Introduction to the Application and Substrate Materials of Fluorine-based High Frequency Printed Circuit Boards

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Introduction to the Application and Substrate Materials of Fluorine-based High Frequency Printed Circuit Boards [Copy link]

The high frequency of electronic equipment is a 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 large capacity and fast wireless transmission of voice, video and data standardization. Therefore, the new generation of products requires high-frequency substrates. The applications of high-frequency printed circuit boards are as follows: Application Frequency of use Cellular & Pager Telecom. 1 ~ 3 GHz Individuals receive base station or satellite transmissions 13 ~ 24 GHz Car Collision Avoidance System (CA) 75GHz Direct Broadcast Satellite System (DBS) 13GHz Satellite Downconverter (LNB/LNA) 2 ~ 3GHZ Home satellite reception 12 ~ 14GHz Global Positioning System (GPS) -40 ~85℃ 1.57/1.22GHz Satellite reception for cars and individuals 2.4GHz Wireless portable communication antenna system 14GHz Satellite Small Earth Station (VSAT) 12 ~ 14GHz Digital microwave system (base station to base station reception) 10 ~ 38GHz For the satellite systems, mobile phone receiving base stations and other communication products listed in the table above, high-frequency circuit boards must be used. They will inevitably develop rapidly in the next few years, and there will be a large demand for high-frequency substrates. The basic characteristics of high-frequency substrate materials are as follows: (1) The dielectric constant (Dk) must be small and very stable. Generally, the smaller the better. The signal transmission rate is inversely proportional to the square root of the dielectric constant of the material. A high dielectric constant is likely to cause signal transmission delay. (2) The dielectric loss (Df) must be small, which mainly affects the quality of signal transmission. The smaller the dielectric loss, the smaller the signal loss. (3) The thermal expansion coefficient should be as consistent as possible with that of the copper foil, because inconsistency will cause the copper foil to separate during hot and cold changes. (4) The water absorption should be low. High water absorption will affect the dielectric constant and dielectric loss when it is damp. (5) Other heat resistance, chemical resistance, impact strength, peel strength, etc. must also be good. Generally speaking, high frequency can be defined as frequencies above 1GHz. Currently, the most commonly used high-frequency circuit board substrates are fluorine-based dielectric substrates, such as polytetrafluoroethylene (PTFE), commonly known as Teflon, which are usually used above 5GHz. In addition, there are FR-4 or PPO substrates, which can be used for products between 1GHz and 10GHz. The physical properties of these three high-frequency substrates are compared as follows. Physical properties Fluorine-based polymers Ceramic PPO Epoxy FR-4 Dielectric constant (Dk) 0.001~3.0 0.04~ 3.38 0.05~ 4.4 Dielectric loss (Df) 10GHz 0.0013 0.0027 0.02 Peel strength (N/mm) 1.04 1.05 2.09 Thermal conductivity (W/m/0K) 0.50 0.64 -- Frequency range 300MHz ~ 40GHz 800MHz ~ 12GHz 300MHz ~ 4GHz Temperature range (℃) -55 ～ 288 0 ～ 100 -50 ～ 100 Transmission speed (In/sec) 7.95 6.95 5.82 Water absorption (%) Low middle high At present, the three major types of high-frequency substrate materials used are epoxy resin, PPO resin and fluorine resin. Epoxy resin is the cheapest, while fluorine resin is the most expensive. In terms of dielectric constant, dielectric loss, water absorption and frequency characteristics, fluorine resin is the best and epoxy resin is worse. When the frequency of the product application is higher than 10GHz, only fluorine resin printed boards can be used. Obviously, the performance of fluorine resin high-frequency substrates is much higher than other substrates, but its shortcomings are poor rigidity and large thermal expansion coefficient in addition to high cost. For polytetrafluoroethylene (PTFE), in order to improve the performance, a large amount of inorganic substances (such as silicon dioxide SiO2) or glass cloth is used as reinforcing filling materials to increase the rigidity of the substrate and reduce its thermal expansion. In addition, due to the molecular inertness of polytetrafluoroethylene 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 methods include chemical etching or plasma etching on the surface of polytetrafluoroethylene to increase the surface roughness or adding an adhesive film layer between the copper foil and the polytetrafluoroethylene resin to improve the bonding strength, but this may affect the dielectric performance. The development of the entire fluorine-based high-frequency circuit substrate requires cooperation from raw material suppliers, research institutions, equipment suppliers, PCB manufacturers, and communication product manufacturers to keep up with the rapid development of the high-frequency circuit board field.