Electronic Design - The reason for high-speed PCB design serpentine line?

Serpentine line is very common,there are a number of uses, and delay compensation (timing isometric) in the conventional design of the most widely used; through the winding of the wire to control the length of the wiring, the control of the wire width, the winding of the wire spacing, known as the serpentine alignment.

Before explaining the PCB wiring before completing the inspection work,I will first introduce you to three special wiring techniques for PCB design.

The PCB layout line will be explained from three aspects: right-angled line,differential wiring, and serpentine line:

Right-angle line (three aspects).The influence of the right-angle line on the signal is mainly reflected in three aspects:First, the angle can be equivalent to the capacitive load on the transmission line, which slows down the rise time, and the second is that the impedance discontinuity will cause Signal reflection,the third is the right-angle tip generated by EMI, which exceeds the 10GHz RF design field,these small right angles may become the focus of high-speed problems.

Differential wiring ("equal length,equidistant,reference plane").What is a differential signal (differential signal)? In popular terminology, the driver sends two equivalent inverted signals, and the receiver determines the logic state "0" or "1" by comparing the difference between the two voltages. And the pair of wires carrying differential signals are called differential wires.

Compared with ordinary single-ended signal wiring, the most obvious advantage of differential signal lies in the following three aspects:

1) The anti-interference ability is strong, because the coupling between the two differential lines is very good. When there is noise interference from the outside,they are coupled to the two lines almost at the same time, and the receiving end only pays attention to the difference between the two signals, so the external common Mode noise can be completely canceled.

2) It can effectively suppress EMI.For the same reason,due to the polarity of the two signals,their external radiated electromagnetic fields can cancel each other.The closer the coupling, the less electromagnetic energy released to the outside world.

3) Precise timing positioning, because the change of the differential signal switch is located at the intersection of the two signals,unlike ordinary single-ended signals that rely on the high and low threshold voltage judgments, so through this process,the temperature effect is small and can be reduced timing error is also more suitable for low-amplitude signal circuits.

The current popular LVDS (Low Voltage Differential Signaling) refers to this small amplitude differential signal technology.

3.Snake line (adjust delay).Snake line is a wiring method commonly used in layout.Its main purpose is to adjust the delay to meet the system timing design requirements.The most critical of the two parameters are the parallel coupling length (Lp) and the coupling distance (S).Obviously,when the signal is transmitted on the serpentine line,in the differential mode,there will be coupling between the parallel line segments,and the smaller S is the greater the Lp,the greater the degree of coupling.It may lead to a reduction in transmission delay and greatly reduce the signal quality caused by crosstalk.The mechanism can refer to the analysis of common mode and differential mode crosstalk.

Serpentine line application is different, its role is also different, roughly the following several major roles:

Adjust the delay to meet the timing requirements    

Due to the high-speed circuit design, part of the signal transmission speed is very fast, so the signal transmission time (i.e., delay) in different lengths of the line will be different.

If the signals do not arrive at the same time, this may result in data errors or system instability.

Obviously, snaking can be used to increase the length of a particular line, and the length and shape of the snaking can be precisely controlled according to design requirements. By adjusting the degree and number of bends in the serpentine, layout engineers can accurately increase or decrease the length of the line to match the delay of other critical paths, so that the critical paths in the entire circuit have the same delay to meet the timing design requirements of the system.

Reducing Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI)    

The sinuous shape of the snake alignment can spread the radiant energy originally concentrated in one area to a larger area, which helps to reduce the intensity of radiation in a specific area, thus reducing the interference to the surrounding environment.

At the same time, more importantly, when two neighboring serpentine alignment at the same time with equal size, opposite direction of the current, the magnetic field generated by them will cancel each other, thus reducing electromagnetic radiation, reducing the interference of other circuits or equipment.

Of course, in high-density circuit design, there may be crosstalk and coupling between different signal lines. Serpentine routing can reduce the physical proximity between signal lines by changing the shape and layout of the lines, thus reducing the crosstalk and coupling effects between signals.

Impedance Matching    

Impedance matching is an important design consideration in certain circuits, such as radio frequency (RF) circuits or high-speed data transmission circuits.

Serpentine routing,along with adjusting the width of the wire and thickness of the copper foil, can be used to adjust the impedance of the line to match the impedance between the source and the load,thereby reducing reflections and losses and improving signal transmission efficiency.

High Frequency Suppression    

Serpentine alignment is equivalent to a simple inductive coil, which can suppress some high frequency signals due to the principle of electromagnetic induction.

 Disadvantages   

Serpentine line can make the wiring complex and require a lot of calculations and optimization, which increases the difficulty of design and manufacturing, and may lead to additional cost and time overhead.

Meanwhile,the signal quality decreases, which may change the transmission quality of the signal.Due to the bending and meandering of the signal wires in the serpentine alignment, the signal may be subject to more loss and interference during transmission, leading to a decrease in signal quality,such as signal distortion and increased noise.

The following are some suggestions for layout engineers to deal with serpentine lines: 

1) Maximize the distance (S) of the parallel segment,at least greater than 3h,where h refers to the distance from the signal line to the reference plane.It is popular to bypass the Big Bend line.As long as s is large enough,the coupling effect between each other can be almost completely avoided.

2) Reduce the coupling length LP.When the LP delay is twice as close to or exceeds the signal rise time,the resulting crosstalk will reach saturation.

3) The signal transmission delay caused by the serpentine line of the strip line (strip line) or buried microstrip line (embedded microstrip) is less than the signal transmission delay of the microstrip line (microstrip).In theory,due to differential mode crosstalk,the stripline will not affect the transmission rate.

4) The high-speed and strict timing requirements of the signal line,try not to take the serpentine line,especially on the small-scale winding line.

5) It is often possible to use serpentine wiring at any angle,which can effectively reduce the coupling between each other.

6) High-speed PCB design,the serpentine line has no so-called filtering or anti interference ability,which will only reduce the signal quality,so it can only be used for time series matching and has no other purpose.

7) Sometimes the winding method of spiral wire can be considered,and the simulation display effect is better than that of ordinary serpentine wire.After PCB routing, the wiring is completed? Obviously,no!It is also very necessary after the PCB wiring inspection work,so how to check the PCB design and wiring,but also to learn more about other related knowledge.

Serpentine line is a common PCB wiring method, can help reduce electromagnetic interference and signal crosstalk, improve circuit stability and reliability. In the design of high-frequency and high-speed circuits, the reasonable application of serpentine alignment can play an important role, is one of the important technical means in PCB design.