[Experience] Some classic questions and answers about high-speed PCB design

Today, as signal rates continue to increase, high-speed PCB design has become an essential skill that every PCB engineer should pay attention to and master, including basic theoretical knowledge and practical design experience. Next, 2PCB will share with you some classic questions and answers about high-speed PCB design, which are all the experience and wisdom of seniors~

1. When designing high-speed multi-layer PCB, what are the issues that should be paid attention to most?

The most important thing to pay attention to is the design of your layers, that is, how you divide signal lines, power lines, ground, and control lines into each layer. The general principle is that analog signals and analog signal grounds must be at least on a separate layer. It is also recommended to use a separate layer for the power supply.

2. For high-speed PCB, how to avoid via holes during wiring? Do you have any good suggestions?

For high-speed PCBs, it is best to drill fewer vias and add signal layers to solve the need for more vias.

3. How to solve the problem of signal integrity in high-speed PCB design?

Signal integrity is basically a matter of impedance matching. The factors that affect impedance matching include the structure and output impedance of the signal source, the characteristic impedance of the wiring, the characteristics of the load end, the topology structure of the wiring, etc. The solution is to rely on termination and adjusting the wiring topology.

4. In high-speed PCB design, the blank area of the signal layer can be covered with copper. How should the copper of multiple signal layers be distributed in terms of grounding and power supply?

Generally, most of the copper in the blank area is grounded. Just pay attention to the distance between the copper and the signal line when applying copper next to the high-speed signal line, because the copper applied will reduce the characteristic impedance of the line a little. Also be careful not to affect the characteristic impedance of other layers, such as in a dual strip line structure.

5. How to consider impedance matching issues when designing high-speed PCB design schematics?

When designing high-speed PCB circuits, impedance matching is one of the design elements. The impedance value has an absolute relationship with the wiring method, such as whether it is on the surface layer (microstrip) or inner layer (stripline/double stripline), the distance from the reference layer (power layer or ground layer), trace width, PCB material, etc. All will affect the characteristic impedance value of the trace. That is to say, the impedance value cannot be determined until after wiring. Generally, simulation software cannot consider some wiring situations with discontinuous impedance due to limitations of the circuit model or the mathematical algorithm used. At this time, only some terminators (terminations), such as series resistors, etc., can be reserved on the schematic diagram. Mitigates the effects of trace impedance discontinuities. The real fundamental solution to the problem is to try to avoid impedance discontinuity when wiring.

6. How to ensure the stability of signals above 50M during layout and wiring?

The key to high-speed digital signal wiring is to reduce the impact of transmission lines on signal quality. Therefore, when laying out high-speed signals above 100M, signal traces are required to be as short as possible. In digital circuits, high-speed signals are defined by signal rise delay time. Moreover, different types of signals (such as TTL, GTL, LVTTL) have different methods to ensure signal quality.

7. How to solve the contradiction between manual wiring and automatic wiring of high-speed signals?

Most of the automatic routers in today’s powerful wiring software have set constraints to control the winding method and the number of vias. The winding engine capabilities and constraint setting items of various EDA companies are sometimes very different. For example, are there enough constraints to control the way the serpentine lines meander, can they control the trace spacing of differential pairs, etc. This will affect whether the routing pattern produced by automatic routing can conform to the designer’s ideas. In addition, the difficulty of manually adjusting the wiring is absolutely related to the capabilities of the winding engine. For example, the pushing ability of traces, the pushing ability of vias, and even the pushing ability of traces on copper coating, etc. Therefore, choosing a router with a strong winding engine is the solution.

8. Will adding test points affect the quality of high-speed signals?

Whether it will affect the signal quality depends on how you add test points and how fast the signal is. Basically, additional test points (without using the existing via or DIP pin on the line as test points) may be added to the line or a short section of wire may be pulled from the line. The former is equivalent to adding a small capacitor on the line, while the latter is an extra branch. Both of these situations will have some impact on high-speed signals, and the degree of impact is related to the frequency speed of the signal and the edge rate of the signal. The magnitude of the impact can be known through simulation. In principle, the smaller the test point, the better (of course it must meet the requirements of the test equipment) and the shorter the branch, the better.

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