Circuit board serpentine line (also known as snaking, meandering or zigzagging wiring) is a common technique in PCB design. Circuit board serpentine is used to connect two or more points on the circuit board wire, wave-shaped or snake-shaped distribution, it can ensure good signal transmission at the same time, reduce interference, improve the quality of signal transmission, protection of the motherboard and so on. This alignment has its unique advantages in signal integrity, electromagnetic compatibility and timing control.
Designers must be aware of the fact that the serpentine wire will destroy the signal quality and change the transmission delay, and should be avoided as much as possible when wiring. However, in actual design, wire wrapping often has to be done intentionally in order to ensure that signals have sufficient hold-up time, or to minimize time offsets between signals of the same group.
The two most critical parameters in a serpentine line are the parallel coupling length (Lp) and the coupling distance (S). Signal transmission in the serpentine alignment, parallel to each other between the line segments will be coupled, in the form of differential mode, the smaller the S, the larger the L, the greater the degree of coupling is also greater, which may lead to a reduction in transmission delay as well as due to crosstalk and greatly reduces the quality of the signal.
The high-speed digital PCB board, such as line length is to make the signal delay difference to stay within a range to ensure that the system reads the data in the same cycle of the validity (delay difference of more than one clock cycle will be wrong to read the data of the next cycle), the general requirements of the delay difference of no more than 1/4 clock cycle, the unit length of the line delay difference is also fixed, the delay with the line width, line length, copper thickness, the board structure is related to the delay, but line Too long will increase the distribution of capacitance and distribution of inductance, so that the signal quality, so the clock IC pins are generally connected to the RC termination, but the serpentine alignment is not to play the role of inductance, on the contrary, the inductance of the signal in the rising element of the high harmonics phase shift, resulting in deterioration of the signal quality, so the serpentine spacing of at least two times the width of the line, the smaller the rise of the signal time, the more susceptible to the distribution of capacitance and the distribution of the inductance of the impact.
Because of different applications with different roles, if the serpentine alignment in the pcb board, its main role as a filter inductance, to improve the circuit’s anti-jamming ability, the serpentine alignment in the computer motherboard, mainly used in some of the clock signals, such as PCIClk, AGPClk, and its role has two points: 1, impedance matching, 2, filtering inductance. For some important signals, such as INTEL HUB architecture in the HUBLink, a total of 13, running 233MHz, the requirements must be strictly equal to eliminate time lag caused by the hidden trouble, winding is the only solution. Generally speaking, the serpentine alignment of the line spacing >= 2 times the width of the line. PCI board on the serpentine line is in order to adapt to the PCI 33MHzClock line length requirements. If in the general ordinary PCB board, is a distribution parameters of the LC filter, but also as a radio antenna inductance coil, short and narrow serpentine alignment can be made fuses and so on.
The role of serpentine wire
- The main role is to compensate for the “same group of related” signal lines in the delay of the smaller part, these parts are usually no or less than other signals through another logic process. The most typical is the clock line, usually it does not need to go through any other logic processing, so its delay will be smaller than other related signals.
- Filtering effect, in the RF circuit, the snake alignment can form a LC circuit, a certain frequency of the signal has a filtering effect.
- Receiving antenna,in some cell phones or radios we can see this role. Some antennas are made with PCB alignment.
- Serpentine alignment in different applications with different roles, if the serpentine alignment in the computer board, its main role as a filtering inductance, to improve the circuit’s anti-interference capability. Computer motherboard snake alignment, mainly used in some clock signals, such as PCIClk, AGPClk, its role has two points: a, impedance matching; b, filter inductance.
- Short and narrow serpentine alignment can be used as a fuse.
Application scenarios for circuit board serpentine line
- High-speed data transmission
High-speed data transmission, to ensure the quality of data transmission to avoid interference. In this case, the use of serpentine wire design circuit boards, you can attenuate the mutual interference of signals, thereby improving the quality of data transmission. - High-capacity current transmission
For high-capacity current transmission circuit boards, the use of serpentine design can make the circuit board more solid, protect the motherboard from current damage, and can better transmit current to improve transmission efficiency. - Miniaturized Boards
For miniaturized circuit boards, the use of serpentine wires can help save space and reduce interference between boards, improving board stability and reliability.
Designers must be aware of the fact that snaking wires can damage signal quality and change transmission delay, and should be avoided as much as possible when wiring. However, in actual design, in order to ensure that the signal has enough hold time, or to reduce the time offset between the same group of signals, often have to deliberately winding.
Some suggestions for PCB Layout engineers when dealing with serpentine wire:
√ Maximize the distance S between parallel line segments, which is at least greater than or equal to 4H, with H referring to the distance from the signal alignment to the reference plane.
√ Minimize the coupling length Lp, the delay of Lp should be less than or equal to 6Tr.
√ Do not use serpentine line if possible.
√ If possible, use a serpentine with an arbitrary angle to effectively minimize mutual coupling.
√ The signal transmission delay caused by the serpentine of a ribbon cable is less than that of a microstrip cable.
√ High-speed and more stringent timing requirements of the signal line, try not to take the serpentine line.
Coupling distance d: greater than or equal to 4H
Parallel coupling length Lp: less than or equal to Tr (inches)
(take the delay and Tr 6 times the relationship)
A PCB on the serpentine line more does not mean that the more “advanced”, it has “advantages” and “disadvantages”, PCB engineers should be careful.
Serpentine line as a challenging and full of creative technical means, its application needs to carefully weigh the pros and cons. It is not only engineers to solve the signal synchronization, filtering and anti-jamming issues such as a powerful assistant, but also to achieve high-speed, high-quality circuit design is a key part. Serpentine wire requires designers to pursue high performance at the same time, not forgetting the signal integrity and electromagnetic compatibility of the strict control.
