High-speed PCB engineers must know and know-PCB design reflow path analysis

“High-speed design has become the focus of more and more PCB designers. When designing high-speed PCBs, every engineer should pay attention to its signal integrity and always consider the return path of its signal Circuit, because a poor return path can easily lead to signal integrity problems such as noise coupling. If the current has to travel a long path to return, the inductance loop of the signal path will increase. When the inductance loop in the system is larger, these signals are more likely to absorb noise from any other Net in the system.

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High-speed design has become the focus of more and more PCB designers. When designing high-speed PCBs, every engineer should pay attention to its signal integrity and always consider the return path of its signal circuit, because a poor return path can easily lead to signal integrity problems such as noise coupling. If the current has to travel a long path to return, the inductance loop of the signal path will increase. When the inductance loop in the system is larger, these signals are more likely to absorb noise from any other Net in the system.

Generally, the problem of discontinuity in the return path is often caused by the lack of ground vias, gaps in the ground plane, lack of decoupling capacitors, or the use of wrong Net. And as your PCB design becomes more and more complex, it becomes more difficult to find these problems quickly.

This article will use design examples to explain in detail how to use the ReturnPath analysis function of IDA (In-Design Analysis) in Allegro PCBDesigner to analyze the return path during the PCB design process to help engineers quickly find out whether the return path of those high-speed signals is appropriate. In order to ensure the quality of the layout and reduce the significant losses that need to be recalled due to unstable signals after mass production of the product, the design can be successful at one time.

The focus of this article

Interpretation of reflux path
Importance of ReturnPath analysis
Detailed ReturnPath analysis examples
Analysis of ReturnPath analysis results

1. What is a return path (ReturnPath)

The operation of electrical products requires a signal circuit to operate, as shown in the figure below (1), the negative electrode of the battery also needs to be connected to the blue wire before the light will turn on. In the early days, we can see that the telegraph system used the “earth” as the ground plane of the signal loop, and another ground wire could be laid out to reduce expensive costs. Or if the similar situation in modern life is when the car needs to be equipped with light bulbs, we can regard the “car shell” as the ground of the signal circuit, and connect the negative pole of the light bulb directly to the car shell. One line is troublesome, and there is no need to consider the return path problem.

However, if you want to connect to various sensors or processors on the driving system, CAN (vehicle network system), or ADAS (advanced driver assistance system), it is not as simple as connecting directly and eliminating the need for wires. It is easy to involve high-frequency/high-speed transmission, and it is necessary to pay attention to the integrity of its return path.

same! For PCB design, if it is a low-frequency signal, the return path will return with the lowest impedance, but as the frequency increases, the current needs to return to the source in a closed loop, so the return path with the lowest inductance will be more considered, and it will usually correspond The return path of the upper and lower layers of the wiring is shown in the left picture (2) below, to avoid the problem of the return path circuitous due to the inner layer cutting (2), so the return path of the high-speed signal is more important.

2. What needs ReturnPath analysis?

As mentioned above, it is very important to consider the return path of high-speed signals, because a little carelessness will greatly reduce the circuit function.

Generally speaking, because the DRC inspection of the standard PCB only checks whether the mouse wire is not connected and the safety distance is not enough, so the analysis like ReturnPath is not easy to achieve, and it often requires experienced veterans to open the relevant diagrams. The layers follow the adjacent layers of the high-speed signal traces to ensure the return path and control the layout quality. Or set some rules on how to add StitchingVia to the Layout. The diagram of STItchingVia is as shown in the figure (3). As for the differential signal to hit Via, there are several STItchingVias next to it. That is another story!

Even in the end, it is necessary to add stitching capacitors to fill the moats (Moat) that cannot be crossed, which leads to increased costs to improve the return path, as shown in the following figure (4) TI specification.

So if we have an intuitive auxiliary analysis tool that will analyze the return path according to the geometry of the signal, and without Models, calculate the inductance ratio RPQF (Return Path Quality Factor, return path quality factor) as shown in Figure 5 below.

When the RPQF value is closer to 1, it means that the signal wiring is closer to the return path, and the higher it is, the more tortuous and farther the return path is.

Moreover, after the analysis is completed, the RPQF value of the relevant signal can be directly listed as shown in Figure 6 below, allowing us to quickly identify the severity of each signal and correct the undesirable part.

Note: The other Impedance impedance analysis and coupling interference analysis in IDA (In-Design Analysis) are also the same. The quality control of various layouts can be quickly achieved by following the inspection process without Models. Fast screening analysis.

3. How to perform ReturnPath analysis

Now Allegro has imported Sigrity’s professional analog analysis technology, bringing IDA (In-Design Analysis, design synchronization analysis) into the PCB design process, helping PCB engineers perform analysis in the design simultaneously, and find out common reflow path discontinuities in advance Problems can be solved in real time, and the quality of the signal return path can be quickly ensured, so that the design efficiency is improved and the probability of failure is reduced. It is also important that the ReturnPath check does not require Models and can be easily implemented with a simple process!