The substrate materials of High-frequency PCBs

High-frequency PCBs show excellent performance in the design and application of modern electronic products, especially in wireless communication and high-speed digital application. With benefits like high efficiency, fast speed, lower attenuation, strong tolerance, and constant dielectric properties, high-frequency PCBs perform well in electronic products with special signal requirements. The substrate materials of High-frequency PCBs include Rogers, Arlon, F4B., etc.

The parameter characteristics of substrate material

To provide faster signal flow rates and higher transmission frequencies, selecting the right substrate material for the high-frequency PCBs is particularly important.

(1) The thermal expansion coefficient of the high-frequency circuit board substrate and the copper foil must be consistent, and if not constant, it will cause the copper foil to separate during the change in heat and cold.

(2) High-frequency circuit board substrate material needs a low water absorption. High water absorption will cause dielectric constant and dielectric loss at the time of being affected with damp.

(3) The dielectric constant (Dk) of high-frequency board substrate material must be small and stable. Generally, the smaller, the better. Because signal transmission rate and the material dielectric constant square root are inversely proportional, and high dielectric constant leads to signal transmission delay easily.

(4) The dielectric loss (Df) of high-frequency board substrate material must be low, which mainly affects the quality of signal transmission. The low the dielectric loss, the less the signal loss.

(5) Other performance of the high-frequency board substrate material like heat resistance, chemical resistance, impact strength and peel strength should also be good.

The types of high-frequency PCB substrate materials

Epoxy resin, PPO resin, and fluorine-based resin are the three widely used high-frequency PCB substrate materials. Epoxy resin is the cheapest, and fluorine-based resin is the most expensive. The fluorine-based resin is the best from the aspects of the dielectric constant, dielectric loss, water absorption, and frequency characteristics; epoxy resin performs poorly.

When the frequency of electronics product application is higher than 10GHz, the only suitable material is fluorine-based resin. The performance of the high-frequency substrate of fluorine-based resin is much higher than that of other substrates. Still, its disadvantages are high cost, poor rigidity, and a large coefficient of thermal expansion. For polytetrafluoroethylene (PTFE), inorganic materials or glass cloth can be used as reinforced filling materials to reduce its thermal expansion and improve the rigidity of the substrate.

In addition, due to the molecular inertia of PTFE resin itself, it is not easy to combine with copper foil, so it needs special surface treatment to connect with copper foil. The treatment methods include chemical etching or plasma etching on the surface of PTFE, or increasing the surface roughness, adding a layer of adhesive film between the copper foil and PTFE resin to improve the binding force, but it may affect the performance of the dielectric.

To keep up with the rapid development of high-frequency PCBs, developing substrate material of high-frequency PCBs requires cooperation from raw material suppliers, research units, equipment suppliers, PCB manufacturers to communications product manufacturers.

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