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Can PCB design high speed signal layout be routed at 90°?
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Can PCB design high speed signal layout be routed at 90°?

When designing PCB layout and wiring, the Angle of high-speed signal trace will be mentioned. They will say that high-speed signals should not be tracked at a right Angle, but at a 45 degree Angle, and can say that the arc will be greater than 45 degrees. The corners are better.

Is that right? How to set PCB trace Angle, preferably 45 degrees or arc? Is 90 degree Angle all right?

Can PCB design high speed signal layout be routed at 90°?

Can the PCB layout be wired 90 degrees?

High frequency and high speed signal lines should avoid routing at a 90° Angle, which is strongly required in various PCB design guidelines, because high frequency and high speed signal lines need to maintain consistent characteristic impedance, and using 90° Angle routing at the corner of the transmission line will change the line width. The line width at the 90° Angle is about 1.414 times the normal line width. Because the line width changes, the signal will be reflected. At the same time, the extra parasitic capacitance around the corner will result in time-consuming signal transmission. Delay effect.

Of course, when the signal travels along a uniform interconnection, the transmitted signal will be free of reflection and distortion. If there is a 90° Angle on the uniform interconnection, it will cause a change in the width of the pcb transmission line at the Angle. According to the relevant electromagnetic theory calculation shows that this will certainly bring the signal reflection effect.

That's the theory, but a theory is a theory. Is the effect of 90° Angle on high-speed signal transmission line important in practice?

It is theoretically certain that 90° Angle will have a negative effect on high-speed signal transmission lines, but is the effect fatal? Does 90° Angle have the same effect on high speed digital signal and high frequency microwave signal transmission lines?

For high-speed digital signal, 90° Angle will have some influence on high-speed signal transmission line. For our current high density and high speed PCBS, the typical trace width is 4-5mil and the capacitance at 90° Angle is about 10fF. It is estimated that the cumulative delay caused by this capacitor is about 0.25ps, so a 90° Angle on a wire with a 5mil line width will not have much effect on the current high speed digital signal (100 psec rising edge time).

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For high frequency signal transmission lines, wider signal transmission lines such as 50Ω impedance and 100mil line width are usually used to avoid signal damage caused by skin effects. The line width at the 90° Angle is about 141mil and the signal delay due to the parasitic capacitance is about 25ps. At this point, the 90° Angle will have a very serious effect.

At the same time, microwave transmission lines always want to minimize signal loss. Impedance discontinuity at 90° and external parasitic capacitance will lead to phase and amplitude errors, mismatches between input and output, and possible parasitic coupling of high frequency signals, resulting in deterioration of circuit performance and affecting the transmission characteristics of PCB circuit signals.

Regarding 90° signal wiring, our view is to avoid 90° wiring whenever possible. Doesn't it say that the effect of a 90-degree Angle on a high-speed digital signal is negligible?

Of course, the effect of a single 90° Angle on the signal quality of a high-speed digital transmission line, the deviation with respect to the height of the wire and the reference plane, the deviation of line width and line spacing uniformity during the etching of the wire itself, and the effect of the dielectric constant of the sheet on the variation of the frequency signal, even through parasitic effects, are far more problematic than the 90° Angle.

However, today's high-speed digital circuit transmission lines always have to be wound around the same length. When more than ten or twenty angles overlap, the signal rise delay caused by the cumulative effect of these 90° angles becomes non-negligible. High-speed signals are always transmitted along the impedance path, and the 90° Angle is of equal length, so that the actual signal transmission path will eventually be slightly shorter than the original path.

Now the high-speed digital signal transmission rate is getting higher and higher, the current HDMI2.0 standard, the transmission bandwidth has reached 18Gbps, 90° Angle wiring will no longer meet the requirements, now is the 21st century, now 45° EDA software has been well supported, even those who use it.

At the same time, wiring cables at 90° angles does not meet people's aesthetics in terms of engineering aesthetics. Therefore, for the current layout, whether you are using high frequency/high speed signal lines, you should try to avoid 90° Angle wiring unless specifically requested.

For high current trace, sometimes we use copper coated plate instead of trace. At the corner of the copper cladding, we also need to replace the 90° Angle with two 45° angles, which is not only aesthetically pleasing, but also EMI free. This is a hidden danger.

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