with GCPW, although the structure of the microstrip line is simple, it will increase the loss at the millimeter wave frequency. At millimeter wave frequencies, microstrip transmission line circuits are easier to radiate energy to the outside world than GCPW circuits, especially in tightly laid out circuits and enclosures, there are potential interference and electromagnetic compatibility (EMC) problems.

However, the final performance application of GCPW also needs to understand the impact of the circuit in the actual processing, because when using various computer-aided (CAE) software to simulate the GCPW circuit, the parameter settings of the material properties are almost ideal. Therefore, these factors may cause a certain difference between the simulation results of the software and the actual measurement results of the GCPW circuit that is actually processed, especially for the design of high-volume millimeter wave circuits.

Even before the circuit is processed, small changes in the PCB material will affect the performance of the GCPW circuit, especially at the small wavelength of the millimeter wave frequency, and the wavelength is very sensitive to these changes. For example, changes in the thickness of the dielectric material and the thickness of the conductor will cause changes in the performance of the GCPW at millimeter wave frequencies. The surface roughness in the copper conductor also affects the GCPW performance, and any other plating layer (such as the PTH plating layer used to make the GCPW circuit) will also affect the GCPW performance.

Process treatment

Although GCPW transmission line technology is very suitable for the production of PCB circuits with high consistency at millimeter wave frequencies, it must still be used in conjunction with high-reliability circuit board materials (such as dielectric constant Dk, loss factor Df). In addition, the processing technology of the millimeter wave circuit must be repeatable to ensure that the circuit can maintain good consistency in mass production. Changes in processing technology may cause changes in PCB performance. For example, the location of the PTH used to connect the two ground planes in the GCPW circuit may vary from circuit to circuit, and this small difference will also become a cause of performance changes.

The shape of the GCPW conductor may vary from circuit to circuit, resulting in performance differences between manufactured GCPW circuits. When modeling copper foil conductors, CAE simulation software usually assumes it as an ideal conductor shape (rectangular from a cross-sectional view). And use this as a basis to predict the performance level of a given circuit. However, in actual processing, most of the surface conductors of GCPW circuits are processed into a trapezoidal shape, and the conductors of different circuits have a certain degree of change. Changes in these conductors will cause changes in the electrical performance of the GCPW circuit, especially the impact on the insertion loss and signal phase angle, and the impact of such changes will increase with the increase in frequency.

Due to the difference between the actual conductor and the ideal conductor, there is a difference between the performance level of the actual circuit (the conductor is trapezoidal after processing) and the ideal circuit (rectangular). As the corresponding signal wavelength becomes smaller at millimeter wave frequencies, it is extremely sensitive to circuits. The ideal circuit conductor reflects the smallest changes in the effective dielectric constant and relative phase response of the circuit, while the standard PCB manufacturing process inevitably has minor changes. Errors, which may also cause performance changes between circuits.

In addition, the GCPW circuit has a different amount of coupling according to the density of the sidewall spacing in the GSG structure. Generally, conductors that are closer together produce tighter coupling. Compared with loosely coupled GCPW transmission lines, tightly coupled GCPW circuits have a greater current density on the sidewalls of coplanar conductors. Loosely coupled GCPW circuits are less sensitive to circuit manufacturing process changes because they cannot obtain additional ground and behave very much like microstrip transmission line circuits.

Any circuit board material used to manufacture millimeter wave GCPW circuits plate, such as RO3003™ laminate of Rogers Corporation (Dk of z-axis is 3.00±0.04, Df at 10 GHz is 0.0010), and its copper foil surface (copper foil and dielectric The roughness of the layer intersection) will affect the performance of circuits made on this material, especially in higher frequencies (such as millimeter wave frequencies) and thinner circuits. The rough copper foil surface will cause the insertion loss of these circuits to increase and the signal phase speed to slow down. The conductor insertion loss is also affected by the relative width of the copper foil conductor and the thickness of the conductor. A wider conductor will show less loss, and a thicker conductor will cause the GCPW transmission line to use more air (with a lower unit Dk value) and transmit with lower loss. Of course, circuit materials with a higher Dk value will also bring about a slower phase velocity.

Metal plating

Manufacturing any type of GCPW circuit will involve electroplating the PCB material. For example: when doing via metallization, some holes will be drilled in the circuit board material, and the hole wall will be electroplated with a layer of copper to realize the conduction between the top and bottom ground layers. In this process, the top and bottom layers are connected. The copper layer of the PCB will inevitably be plated with a layer of copper again. In addition, metal plating may be performed again on the GCPW circuit to form the final surface treatment plating layer and protect the copper conductor. The conductivity of the metal used for electroplating of surface treatment is usually lower than that of copper, which will increase conductor loss and lead to an increase in insertion loss; moreover, the surface of this coating will also affect the phase response, so this effect is necessary at millimeter wave frequencies. Considered.

There is bound to be a difference between the results of computer software simulation and the measurement results of the millimeter wave GCPW circuit actually processed. One of the keys to the successful mass production of millimeter wave GCPW circuits is to minimize various error changes through specific material characteristics and specific circuit characteristics. By understanding how mature circuit board materials (such as RO3003 laminate) will be affected by different GCPW manufacturing processes, it is possible to establish meaningful production performance tolerance standards. Thus, even for a millimeter-wave ADAS circuit of 77 GHz, a high yield can be achieved.