How to Minimize the RF Effect in PCB Interconnect Design

There are a number of different ways to minimize the RF effect in a PCB interconnect design. Some of these include ensuring that the traces are not in close proximity to one another, using a ground grid, and separating RF transmission lines from other traces.

Multilayer configuration

RF effect in PCB interconnect design is a common problem. This effect occurs mainly because of nonideal circuit properties. For example, if an IC is placed on two different circuit boards, its operating range, harmonic emissions, and interference susceptibility will be drastically different.

To minimize this effect, a multilayer configuration is necessary. Such a board should have a reasonable layout, high-frequency impedance, and simple low-frequency wiring. Using the correct substrate material minimizes signal loss, and it helps maintain consistent impedance throughout the circuits. This is crucial because signals transition from the circuit to the transmission lines, and they must have constant impedance.

Impedance is another issue with PCB interconnect design. It is the relative impedance of two transmission lines, beginning at the PCB surface and extending to the connector or coaxial cable. The higher the frequency, the more difficult it is to manage the impedance. Therefore, the use of higher frequencies seems to be a significant design challenge.

Creating a ground grid

One way to reduce the rf effect is to create a ground grid on your PCB. A ground grid is a series of box sections that is connected by traces to ground. Its purpose is to minimize the signal return path, while still maintaining low impedance. The ground grid can be either a single trace or a network of overlapping traces.

The ground plane acts as a reference to calculate the impedance of signal traces. In an ideal system, the return current stays on the same plane as the signal traces. However, in real systems, the return current may deviate from the ideal path due to various factors, including variations in the copper plating of the PCB and the laminate material used.

Separating RF transmission lines from other traces

When designing circuits with multiple traces, it is important to separate RF transmission lines from the rest of the circuit. Separation of these traces is important in order to prevent crosstalk. To achieve this, it is best to space RF transmission lines at least two trace widths apart. This distance reduces the amount of radiated emissions and minimizes the risk of capacitive coupling.

RF transmission lines are typically separated from other traces by striplines. In multi-layer printed circuit boards, striplines are most easily constructed on the inner layers. Like microstrip, striplines have ground planes above and below the RF transmission line. While striplines offer better isolation than microstrip, they tend to have a higher RF loss. For this reason, striplines are typically used for high-level RF signals.

Using PTFE ceramics

RF effect is a very real concern in PCB interconnect design. Due to high frequencies, the signals traveling on a trace can shift. This causes the dielectric constant to change depending on the speed of the signal and the tracing geometry. The dielectric constant of the PCB substrate material also affects the speed of the signal.

When comparing ceramics to solder, PTFE ceramics have an edge over FEP ceramics. While the former is cheaper and easier to fabricate, it will reduce signal reliability. Besides, PTFE ceramics are less likely to absorb moisture. However, if the PTFE ceramics are covered by hydrocarbons, the moisture absorption will increase.

Using symmetric stripline routing

Stripline routing is a common approach in digital circuit design. It uses a dielectric layer sandwiched between two ground planes with signal-carrying conductors in the center. This method is called symmetric stripline. Typical stripline dimensions are s=2.0, w=3.0, t=1.0, and b=5.0.

This method has two major advantages over microstrip. It allows for smaller traces, which provide more protection against aggressor signals. In addition, stripline routing can help minimize RF impact on the interconnect design. However, it requires careful consideration of the board layer stackup and the dielectric materials between ground planes.

As for the PCB track width, it should not exceed two inches. This is important for high-speed logic, which has a rise/fall time of five nanoseconds. It is advisable to terminate high-speed logic PCB tracks with a characteristic impedance, and to avoid voids in the reference plane.