3 special wiring techniques for PCB

- Dec 07, 2019-

3 special wiring techniques for PCB

abstract:     

As electronic products continue to develop in the direction of high density, integration, and high reliability, PCB (printed circuit board), as one of the main components of electronic products, is increasingly active in the electronics industry stage. The 2016 Shenzhen International Circuit Board Procurement Exhibition (CS SHOW 2016) will officially kick off at the Shenzhen Convention and Exhibition Center from August 30th to September 1st. As the only industry exhibition with PCB / FPC procurement as the theme in the industry, thousands of circuit board buyers will gather in Shenzhen to discuss business opportunities with the leading circuit board companies across the Taiwan Strait and the Mainland! Before starting, let's take a look at 3 special routing techniques for PCBs in design and layout.

The quality of the PCB design layout (Layout) will directly affect the performance of the entire system. Most high-speed design theories will eventually be implemented and verified through Layout. It can be seen that the layout is critical in high-speed PCB design. The following will analyze the rationality of some situations that may be encountered in actual wiring, and give some more optimized routing strategies.

It is mainly explained from three aspects: right-angle wiring, differential wiring, and serpentine wiring.

  1. Right-angle routing

Right-angle wiring is generally a situation that is required to be avoided in PCB wiring, and it has almost become one of the standards for measuring the quality of the wiring. So how much influence does right-angle wiring have on signal transmission? In principle, running at right angles will change the line width of the transmission line, causing impedance discontinuities. In fact, not only right-angle wiring, staggered angle, and acute-angle wiring may cause impedance changes.

The impact of right-angled routing on the signal is mainly reflected in three aspects:

First, the corner can be equivalent to a capacitive load on the transmission line, slowing the rise time;

Second, the impedance discontinuity will cause signal reflection;

The third is the EMI generated by the right-angle tip.

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Many people have such an understanding of right-angle wiring. They believe that the tip easily emits or receives electromagnetic waves and generates EMI. This has also become one of the reasons why many people think that right-angle wiring cannot be used. However, the results of many actual tests show that right-angled traces do not produce significant EMI over straight lines. Perhaps the current instrument performance and test level limit the accuracy of the test, but at least one problem is explained, the radiation of the right-angled trace is already smaller than the measurement error of the instrument itself.

Generally speaking, right-angle routing is not as scary as you think. At least in applications below GHz, any effects such as capacitors, reflections, and EMI are hardly reflected in the TDR test. The focus of high-speed PCB design engineers should be on layout, power / ground design, and routing design. Vias and other aspects. Of course, although the impact of right-angle wiring is not serious, it does not mean that we can all follow right-angle wiring in the future. Pay attention to details is a basic quality necessary for every outstanding engineer. Moreover, with the rapid development of digital circuits, PCB The frequency of signals handled by engineers will also continue to increase. In the RF design field above 10GHz, these small right angles may become the focus of high-speed issues.

  2. Differential trace

Differential Signal is used more and more widely in high-speed circuit design. The most critical signals in the circuit are often designed with a differential structure. What makes it so popular? How can you guarantee its good performance in PCB design? With these two questions in mind, let's discuss the next part.

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What is a differential signal? In layman's terms, the driving end sends two equal-value, inverted signals, and the receiving end determines whether the logic state is "0" or "1" by comparing the difference between the two voltages. The pair of traces carrying differential signals is called a differential trace.

Compared with ordinary single-ended signal traces, the most obvious advantages of differential signals are reflected in the following three aspects:

a. Strong anti-interference ability, because the coupling between the two differential traces is very good. When there is noise interference from the outside, it is almost coupled to the two lines at the same time, and the receiver only cares about the difference between the two signals. So the external common mode noise can be completely canceled.

B. It can effectively suppress EMI. For the same reason, because the two signals have opposite polarities, the electromagnetic fields radiated by them can cancel each other out. The tighter the coupling, the less electromagnetic energy is released to the outside.

c. Timing positioning is accurate, because the switching change of the differential signal is located at the intersection of the two signals, unlike ordinary single-ended signals, which rely on two threshold voltages to judge, so it is less affected by process and temperature, which can reduce timing errors At the same time, it is more suitable for circuits with low amplitude signals. The current popular LVDS (low voltage differential signaling) refers to this small amplitude differential signal technology.

PCBFor PCB engineers, the main concern is how to ensure that these advantages of differential routing can be fully utilized in actual routing. Maybe anyone who has been in contact with Layout will understand the general requirements for differential routing, that is, "equal length, equidistant". Equal length is to ensure that the two differential signals maintain opposite polarities at all times to reduce common mode components; equal distance is mainly to ensure that the differential impedance of the two is consistent and reduce reflections. "Putting it as close as possible" is sometimes one of the requirements for differential routing.

In PCB circuit design, the coupling between differential traces is generally small, often accounting for only 10 to 20% of the degree of coupling, and more is coupled to ground, so the main reflow path for differential traces still exists on the ground plane. . When there is discontinuity in the local plane, the coupling between differential traces will provide the main return path when there is no reference plane, as shown in Figure 1-8-17. Although the discontinuity of the reference plane does not affect the differential traces as severely as the ordinary single-ended traces, it will still reduce the quality of differential signals and increase EMI, which should be avoided as much as possible. Some designers believe that the reference plane under the differential trace can be removed to suppress some common-mode signals in differential transmission, but this approach is theoretically not desirable. How can the impedance be controlled? Not providing a ground impedance loop for common-mode signals will inevitably cause EMI radiation, and this approach will do more harm than good.

Differential traces can also be routed in different signal layers, but this method is generally not recommended, because differences such as impedance and vias generated by different layers will destroy the effect of differential mode transmission and introduce common mode noise. In addition, if the adjacent two layers are not tightly coupled, the ability of the differential trace to resist noise will be reduced, but if the proper spacing from the surrounding traces is maintained, crosstalk is not a problem. At general frequencies (below GHz), EMI will not be a serious problem. Experiments show that the differential energy attenuation between 500Mils and 3 meters away has reached 60dB, which is enough to meet the FCC electromagnetic radiation standards. Designers do not have to worry too much about the incompatibility of the differential lines and cause electromagnetic incompatibility.

2. 3. Serpentine

Snake lines are a type of routing method often used in Layout. Its main purpose is to adjust the delay and meet the system timing design requirements. The designer must first have this understanding: the serpentine line will destroy the signal quality, change the transmission delay, and try to avoid it when wiring. However, in actual design, in order to ensure that the signal has a sufficient holding time, or to reduce the time offset between the same group of signals, it is often necessary to intentionally perform winding.

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So, what effect does the serpentine have on signal transmission? What should I pay attention to when routing? The two most important parameters are the parallel coupling length (Lp) and the coupling distance (S), as shown in Figure 1-8-21. Obviously, when the signal is transmitted on the serpentine trace, the parallel segments will be coupled in the form of differential mode. The smaller S, the larger Lp, the greater the degree of coupling. It may result in reduced transmission delay and greatly reduced signal quality due to crosstalk. The mechanism can refer to the analysis of common mode and differential mode crosstalk in Chapter 

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