What Are High-Speed Materials for PCB?
Due to the development of technology and 5G cellular communication systems, the creation of high-speed digital is increasingly important. For high-speed printed circuit boards (PCB), they must be able to provide excellent performance in terms of signal and frequency transmission since they are required to handle mechanical and electrical characteristics without impeding their operational capabilities.
The major concern of high-speed PCB is how to maintain the integrity of the digital signals over a wide range of high-speed frequencies. The reason is that a high-speed signal is a square-wave signal composed of a combination of several sine waves. A high-speed digital signal contains a fundamental signal, a third-harmonic signal, a fifth harmonic signal, a seventh harmonic signal, and many other add harmonic signals of higher frequencies. Consequently, it is essential to transfer millimeter-wave (MMW) signals with the lowest possible loss and distortion in order to maintain a digital signal’s integrity and the sharpness of its rise and fall times.
If we choose inappropriate materials for high-speed PCB, it will result in poor plated-through holes and even impedance discontinuities in transmission lines on multi-layer PCB. Therefore, designers use a series of material parameters such as the dissipation factor (Df) and the dielectric constant (Dk) to determine the suitability of high-speed PCB materials because they are related to high-frequency performance. For example, compared with a medium-loss material with a dissipation factor of 0.010, a low-loss material with a dissipation factor of 0.005 or even lower is more suitable for digital signals at 10Gbps speeds. In high-speed PCB, the dissipation factor(Df) is a measure of loss issues, signal integrity, and signal distortion minimization. As for the dielectric constant (Dk), it mainly affects the impedance of transmission lines on the substrate, thereby changing the performance of high-speed digital circuits.
FR-4 is a favored PCB material in many applications, but it is not acceptable as a PCB material to high-speed digital circuits and microwave and millimeter-wave signals. The main reason is that FR-4 material cannot satisfactorily introduce insertion loss and distortion. In order to select the proper material for high-speed PCB, the designer starts with the dielectric constant(Dk). Thus, material suppliers describe their materials according to the following aspects: dielectric constant in the z-axis or the x-y plane and at a typical test frequency such as 1GHz. Furthermore, high-speed designs have a high requirement in channels such as close matching with phase and amplitude. Finally, PCB materials able to handle high-speed digital signals must be able to manage signals rich in harmonic content. Enhanced epoxy is a material with better electrical properties, which is suitable for multi-layer high-speed PCB. The applications utilizing high-speed PCB include servers, routers, storage area networks, power amplifiers, transceiver modules, and high-speed data channels.
There are two typical high-speed PCB materials: Rogers 4350B and Megtron 6, both of them are low-loss materials with similar low dissipation factor (Df) and dielectric constant (Dk) values. Both materials are based on hydrocarbon resins and are more costly than ordinary FR-4 laminates. Neither of these laminates can be clad with a quarter ounce of copper. In order to prevent high-frequency signal reflection, they are available with low-profile foils. For Rogers 4350B, it is much more expensive than Megtron 6 because prepregs used for Rogers 4350 core need higher pressure, and Rogers 4350 core material is ideally flat and repeatable, which helps with impedance control. As for Megtron 6, laminates of Megtron 6 are just like conventional FR-4 materials, which do not involve incompatible pressure, temperature, movement, or curing time. In the stack-up, the hybrid board can be constructed in a single laminate with an inner layer of less costly FR-4 material and an outer layer or multiple layers of Megtron 6 using a foil or a cap structure. In addition, a wider selection of Megtron 6 core material, prepreg thickness and resin content simplifies stack-up development and impedance control. The applications of Rogers 4350B are automotive radar and sensor, etc. The applications of Megtron 6 are high-speed transfer and computing, etc.