Effect of vias on signal transmission

Simply put, every hole in the PCB can be called a via. In terms of function, the vias can be divided into two types: one is used as an electrical connection between the layers; the other is used for device fixing or positioning. In terms of process, these vias are generally divided into three categories, namely blind vias, buried vias, and through vias. The blind holes are located on the top and bottom surfaces of the printed circuit board and have a certain depth for the surface lines and the underlying inner layer connections, and the hole depth usually does not exceed a certain ratio (aperture). Buried hole refers to the connection hole in the inner layer of the printed circuit board, which does not extend to the surface of the circuit board. The above two types of holes are located in the inner layer of the circuit board, and are completed by a through hole forming process before lamination, and several inner layers may be overlapped during the formation of the via holes. The third type is called a through hole, and the hole passes through the entire circuit board and can be used to implement internal interconnection or as a component mounting positioning hole. Since the vias are easier to implement in the process and lower in cost, most printed circuit boards use it without the need for two other vias. The via holes described below are not specifically described and are considered as through holes.

From a design point of view, a via is mainly composed of two parts, one is a drill hole and the other is a pad area around the hole. The size of these two parts determines the via size. Obviously, in high-speed, high-density PCB design, designers always want the smaller the via, the better, so that more wiring space can be left on the board. In addition, the smaller the via, the smaller its own parasitic capacitance. More suitable for high speed circuits. However, the reduction in pore size leads to an increase in cost, and the via size cannot be reduced indefinitely. It is limited by process techniques such as drilling and plating: the smaller the hole, the more time it takes to drill. The longer it is, the easier it is to deviate from the center position; and when the hole depth exceeds 6 times the diameter of the hole, there is no guarantee that the hole wall can be uniformly plated with copper. For example, if a normal 6-layer PCB thickness (through-hole depth) is 50Mil, then under normal conditions, PCB manufacturers can provide a minimum diameter of 8Mil. With the development of laser drilling technology, the size of the hole can be smaller and smaller. Generally, the diameter is less than or equal to 6Mils. We call it micro hole. Micropores are often used in HDI (High Density Interconnect Structure) designs, which allow the vias to be placed directly on the pads (Via-in-pad), which greatly improves circuit performance and saves wiring space.

Vias appear as discontinuous breakpoints on the transmission line, causing signal reflections. Generally, the equivalent impedance of the via is about 12% lower than that of the transmission line. For example, the impedance of the 50 ohm transmission line is reduced by 6 ohms when passing through the via (specifically, the via size is also related to the thickness of the board, and is not absolutely reduced). However, the reflection of the via hole due to the discontinuity of the impedance is negligible. The reflection coefficient is only: (44-50) / (44 50) = 0.06. The problem of via hole is more concentrated on the parasitic capacitance and inductance.