In a high-speed printed circuit board (PCB), through holes have been criticized for degrading signal integrity performance. However, the use of vias is inevitable. On a standard circuit board, the components are placed on the top layer, while the traces of the differential pair are on the inner layer. The electromagnetic radiation of the inner layer and the crosstalk between pairs are low. Vias must be used to connect the components on the plane of the circuit board to the inner layer.

Fortunately, a transparent via can be designed to minimize the impact on performance.

  1. Basic knowledge of via structure

  Let's start by examining the components that connect the top transmission line to the inner layer in a simple via. Fig. 1 is a 3D diagram showing the via structure. There are four basic components: signal vias, via stubs, via pads and isolation pads.

  Vias are metal cylinders plated outside the through holes between the top and bottom layers of the circuit board. Signal vias connect transmission lines on different layers. The via stub is the unused part of the via. Via pads are ring-shaped spacers that connect the via to the top or internal transmission line. Isolation disks are annular gaps in each power or ground plane to prevent short circuits to the power and ground planes.

By balancing the size of the inductance and parasitic capacitance, a via with the same characteristic impedance as the transmission line can be designed, so that it will not have a special impact on the operation of the circuit board. There is no simple formula to convert between via size and C and L components. The 3D electromagnetic (EM) field calculation program can predict the structural impedance based on the dimensions used in the PCB layout. By repeatedly adjusting the structure size and running 3D simulation, the via size can be optimized to achieve the required impedance and bandwidth requirements.

3. Design a transparent differential via

  We have discussed in the previous post that when implementing a differential pair, the line A and line B must be highly symmetrical. These pairs are routed in the same layer. If a via is required, a hole must be punched in the vicinity of the two lines. Since the two vias of the differential pair are very close, an elliptical isolation disk shared by the two vias can reduce parasitic capacitance instead of using two separate isolation disks. Ground vias are also placed next to each via so that they can provide ground return paths for the A and B vias.

  Figure 2 shows an example of a ground-signal-signal-ground (GSSG) differential via structure. The distance between two adjacent vias is called the via pitch. The smaller the via pitch, the more mutual coupling capacitance.