RF PCB Designs Bend in Both Directions
For RF PCBs, which are used to transmit and receive electromagnetic signals in cellular phones, wireless sensor networks, and other IoT devices, precision is critical. RF circuitry requires tight impedance control to prevent interference and signal loss. This means ensuring that the characteristic impedance of a signal trace is not changed by the circuit layout, PCB material choices, or design techniques.
A good way to achieve this is to use a stripline transmission line, which has the signal trace on top and the return path or ground plane underneath. Dielectric material between the two layers helps to keep the characteristic impedance low. In this type of layout, the signal trace width, spacing, and thickness are all carefully chosen to meet impedance requirements.
It’s important to note that, even when the signal’s characteristic impedance is controlled, interference can occur if other components are placed too close together. This is especially true at high microwave frequencies. This interference is called crosstalk, and it can be caused by the coupling of energy from a signal to nearby lines or to other conductive objects in the environment around the RF circuit. This is why RF circuits are generally designed to be as isolated as possible from other components, ideally with no adjacent tracks or power supplies.
In addition to keeping traces separated, another way to minimize interference is to place a large decoupling capacitor at the end of each power supply line (i.e., the main supply decoupling capacitor). This will help to reduce the effect of the circuit on its surroundings. Typically, this will be several tens of uFds in value and mounted on the inner layer of the board.
Can RF PCB Designs Bend in Both Directions?
RF circuits are a lot more sensitive to noise than traditional digital electronics, which makes achieving impedance control even more challenging. This is because the circuits have more moving parts, and their characteristics may change as they move. This is why a careful layout is required for rf pcb design.
In general, a bending radius of 3x the trace width is recommended for a dynamic bend, while 10x the trace width is recommended for one-time bending. For a PCB that will be subject to frequent flexing, it’s also important to consider the material choice. PTFE-based PCB materials like Rogers RT/duroid 5880 are great options, as they provide RF performance and durability. These materials do not have glass fabric reinforcement, which means they’re less prone to cracking or brittleness with flexing. Additionally, they can be formed into complex shapes without the need for a metal mandrel.
It’s also a good idea to avoid placing viaholes or plated-through holes (PTHs) in the area of the PCB that will undergo the most flexure. This will protect them from stress and cracking, which could damage the underlying copper layer. This is because flexure causes the surface of the metal to expand and contract, which can cause metallurgical defects. As a result, avoiding these areas is the best way to minimize reliability problems with a flexible RF circuit.
