Influence of vias in high-speed PCB In high-speed PCB multilayer boards, when signals are transmitted from one layer of interconnects to another, they need to be connected through vias. When the frequency is lower than 1GHz, the via can play a good connection role, and its parasitic capacitance and inductance can be ignored. When the frequency is higher than 1GHz, the influence of via parasitic effect on signal integrity can not be ignored. At this time, the via appears as a breakpoint with discontinuous impedance on the transmission path, which leads to signal integrity problems, such as signal reflection, delay and attenuation. When the signal is transmitted to another layer through the via, the reference layer of the signal line also acts as the return path of the via signal, and the return current will flow between the reference layers through capacitive coupling, resulting in problems such as grounding elasticity. Types of via holes are generally divided into three categories: through holes, blind holes and buried holes. Blind hole: It is located on the top and bottom surfaces of the printed circuit board and has a certain depth. It is used to connect the surface circuit with the inner circuit below, and the depth and aperture of the hole usually do not exceed a certain ratio. Buried hole: refers to the connection hole located in the inner layer of the printed circuit board, which does not extend to the surface of the circuit board. Through-hole: This kind of hole passes through the whole circuit board and can be used for internal interconnection or as an installation positioning hole for components. Because through holes are easier to realize in technology and lower in cost, they are commonly used in printed circuit boards. Parasitic Capacities of Via There is a parasitic capacitance to the ground in the via itself. If the diameter of the isolation hole on the ground floor of the via is D2, the diameter of the via pad is D1, the thickness of the PCB is T, and the dielectric constant of the board substrate is ε, the parasitic capacitance of the via is approximately: C =1.41εTD1/(D2-D1) The parasitic capacitance of the via mainly affects the circuit by prolonging the rising time of the signal and reducing the circuit speed. The smaller the capacitance, the smaller the impact. Parasitic inductance of vias The vias themselves have parasitic inductance. In the design of high-speed digital circuits, the harm of parasitic inductance in vias is often greater than that of parasitic capacitance. The parasitic series inductance of vias will weaken the function of bypass capacitor and the filtering effect of the whole power supply system. If l represents the inductance of the via hole, h represents the length of the via hole, and d represents the diameter of the center hole, the parasitic inductance of the via hole is about: l = 5.08h [ln (4h/d) 1] From the formula, it can be seen that the diameter of the via hole has little influence on the inductance, but the length of the via hole has the greatest influence on the inductance.
