The high frequency of electronic equipment is the development trend, especially with the increasing development of wireless networks and satellite communications, information products are moving towards high speed and high frequency, and communication products are moving towards the standardization of voice, video and data for wireless transmission with large capacity and speed. Therefore, the development of a new generation of products requires high-frequency substrates. Communication products such as satellite systems and mobile phone receiving base stations must use high-frequency circuit boards. In the next few years, they will inevitably develop rapidly, and high-frequency substrates will be in large demand.
① The high-frequency circuit board substrate should have low water absorption, and high water absorption will cause dielectric constant and dielectric loss when wet.
②The thermal expansion coefficient of the high-frequency circuit board base material and copper foil must be the same. If they are inconsistent, it will cause the copper foil to separate during the cold and hot changes.
③The dielectric constant (Dk) of the high-frequency circuit board substrate must be small and stable. Generally speaking, the smaller the better. The signal transmission rate is inversely proportional to the square root of the material’s dielectric constant. High dielectric constant is likely to cause signal transmission. Delay.
④The dielectric loss (Df) of the high-frequency circuit board substrate material must be small, which mainly affects the quality of signal transmission. The smaller the dielectric loss, the smaller the signal loss.
⑤Other heat resistance, chemical resistance, impact strength, peel strength, etc. of the high-frequency circuit board substrate material must also be good. Generally speaking, high frequency can be defined as the frequency above 1GHz. At present, the most commonly used high-frequency circuit board substrate is a fluorine-based dielectric substrate, such as polytetrafluoroethylene (PTFE), which is usually called Teflon, and is usually used above 5GHz. In addition, FR-4 or PPO substrates are also used, which can be used for products between 1GHz and 10GHz.
At this stage, the three types of high-frequency circuit board substrate materials: epoxy resin, PPO resin and fluorine-based resin, epoxy resin is the cheapest, and fluorine-based resin is the most expensive; and the dielectric constant, dielectric loss, Considering water absorption and frequency characteristics, fluorine resin is the best and epoxy resin is inferior. When the frequency of the product application is higher than 10GHz, only the fluorine-based resin printed board can be applied. Obviously, the performance of fluorine-based resin high-frequency substrates is much higher than other substrates, but its shortcomings are low rigidity and large thermal expansion coefficient in addition to high cost. For polytetrafluoroethylene (PTFE), in order to improve performance, a large amount of inorganic substances (such as silica SiO2) or glass cloth are used as reinforcing fillers to increase the rigidity of the substrate and reduce its thermal expansion.
In addition, due to the molecular inertness of the PTFE resin itself, it is not easy to bond with copper foil, so special surface treatment of the bonding surface with copper foil is required. The treatment method includes chemical etching or plasma etching on the surface of PTFE to increase the surface roughness or add a layer of adhesive film between the copper foil and the PTFE resin to improve the bonding force, but it may affect the performance of the medium. In order to keep up with the rapid development of high-frequency circuit boards, the development of the entire fluorine-based high-frequency board substrate requires the cooperation of raw material suppliers, research units, equipment suppliers, PCB manufacturers, and communication product manufacturers needs.