PCB Blog - Reflow path analysis during PCB board design

High-speed design has become the focus of more and more PCB board designers. When designing high-speed PCBs, every engineer should pay attention to their signal integrity, and always consider the return path of their signal circuits, because poor return paths can easily lead to signal integrity problems such as noise coupling. If the current has to travel a long path to return, the inductive loop of the signal path increases. As the inductive loops in the system are larger, the more likely these signals are to absorb noise from any other Nets in the system. Common return path discontinuities are often caused by missing ground vias, gaps in the ground plane, missing decoupling capacitors, or using the wrong Net. And as your PCB design becomes more complex, it becomes more difficult to find these problems quickly. This article will explain in detail how to use the ReturnPath analysis function of IDA (In-DesignAnalysis, design synchronization analysis) in Allegro. PCB designer through design examples performs return path analysis during the PCB design process to help engineers quickly find out whether the return paths of those high-speed signals are appropriate. , in order to ensure the quality of Layout and reduce the heavy loss of recall due to unstable signals after mass production, and achieve design success.

Return path definition

The Importance of ReturnPath Analysis

A detailed explanation of the ReturnPath analysis example

Analysis of ReturnPath Analysis Results

1. What is Return Path?

The operation of electrical products requires a circuit with its signal to operate, just like the negative pole of the battery in the figure below (1) must be connected to the blue wire before the light will turn on. In the early days, we can see that the telegraph system uses the "earth" as the ground plane of the signal loop, and another ground wire can be omitted to reduce the expensive cost. Or if a similar situation in modern life is when a light bulb is to be installed on a car, 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 to light up, which can save many cloths. The trouble of a line, and it is not necessary to consider the return path problem. However, if you want to connect various sensors or processors to the driving system, CAN (vehicle network system) or even ADAS (advanced driver assistance system), it is not as simple as directly connecting and omitting the 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. Similarly, for PCB design, if it is a low-frequency signal, its return path will return with the impedance, but as the frequency increases, the current needs to return to the source in a closed loop, so the return path of the inductor will be more considered, and usually Corresponding to the return path of the upper and lower layers of the wiring to avoid the problem of detouring of the return path caused by the inner layer cutting, so the consideration of the return path of the high-speed signal is more important.

2. What needs ReturnPath analysis?

As mentioned above, it is critical to consider the return paths of high-speed signals, as the slightest carelessness can greatly reduce circuit functionality. Generally speaking, because the DRC inspection of a standard PCB only checks whether the mouse wire is connected or not and whether the safety distance is sufficient, an analysis like ReturnPath is not easy to implement, and an experienced veteran is often required 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 make some specifications for the layout of how to add STItchingVia next to the traces. As for the differential signal after the Via is used, several STItchingVia should be placed next to it. That is another story! It is even necessary to add stitching capacitors to fill those moats that cannot be crossed, resulting in an increase in cost to improve the return path. So if we have an intuitive auxiliary analysis tool, it will analyze the return path according to the geometric structure of the signal, and calculate the ratio of its inductance RPQF (ReturnPathQualityFactor, return path quality factor) without Models. When the RPQF value is closer to 1, it means that the signal wiring is closer to the return path, and the higher the value, the more tortuous and farther the return path is. Moreover, after the analysis is completed, the RPQF values of the relevant signals can be listed directly, as shown in Figure 6 below, allowing us to quickly identify the severity of each signal and correct the unsatisfactory parts. Note: The other Impedance impedance analysis and Coupling coupling interference analysis in IDA (In-DesignAnalysis) can also be implemented according to the inspection process without Models. Quality control of various Layouts can be quickly realized by Quick sieve analysis.

3. How to perform ReturnPath analysis

Now Allegro has imported Sigrity's simulation analysis technology, bringing IDA (In-Design Analysis, design synchronous analysis) into the PCB design process, helping PCB engineers to analyze synchronously in the design, and pre-find the common discontinuity of the return path. , solve the problem in real-time, quickly ensure the quality of the signal return path, improve the design efficiency and reduce the probability of failure. Equally important is that the ReturnPath check on the PCB board also does not require Models and can be easily implemented with a simple process!