PCB Fabrication

How to correctly use snake-shaped lines in PCB design

by:A-TECH      2021-03-25
The main purpose of the serpentine line is to adjust the delay and meet the system timing design requirements. It is a type of wiring method often used in Layout.   The designer must know: the serpentine line will destroy the signal quality, change the transmission delay, and try to avoid using it when wiring. However, in actual design, in order to ensure that the signal has sufficient hold time, or to reduce the time offset between the same group of signals, it is often necessary to deliberately wind the wire. The two most critical parameters of the serpentine line's influence on signal transmission are the parallel coupling length (Lp) and the coupling distance (S). When the signal is transmitted on the serpentine trace, the parallel lines will be coupled. In the differential mode, the smaller the S and the larger the Lp, the greater the degree of coupling. This may result in a reduction in transmission delay and a significant reduction in signal quality due to crosstalk. In high-speed PCB design, the serpentine line has no so-called filtering or anti-interference ability, and can only reduce the signal quality, so it is only used for timing matching and has no other purpose. Any trace on the PCB will cause a delay to the signal when it passes a high-frequency signal. The main function of the serpentine trace is to compensate for the less delayed parts of the 'same group of related' signal lines. These parts are usually It is not or less processed by another logic than other signals; the most typical is the clock line, usually it does not need to go through any other logic processing, so its delay will be less than other related signals.    The line delay difference per unit length is also fixed, The delay is related to the line width, line length, copper thickness, and layer structure. However, if the line is too long, it will increase the distributed capacitance and distributed inductance, which will affect the signal quality. Therefore, the clock IC pins are generally connected to the RC termination, but the snake The shape of the trace does not play the role of inductance.On the contrary, the inductance will cause the phase shift of the higher harmonics in the rising element in the signal, causing the signal quality to deteriorate. Therefore, the serpentine line spacing is required to be at least twice the line width. The smaller the rise time, the more susceptible to the influence of distributed capacitance and distributed inductance. Because different applications have different functions, if the serpentine trace appears in the computer board, it mainly acts as a filter inductance to improve the resistance of the circuit. Interference ability, the serpentine trace in the computer motherboard has two main functions: 1. Impedance matching 2. Filtering inductance. For some important signals, the length must be strictly equal to eliminate the hidden dangers caused by time lag. The winding is the only one. The solution.    If it is a distributed parameter LC filter in a general ordinary PCB board, it can also be used as the inductance coil of the radio antenna, and the short and narrow serpentine trace can be used as a fuse. In order to improve the stability of the motherboard and graphics card, it helps to eliminate the inductance phenomenon caused by the long straight line when the current passes, and reduces the crosstalk between the lines. This is especially obvious at high frequencies. Of course, the designer can also The same effect can be achieved by reducing the density of wiring.   Snake-shaped wiring should pay attention to several points:   1. Try to increase the distance (S) of parallel lines, at least greater than 3H, H refers to the distance from the signal trace to the reference plane. In layman's terms, it is to go around a big bend. As long as S is large enough, mutual coupling effects can be almost completely avoided. 2. Reduce the coupling length Lp. When the double Lp delay approaches or exceeds the signal rise time, the crosstalk generated will reach saturation.  3. For high-speed signal lines and those with strict timing requirements, try not to use serpentine lines, especially in small areas.   5. Snake-shaped traces at any angle can often be used, which can effectively reduce mutual coupling.   6. In high-speed PCB design, the serpentine line has no so-called filtering or anti-interference ability, and can only reduce the signal quality, so it is only used for timing matching and has no other purpose.  7. Sometimes you can consider spiral routing for winding. Simulation shows that its effect is better than normal serpentine routing.
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