PCB News - Why do PCB wiring should avoid acute and right angles as far as possible

If it is rf line, if it is a right Angle at the corner, there will be discontinuity, and discontinuity will easily lead to the generation of high order mode, which has an impact on radiation and conductivity. RF signal line if the right Angle, the effective line width at the corner will increase, impedance discontinuous, causing signal reflection. To minimize discontinuity, corners are treated in two ways: corner cutting and corner rounding. The radius of the arc Angle should be large enough.

Route at acute and right angles

Acute Angle wiring is generally forbidden in PCB wiring. Right-angle wiring is generally avoided in PCB board wiring and has almost become one of the standards to measure the quality of wiring. Then how much impact will right-angle wiring have on signal transmission?

In principle, the line width of the transmission line will be changed by the acute Angle and right Angle, resulting in the discontinuity of impedance.

A change in line width results in a change in impedance

When the equivalent width of the line changes, the signal is reflected. We can see:

When we are wiring, if the line width changes, it will lead to a change in wiring impedance.

Microstrip lines? It consists of a strip of wire connected to the ground plane with a dielectric in the middle. If the dielectric constant, the width of the line, and its distance from the ground plane are controllable, then its characteristic impedance is controllable, and its degree will be within ± 5%.

Why do PCB wiring should avoid acute and right angles as far as possible

Stripline

A ribbon line is a strip of copper in the middle of the dielectric between two conducting planes. If the thickness and width of the line, the dielectric constant of the medium, and the distance between the ground planes of the two layers are controllable, the characteristic impedance of the line is controllable, and the accuracy is within 10%.

Why do PCB wiring should avoid acute and right angles as far as possible

If the impedance is discontinuous, it will reflect

Acute Angle difference, right Angle, obtuse Angle, rounded Angle, straight line.

Why do PCB wiring should avoid acute and right angles as far as possible

When the driver sends a signal into a transmission line, the amplitude of the signal depends on the voltage, the internal resistance of the buffer, and the impedance of the transmission line. The initial voltage seen at the driver is determined by the partial voltage between the internal resistance and the line impedance.

Why do PCB wiring should avoid acute and right angles as far as possible

Reflection coefficient? Among them 1 rho or less 1 or less

When ρ=0, no reflection occurs

Full positive reflection occurs when ρ=1(Z 2 =∞, open circuit)

Total negative reflection occurs when ρ=-1(Z 2 =0, short circuit)

Vinitial = 1.33 V

Subsequent reflectance is calculated according to the reflection coefficient formula

The reflectivity of the source is -0.33 based on the source impedance (25 ohms) and transmission line impedance (50 ohms) according to the reflection coefficient formula;

The reflectivity of the terminal is calculated according to the terminal impedance (infinity) and the transmission line impedance (50 ohms) according to the reflection coefficient formula of 1;

We get this waveform by superimposing it on top of the original pulse waveform in terms of the amplitude and delay of each reflection, and that's why impedance mismatches cause poor signal integrity.

The impedance will have to change due to the presence of connections, device pins, wiring width changes, wiring bends, and through holes. So reflection is inevitable.

Is there any other reason besides reflection?

The influence of right-angle alignment on signal is mainly reflected in three aspects

One is that the corner can be equivalent to the capacitive load on the transmission line, slowing down the rise time;

Second, impedance discontinuity will cause signal reflection;

Third, EMI generated by right angles.

Fourth, there is another saying: acute Angle will cause corrosion residue in the production process, which is not easy to process. It should not be difficult for the current PCB processing technology, so it is not taken as a reason.

As the line width of right-angle line increases, the impedance at this point will decrease, so there will be a certain signal reflection phenomenon. We can calculate the equivalent impedance after the line width increases according to the impedance calculation formula mentioned in the section of transmission lines, 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 between 7%-20%, so the reflection coefficient is about 0.1. Moreover, it can be seen from the figure below that the impedance of the transmission line changes to the normal impedance within the length of W/2 lines, and then recovers to the normal impedance after W/2. The time for the impedance change is very short, usually within 10ps. Such a fast and small change is almost negligible for the general signal transmission.

Many people have such an understanding of right-angle routing, believing that it is easy to emit or receive electromagnetic waves and generate EMI, which has become one of the reasons why many people think right-angle routing is not possible. However, many practical test results show that right-angle line does not produce much EMI than straight line. Perhaps the current instrument performance, test level restricted the test, but at least explained a problem, right-angle line radiation has been less than the instrument itself measurement error.

Why do PCB wiring should avoid acute and right angles as far as possible

In general, right-angle alignment is not as terrible as it might seem. At least in non-RF and high speed applications, any effects such as capacitance, reflection, EMI etc. are almost not reflected in TDR tests. The design engineer of high speed PCB should focus on layout, power/ground design, wiring design, perforation and other aspects. Of course, although the impact of right-angle alignment is not very serious, it does not mean that we can all walk right-angle alignment in the future, attention to detail is the basic quality every engineer must have, and, with the rapid development of digital circuit, the signal frequency of PCB engineers will continue to improve, to RF design field above 10GHz. These small right angles can become the focus of high-speed problems.