What is high frequency PCB?
High frequency PCB is kind of special board with higher electromagnetic frequency. Generally speaking, high frequency PCB can be defined as board with the frequency above 1GHz. Its physical properties, accuracy, and technical parameters requirement is high, and which is frequently used in automotive anti-collision systems, satellite systems, radio systems and other fields. The price of high frequency PCB is usually much higher than normal circuit boards.
The features of high frequency PCB
- The impedance control requirements are relatively strict, so the line width control is very strict, with a general tolerance of about 2%.
- Due to the special base material, the adhesion of PTH copper is not high, which usually need to roughen the vias and the surface with the help of plasma processing equipment to increase the adhesion of PTH hole copper and solder mask ink.、
- It couldn’t grind the board before solder mask, otherwise the adhesion will be very poor. It could only use micro-etching potion to roughen it.
- Most of the circuit boards use PFTE (polytetrafluoroethylene) materials, which will be a lot of burrs when use with ordinary milling cutters, so special milling cutters are required.
The parameters of high frequency PCB
It is trend that electronic equipment develop to high-frequency, especially with the increasing development of wireless networks and satellite communications.IT products are moving towards high speed and high frequency, while communication devices are moving towards the standardization of voice, video and data for wireless transmission with large capacity and speed. Therefore, the development of new generation products requires high-frequency boards. Communication device 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 PCB will be in large demand.
- The thermal expansion coefficient of the base material and the copper foil of high-frequency circuit boards must be the same. If they are inconsistent, it will cause the copper foil to separate during the cold and hot changes.
- The high frequency circuit board have low water absorption. If it is with high water absorption will cause dielectric constant and dielectric loss when wet.
- The dielectric constant (Dk) of the high-frequency PCB board 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 easily cause signal transmission delay.
- The dielectric loss (Df) of the high frequency circuit board material must be small, which mainly affects the quality of signal transmission. The smaller the dielectric loss, the smaller the signal loss.
- Also, the heat resistance, chemical resistance, impact strength, peel strength etc. of the high-frequency PCB board substrate material must be good.
Currently, the most commonly used high frequency PCB board substrate is a fluorine-based dielectric substrate, such as polytetrafluoroethylene (PTFE), also called Teflon, and it is usually used for the circuit above 5GHz. There are also FR-4 or PPO substrates, which can be used for products between 1GHz and 10GHz.
- Considering the cost, the three types of high frequency PCB board substrate materials: epoxy resin, PPO resin and fluorine-based resin, epoxy resin is the cheapest, and fluorine-based resin is the most expensive.
- Considering dielectric constant, dielectric loss, water absorption and frequency characteristics, fluorine resin is the best, epoxy resin is inferior.
When the frequency of the PCB application is higher than 10GHz, only the fluorine-based resin substrate can be applied. Obviously, the performance of fluorine-based resin high-frequency substrates is much higher than that of 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 its 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 the copper foil, so special surface treatment of the bonding surface with the 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 PCB circuit boards, the development of the whole fluorine-based high-frequency board substrate requires the cooperation of raw material suppliers, research companies, equipment suppliers, PCB manufacturers and communication devices manufacturers.