No Right Angle Alignment in PCB Layout

Dated:2017-12-04      Popularity:1369


Right-angle alignment is generally avoided in PCB wiring layout, and become one of the criteria to measure the quality of wiring layout. But do right angle-alignment influence the signal transmission?

PCB wiring layout

In principle, right-angle alignment changes the width of the transmission line, resulting in discontinuities in impedance. In fact, not only the right-angle alignment, obtuse angle and acute angle alignment also can cause impedance changes.

The signal influence caused by right-angle alignment is mainly shown in three aspects: one is that the corners can be equivalent to capacitive loads on the transmission line which can slow the rising time; another is discontinuous impedance can cause signal reflections; the last is right-angle tip can generate electro magnetic interference.

The parasitic capacitance caused by the right angle of the transmission line can be calculated from the following formula: C=61W(Er)1/2/Z0. C refers to the corner’s equivalent capacitance (unit: pF), W refers to the trace width (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 at right angles increases, the impedance there will be reduced, so there will definitely have a signal reflection phenomenon. We can calculate the equivalent impedance after the line width increases according to the impedance calculation formula of the transmission line. The reflection coefficient is then calculated according to the formula ρ=(Zs-Z0)/(Zs+Z0). Impedance changes due to the right-angle alignment are often between 7% -20%, so the maximum reflection coefficient is about 0.1. 

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