The Introduction of High Frequency Board

High frequency board refers to the high frequency circuit board which carry high frequency signal. High frequency signal can be defined as the frequency above 1GHz. High frequency board is very expensive, usually $0.6 per square centimeter. The high frequency board comprises a core plate which contain a hollow groove and a clad copper plate adhered to the upper surface and the lower surface of the core plate by flow glue.

The Material of High-Frequency Board

High frequency board are made of low dielectric constant, low high-frequency loss material. Currently most high-frequency board substrate is fluorine resin, such as polytetrafluoroethylene (PTFE), usually called Teflon, usually used in more than 5GHz situation. In addition, there are FR-4 and PPO substrate which can be used in 1GHz ~ 10GHz products, these three high-frequency substrate properties are as follows.

Among these three categories of high-frequency substrate material, epoxy resin, PPO resin and fluorine resin, epoxy resin is the cheapest, and fluorine resin is the most expensive. Take dielectric constant, dielectric loss, water absorption and frequency characteristics into consideration, fluorine resin is the best, epoxy resin is poor. When the frequency of application of the product is higher than 10GHz, only the fluorine resin PCB can be applied. Obviously, fluorine resin high-frequency substrate performance is much better than other substrates, but its shortcomings is high cost, poor rigidity, and thermal expansion coefficient. For polytetrafluoroethylene (PTFE), to improve its performance, factory will add a large number of inorganic materials (such as silica SiO2) or glass cloth to enhance the filling material, to improve the substrate stiffness and reduce its thermal expansion. Also, due to the molecular inertness of the polytetrafluoroethylene resin itself, it is not easy to combine with the copper foil, so it is necessary to perform special surface treatment for the copper foil bonding surface. The surface of the polytetrafluoroethylene can be improved by chemical etching or plasma etching to increase the surface roughness or to increase the adhesion between the copper foil and the polytetrafluoroethylene resin. However, the treatment may affect its dielectric constant. To keep up with high-frequency circuit board rapid development, raw material suppliers, research units, equipment suppliers, PCB board manufacturers and communications products manufacturers and other aspects of cooperation need to work together.

The Characteristics of High-Frequency Board

(1) The dielectric constant (Dk) must be small and stable, usually the smaller the better. The signal transmission rate is proportional to the square root of the material dielectric constant, high dielectric constant is likely to cause signal transmission delay.
(2) Dielectric loss (Df) must be small, which mainly affects the quality of signal transmission, the smaller dielectric loss, the signal loss is also smaller.
(3) The thermal expansion coefficient should be close to copper foil as much as possible, because inconsistencies in the hot and cold changes will cause copper foil separation.
(4) low water absorption will affect the dielectric constant and dielectric loss when be damp.
(5) Other heat resistance, chemical resistance, impact strength, peel strength, etc. must also be good.

The Failure Phenomenon of High-Frequency Board

High frequency board failure phenomenon is mainly caused by low dielectric constant value caused by the impact of high-frequency circuit stability. High-frequency signal has conductor skin effect, the circuit board low dielectric constant will cause high-frequency signal loss, it will make signal weakened, frequency drift, even stop vibration. The overall electrical performance is worse.

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October 2017
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