High-frequency PCB and high-speed signal transmission lines should avoid routing at 90 degree corners, which is strongly required in various PCB Design Guides.Because the high-frequency and high-speed signal transmission line needs to keep the characteristic impedance consistent, and the 90 degree corner routing is used, the line width will change at the corner of the transmission line.The line width at the 90 degree corner is about 1.414 times the normal line width. Due to the change of the line width, the signal will be reflected. At the same time, the extra parasitic capacitance at the corner will also affect the transmission of the signal.
Of course, when the signal propagates along the uniform interconnect, there will be no reflection and distortion of the transmitted signal. If there is a 90 degree corner on the uniform interconnect, it will cause a change in the width of the PCB transmission line at the corner. According to the relevant electromagnetic Theoretical calculation shows that this will definitely bring about the reflection effect of the signal.
This is the case in theory, but theory is theory after all. Is the impact of 90 degree corners on high-speed signal transmission lines important in practice?
According to King Sun rigid flex circuit board engineering, 90 degree corners will have a negative impact on high-speed signal transmission lines, which is theoretically certain, but is this impact fatal? The impact of 90 degree corners on high-speed digital signal and high-frequency microwave signal transmission lines Is it the same?
For high-speed digital signals, a 90 degree corner will have a certain impact on high-speed signal transmission lines. For our current high-density and high-speed pcb, the general trace width is 4-5mil, and the capacitance of a 90 degree corner is about 10fF. It is estimated that the cumulative delay caused by this capacitor is about 0.25ps, so a 90 degree corner on a wire with a 5mil line width will not have a great impact on the current high-speed digital signal (100-psec rising edge time).
For high-frequency signal transmission lines, in order to avoid signal damage caused by the skin effect, a wider signal transmission line is usually used, such as 50Ω impedance and 100mil line width. The line width at the 90 degree corner is about 141mil, the signal delay caused by parasitic capacitance is about 25ps. At this time, the 90 degree corner will have a very serious impact.
At the same time, microwave transmission lines always hope to minimize the loss of the signal. The impedance discontinuity at the 90 degree corner and the parasitic capacitance outside will cause the phase and amplitude errors of the high-frequency signal, the mismatch between the input and the output, and the possible existence of Parasitic coupling, which in turn leads to the deterioration of circuit performance, affects the transmission characteristics of PCB circuit signals.
Regarding 90 degree signal routing, our point of view is to avoid routing at 90 degree as much as possible. Didn’t it say that the impact of 90 degree corners on high-speed digital signals can be ignored?
According to King Sun rigid-flex circuit board engineering, of course, the influence of a single 90 degree corner on the signal quality of the high-speed digital transmission line, relative to the deviation of the height of the wire and the reference plane, the deviation of the uniformity of the line width and line spacing during the etching process of the wire itself, The change of the dielectric constant of the sheet on the frequency signal, and even the parasitic parameters of the vias, are much larger than the problems caused by the 90 degree corner.
However, today’s high-speed digital circuit transmission lines always have to be wound around the same length. When more than ten or twenty corners are superimposed, the signal rise delay caused by the cumulative effect of these 90 degree corners will become non-negligible. The high-speed signal is always transmitted along the impedance path, with 90 degree corners of equal length, and the final actual signal transmission path will be slightly shorter than the original.
And now the high-speed digital signal transmission rate is getting higher and higher, the current HDMI2.0 standard, the transmission bandwidth rate has reached 18Gbps, the 90 degree corner wiring will no longer meet the requirements, and it is now the 21st century, now The EDA software of 45 degree has been well supported even for those who use it.
At the same time, routing cables at 90 degree corners is not in line with people’s aesthetics in terms of engineering aesthetics. Therefore, for the current layout, whether you are using high-frequency/high-speed signal lines, we should try to avoid routing with 90 degree corners unless there are special requirements.
According to King Sun PCB engineering, for high-current traces, sometimes we will replace the traces with copper-clad copper sheets. Not only beautiful, but also there is no EMI hidden danger.