RF&Microwave PCB manufacturing microwave high frequency | YMSPCB

RF PCB standard：

1. Low power RF PCBmainly uses standard FR4 material (good insulation properties, uniform material, dielectric constant ε=4, 10%).
2. In the RF PCB design, each component should be closely arranged to ensure the shortest connection.
3. For a mixed-signal PCB, the RF and analog partsshould be far away from the digital part (the distance is usually more than 2cm, at least 1cm), and the ground of the digital part should be separated from the RF part.
4. When selecting components to work in a high-frequency environment, use SMD(Surface Mounted Devices) as much as possible. SMD are small in size, and the component pins are generally very short.

This article provides guidelines and suggestions for radio frequency (RF) printed circuit board (PCB) design and layout, including some discussion of mixed-signal applications, such as digital, analog, and RF components on the same PCB. The content is organized by topic areas and provides “best practice” guidelines, which should be applied in conjunction with all other design and manufacturing guidelines. These guidelines may also apply to specific components, PCB manufacturers, and materials.

RF Transmission Lines

This transmission line includes a fixed-width metal trace (conductor) and a ground area located directly underneath (on the adjacent layer). For example, a microstrip on layer 1 (top metal) requires a solid ground area on layer 2 (Figure 1). The width of the trace, the thickness of the dielectric layer, and the dielectric determine the characteristic impedance (usually 50Ω or 75Ω)

Ground Area

If layer 1 is used for radio frequency components and transmission lines, it is recommended to use a solid (continuous) ground area on layer 2. For strip lines and offset strip lines, the upper and lower intermediate conductors require grounding areas. These areas must not be shared or allocated to signal or power networks but must be allocated to the ground. Sometimes limited by design conditions, a local ground area on a specific layer must be located under all radio frequency components and transmission lines. The ground area must not be disconnected under the transmission line.

A large number of ground vias should be placed between different layers of the RF part of the PCB. This helps prevent ground current loops from increasing parasitic ground inductance. Vias also help prevent cross-coupling of RF signal lines and other signal lines on the PCB.

Special Considerations for power and ground layers

For the circuit board layers allocated to the system power (DC power) and ground, the loop current of the components must be considered. The general principle is to avoid placing signal lines on the circuit board layer between the power and ground layers.

Figure 7. Incorrect circuit board layer assignment: There are signal layers between the power layer and the ground current loop on the ground layer. Bias line noise can couple to the signal layers.

Figure 8. Better layer assignment: There is no signal layer between the power layer and the ground layer.

Power (Bias) Traces and Power Decoupling

If the component has multiple power connections, a common practice is to use power wiring in a “star” configuration (Figure 9). Install larger decoupling capacitors (tens of µF) at the “root” node of the star configuration, and install smaller capacitors on each branch. The value of these small capacitors depends on the operating frequency of the RF IC and its specific functions (i.e., decoupling between stages and the main power supply). An example is shown below:

Figure 9. If a component has multiple power connections, the power wiring can be in a star configuration.

Compared with the configuration in which all pins are connected to the same power network in series, the “star” configuration avoids long ground loops. Long ground loops will cause parasitic inductance, which can cause unexpected feedback loops. The key consideration for power supply decoupling is that the DC power supply must be electrically connected to an AC ground.

Selection of Decoupling and Bypass Capacitors

Due to the existence of self-resonant frequency (SRF), the capacitor’s effective frequency range, in reality, is limited. SRF can be obtained from the manufacturer, but sometimes must be characterized by direct measurement. Above SRF, the capacitor is inductive, so it has no decoupling or bypassing function. If broadband decoupling is required, the standard approach uses multiple (capacitance values) of increased capacitors, all in parallel. The SRF of a small capacitor is generally larger (for example, the SRF of a 0.2pF, 0402 SMT package capacitor = 14GHz). The SRF of a large capacitor is generally smaller (for example, the SRF of a 2pF capacitor in the same package = 4GHz). Table 2 lists typical configurations.

Table 2. Effective frequency range of capacitors

Bypass capacitor layout considerations

Since the power lines must be AC ground, it is essential to minimize the parasitic inductance of the AC ground loop. The component layout or placement direction may cause parasitic inductance, such as the ground direction of decoupling capacitors. There are two ways to place the bypass capacitor, as shown in Figure 10 and Figure 11:

Figure 10. The total area of the bypass capacitor and related vias is the smallest.

Figure 11. This configuration requires a larger PCB area.

In this configuration, connecting the VCC pad on the top layer to the via hole of the inner power supply region (layer) may hinder the AC ground current loop, forcing a longer loop, resulting in higher parasitic inductance. Any AC flowing into the VCC pin passes through the bypass capacitor, reaches its ground side, and then returns to the inner ground plane. In this configuration, the total footprint of the bypass capacitor and related vias is the smallest. In another configuration, the AC ground loop is not restricted by the vias in the power supply area. Generally speaking, this configuration requires a slightly larger PCB area.

Grounding of crowbar-connected components: For crowbar-connected (grounded) components such as power supply decoupling capacitors, it is recommended to use at least two ground vias for each component (Figure 12), reducing the influence of via parasitic inductance. The short-circuit connection component group can use via grounding “island.”

Figure 12. Using at least two ground vias per component can reduce the effect of via parasitic inductance.

IC Ground Area (“pad”)

Most ICs require a solid ground area on the component layer (the top or bottom layer of the PCB) directly below the component. This ground area will carry DC and RF return flow through the PCB to the distributed ground area. The second function of the component “ground pad” is to provide a heat sink, so the pad should include the maximum number of vias as permitted by the PCB design rules.

 In the example shown in the figure below, a 5 × 5 via array is installed in the middle ground area (on the component layer) directly below the RF IC (Figure 13). Where other layout considerations allow, the maximum number of vias should be used. These vias are ideal vias (through the entire PCB). These vias must be plated. If possible, use the thermal paste to fill the vias to improve heat dissipation (fill the thermal paste after plating the vias before finally plating the circuit board).

Figure 13. A 5 × 5 via array on the middle ground area directly below the RF IC

　　All high-frequency PCB operating beyond 100 MHz are termed as RF PCBs whereas a Microwave RF PCB operates above 2GHz. The development procedures involved in RF PCBs are different as compared to conventional PCBs. RF Microwave PCBs are much more sensitive towards various parameters which have no effects over ordinary PCBs. So, the development has too carried out in a controlled environment with the required expertise.

RF Microwave PCB applications