Typically Used in RF Boards
RF boards facilitate wireless communication standards like Bluetooth, WiFi, ZigBee, RFID, LoRa and cellular. They offer precision 50- or 75-ohm impedance-matched transmission lines that reduce interference and increase power transfer efficiency, range and signal integrity. They are designed with specialized laminates and advanced substrate materials with high dielectric constant, low loss tangent and low coefficient of thermal expansion. This material selection satisfies the specific requirements of RF PCB applications.
PCBs are made of several layers of copper and insulator materials that support conductive traces and signal path connectivity. The choice of materials is based on several factors including the circuit’s intended functionality and environmental conditions. The right insulator can significantly affect the quality of an RF board and its performance.
The ideal PCB material for an rf board should have a high dielectric constant (Dk), low dissipation factor, and a stable Dk over a wide range of frequency. It should also have a low coefficient of thermal expansion and contraction, which ensures that the board is structurally sound and has minimal distortion at extreme temperatures. The material should also have a low dielectric loss and good thermal conductivity to minimize heat buildup in the assembly, which can cause component degradation and shorten a circuit’s lifespan.
What Materials Are Typically Used in RF Boards?
For RF and microwave PCBs, the dielectric loss of a PCB material is affected by its copper foil thickness. This is because a conductor’s surface roughness will create greater losses than would be the case for a smooth conductor, owing to the skin effect that causes current to occupy only the outermost edges of a conductor’s cross-section. Using higher-Dk PCB materials helps to overcome this effect and improves signal transmission efficiency.
Typical FR-4 PCBs are suitable for RF applications up to WiFi frequencies, but they become unsuitable beyond this point. Choosing a higher-Dk insulator like PTFE, Rogers, or ceramic substrates will ensure that your circuit will operate at the desired frequency without any signal-loss problems.
While the choice of RF PCB materials is important, it’s equally critical to understand how these materials will behave during the fabrication process. RF PCBs are typically exposed to intense soldering stress and harsh environments that can affect their structural integrity. For this reason, it’s essential to choose a durable PCB material that can withstand high levels of soldering while maintaining its electrical and thermal properties.
PTFE with micro-glass fiber or woven glass is the most common RF PCB material. It offers the best combination of high-Dk and cost, but it isn’t suited to all budgets. For those with a more limited budget, there are options like ceramic filled PTFE that offer similar characteristics at a lower price. Alternatively, multilayer RF boards can be designed with different materials for each layer. This enables designers to use expensive Rogers or PTFE material on the RF layers and save money by using cheaper FR-4 material in the other layers of the board. This provides the best of both worlds and gives engineers the freedom to create more component-dense RF circuits.