Design Guidelines for RF and Microwave PCB Assembly

The RF and Microwave industry is fast-paced, and the push to produce PCB assembly faster and cheaper continues. This aggressive pace has led to PCB design rules conflicting with component requirements. A disconnect between designers and manufacturing causes these rules to be followed blindly, eliminating opportunities to improve performance.

These issues require the designer to be aware of potential problems and pitfalls to avoid them. This guide discusses some of the challenges specific to RF and microwave circuit design and some suggestions on how to deal with them, covering everything from essential design tips to practical recommendations for high-volume RF assembly.

Essential Design Guidelines for RF And Microwave PCB Assembly

Here are some of the pitfalls of this process in an attempt to prevent them from happening.

Component Placement

Components should be placed as close to the center of the board as possible. This minimizes parasitic inductance, which can degrade performance at higher frequencies. Components should also be placed as close to their mating connectors as possible. This decreases coupling between traces and improves signal integrity. For example, if you are designing a high-speed digital interface board, keeping traces on opposite sides of the board at a minimum distance from each other is crucial. This minimizes crosstalk between signals on different layers of the board.

Poor Grounding

Grounding is a critical aspect of any electrical circuit. If there are any poor grounds in your design, you may end up causing interference to other circuits or even damage your circuit. For instance, if you have a trace running along both sides of your board, then one side may have a better connection than another side (due to different environmental conditions). This can cause problems. To avoid this situation, ensure that all traces are well-grounded throughout their length by using multiple vias and thick traces.

Routing optimization

 Routing paths can be optimized to reduce noise by eliminating vias (interconnection points) in areas that are not needed. Vias are often used as ground planes that help reduce radiated emissions from aboard. However, vias can also act as antennas if they are located near a high-impedance point on the board, such as power supply lines. In this case, they may radiate electromagnetic energy into free space or other nearby conductors such as traces or cables the ground plane.

Loop Areas

Loop areas cause signal degradation due to their high impedance compared to linear traces. So minimize loop areas by using multiple vias instead of single vias where possible, or place them as close as possible to component pads so that there is a minimal length between them and component pads that need to be connected with a via.

Conducted Noise

Conducted noise travels through a conductive path, such as a trace or ground plane. Conducted noise can be caused by switching transistors, microcontrollers, or other electronic devices on your circuit board. When these devices switch off, they can send out a spike of current that can create electromagnetic fields that radiate from your circuit board. These spikes are called transient current spikes, and they can travel through the ground plane on your circuit board, creating electrostatic discharge (ESD) events if they reach the antenna at the end of your circuit board.

Using Multilayer Boards

The more layers you add to your board, the more copper surface area will be available to dissipate heat and provide current return paths. This means that multilayer boards are ideal for high-frequency applications such as RF/microwave systems, where it is essential to dissipate heat efficiently while maintaining a low impedance return path for high-frequency currents such as those found in antennas.

Return Signal Loss

An antenna's loss of the return signal is a significant consideration in RF design. The return loss of any RF board is defined as the power ratio between the forward and reflected waves at a given frequency. In an ideal situation, this value should be greater than -20 dB across all frequencies of interest. However, in practice, you should consider -10 dB as a minimum requirement for most applications.

The main contributors to return loss include impedance mismatches and leakage currents in the ground plane. The impedance mismatch between the antenna and its associated components can cause reflections that result in reduced efficiency and poor performance. For example, ensuring that the impedance match between an antenna and a coaxial cable is within +/- 2% will help ensure good efficiency.

Leakage currents can also cause considerable losses by reflecting signals into their source circuit or radiating them out into free space, causing interference with other devices on your board or your system.

PCB Laminate Properties

PCB laminate properties are fundamental to the performance of your circuit board. The available types of laminates have different characteristics, including dielectric constant (aka k-factor), loss tangent, and temperature coefficients. These factors all play a role in determining how well your circuit board performs over its lifetime. These properties can be affected by many things, including the thickness of the PCB material, the type of substrate used, and even the number of layers present on the circuit board itself.

The most important factor when choosing a PCB laminate is ensuring it is compatible with your design requirements. For example, if you need high-frequency performance, you will likely want to select a high dielectric constant material such as Rogers Core or Rogers 3705 PTFE laminate. However, if you do not need high-frequency performance, a lower k-factor material such as FR4 may be sufficient for your needs.

Transmission Lines and Vias

Transmission lines are used to conduct RF signals from one place to another. The transmission lines have different impedance values and can be classified according to their electrical length. The impedance is the resistance to current flow caused by inductance, capacitance, and other electrical properties. Transmission lines typically consist of a single conductor or a pair of conductors (shielded or unshielded). The impedance of the transmission line depends on its physical length and the frequency of operation.

The primary purpose of transmission lines is to reduce signal reflection during the transition between two devices on a PCB. To do this, you must ensure no discontinuity in the line that could cause reflections or standing waves. This requires careful design planning when placing components on your board layout and choosing the right transmission line design for your application.

There are three transmission lines: coaxial cable, microstrip line, and slotline.

Coaxial Cable

Coaxial cables are used for high-frequency applications up to 40 GHz since they can handle high currents with little loss over their entire bandwidth range. Coaxial cables are usually made from copper wire but other materials such as silver-plated copper can also be used depending on the frequency band and power requirements of the application.

Microstrip

Microstrip is the most commonly used transmission line. It consists of a ground plane on one side and an elevated signal trace on the other side of the PCB laminate. The ground plane should be wider than the signal trace, so there is no interaction between them. The thickness of the dielectric layer should be a minimum of 1/4 inch thick for millimeter-wave frequencies and 1/2 inch thick for UHF or higher frequencies. At lower frequencies, you can use thinner dielectrics such as FR-4 or Rogers 4350 without increasing losses due to fringing fields between the ground plane and the signal trace.

Stripline

Stripline is similar to microstrip except that it has no ground plane on one side or both sides of the PCB laminate. It has less loss than microstrip because there are no fringing fields between two conductors. Stripline is often used when high-speed signals must be transmitted over long distances without causing reflections from impedance mismatch at bends in the stripline or connectors mounted on its ends.

Power Plane Clearance

The power plane clearance is the distance between two conductive layers. The purpose of designing such a clearance is to reduce the unwanted coupling between different signal layers or between a signal layer and ground plane or reference plane. It also limits interference between neighboring traces and prevents crosstalk between adjacent signal layers. It also helps to avoid arcing or sparking at sharp corners, which is dangerous and affects the circuit's performance.

The minimum recommended clearance for a power plane is 0.15 mm from any other signal layer, including vias on that same layer. For example, if there are two signal layers in close proximity, then you should make sure that there is at least 0.15 mm between them so as not to cause any interference problems between them due to capacitive coupling or inductive coupling effects from nearby vias or traces on these two layers.

Key Takeaway

The purpose of this article was to provide a general introduction to the design and assembly challenges associated with RF and microwave PCB manufacturing. The challenges vary widely depending on whether you are designing complex boards or working with subminiature components. Although what we have presented here is far from comprehensive, hopefully, it provides a useful starting point for anyone interested in learning more about RF and microwave PCBs. Find more information at Hemeixin to get started on your next project.