High frequency electronic equipment is the development trend, especially in the increasing development of wireless network, satellite communications, technology information products trend high speed and high frequency, and communication products trend large capacity high speed wireless transmission of voice, video and data standardization, so the development of a new generation of products require high frequency circuit board.
The communication products such as satellite system, mobile phone receiving base station and so on, must be equipped with high frequency circuit boards, which will develop rapidly in the next few years. There will be large demand for high-frequency substrates.
The basic characteristics of high frequency substrate materials require the following points:
1.The dielectric constant (Dk) must be small and very stable, usually is the square root of the transfer rate and the dielectric constant of the smaller the better signal is inversely proportional to the high dielectric constant, easy to cause the signal transmission delay.
2.The dielectric loss (Df) must be small, which mainly affect the signal transmission quality, the less the dielectric loss so that the signal loss is small.
3.The thermal expansion coefficient of copper foil is as consistent as possible, because the inconsistency causes the separation of copper foil in the cold and heat changes.
4.The low water absorbability and high water absorption will affect the dielectric constant and dielectric loss at damp.
5.The heat-resistant, anti chemical, impact strength, peeling strength and so on must be good.
In general, the high frequency can be defined as above 1GHz. The high frequency circuit board substrate is always use the fluoride substrate, such as polytetrafluoroethylene (PTFE), usually known as Teflon, usually used in more than 5GHz.In addition, it also use FR4 or PPO substrate, which can be used in 1GHz ~ 10GHz products. These three kinds of high frequency substrates are compared as follows:
The three kinds of high frequency substrate material Epoxy resin, PPO resin and fluorine resin which are used at present,epoxy resin is the cheapest, and the most expensive is fluorine resin; consider dielectric constant, dielectric loss, water absorption rate and the frequency characteristics,fluorine resin is the best, epoxy resin is the poor.When the frequency of product application is higher than 10GHz, only fluorine resin printed board can be applied. Obviously, fluorine resin high frequency substrate performance is far higher than that of other substrates, but its shortcomings is except high cost also poor rigidity, and high thermal expansion coefficient.For polytetrafluoroethylene (PTFE), in order to improve the performance, a large number of inorganic materials (such as silica SiO2) or glass cloth are used as reinforcing filler materials to improve the rigidity of the substrate and reduce its thermal expansion.In addition, because of the molecular inertia of the polytetrafluoroethylene resin, it is not easy to combine with copper foil, so it is more necessary to deal with the special surface of the copper foil.The treatment methods include polytetrafluoroethylene surface chemical etching or plasma etching, increasing the surface roughness or increasing adhesion between copper foil and polytetrafluoroethylene resin, but it may affect the performance of the dielectric.
The development of fluorine based high frequency circuit board requires cooperation among raw material suppliers, research institutes, equipment suppliers, PCB manufacturers and communication products manufacturers, so as to keep up with the rapid development of high frequency circuit boards.
