Influence of Right-Angle Traces in PCB Layout

Right-angle wiring is generally a situation that needs to be avoided as much as possible in PCB wiring, and it has almost become one of the standards for measuring the quality of wiring. So how much impact does right-angle wiring have on signal transmission? In principle, right-angle traces will change the line width of the transmission line, resulting in discontinuities in impedance. In fact, not only right-angle traces, but also sharp-angle traces may cause impedance changes.

The impact of right-angle traces on the signal is mainly reflected in three aspects: 1. The corner can be equivalent to a capacitive load on the transmission line, slowing down the rise time. 2. The impedance discontinuity will cause signal reflection. 3. EMI generated by the right-angle tip. The parasitic capacitance brought by the right angle of the transmission line can be calculated by the following empirical formula: C=61W (Er) 1/2/Z0 In the above formula, C refers to the equivalent capacitance of the corner (unit: pF), W refers to the width of the trace (unit: inch), εr refers to the dielectric constant of the medium, and Z0 is the characteristic impedance of the transmission line.

As the line width of the right-angle trace increases, the impedance there will decrease, so a certain signal reflection phenomenon will occur. We can calculate the equivalent impedance after the line width increases according to the impedance calculation formula mentioned in the transmission line chapter, and then calculate the reflection coefficient according to the empirical formula: ρ=(Zs-Z0)/(Zs+Z0). Generally, the impedance change caused by right-angle wiring is between 7% and 20%, so the maximum reflection coefficient is about 0.1.