PCB smt layout tips and considerations

For hardware electronics designers, the PCB smt design is becoming more and more complex. Advanced package devices with increasingly dense pins are used, and the network density per unit area continues to increase, bring more pressure on the layout. At the same time, more engineers cannot meet the 100% coverage rate of the autorouter, and hope to be able to conduct electrical rules to constrain the wiring to meet the signal integrity requirements.

More than 70% of engineers care about signal integrity, including transmission line analysis, signal return paths, matching, crosstalk, topology, and EMI/EMC issues. In addition, questions about pcb smt manufacturability and software usage were also asked. It seems that the engineers are very concerned about high speed. On the contrary, it seems that they are not very concerned about high-density wiring.

This seems to be similar to the current domestic design situation. Many high-speed designs are still in the discussion stage of the program, or in the prototype stage. Engineers foresee or have observed some problems, such as overshoot, undershoot, oscillation, crosstalk, etc. However, for the cause of the entire high-speed problem, the high-speed analysis and simulation methods are not well understood, and high-speed digital circuit design and RF design are often treated as the same problem. I feel that in real-time high-speed analysis, timing analysis is also a key in digital circuit design but is often overlooked. In addition to analysis, in order to reduce the transmission line effect and reduce the PCB smt area, high-density design implementation is also a problem. However, in China, due to concerns about crosstalk, development cycles or commissioning problems, multiple boards are often used to reduce the design density.

For high-speed PCB smt design, pre-simulation analysis is critical, including device selection, model editing, signal distribution, matching strategies, and cascading designs. Pre-simulation analysis cannot stay only in the schematic stage. Edit with topology,you must also use the PCB smt LAYOUT tool for layout detection and pre-layout analysis to estimate the implementation difficulty and routing method. At the same time, for the allocated plane layer, power supply is first divided, the priority of the wiring layer is arranged, and the cross-segmentation of key signals is avoided to the utmost.

After completing the schematic, the general engineer can't wait to start the PCB smt design. I suggest you can spend a day or two checking the schematic. Because at the end of the day, if you find that the schematic design is wrong, the cost of the change is too great. Start PCB smt design, don't be busy wiring, first layout. According to experience, the PCB smt design time is generally allocated according to the three-three system, the layout accounts for 1/3 of the time, the wiring accounts for 1/3, and 1/3 is checked. The layout is quite critical, and experienced engineers have a critical wiring rule at the same time layout. The layout is generally first big and then small, first key and then secondary, first place the device with positioning requirements, in addition to considering the quality of the wiring, the test, processing and other issues should also be considered.

Manual routing is still the choice of most engineers, because many people are not very satisfied with the results of automatic routing. In fact, in some advanced EDA tools, automatic routing has been quite intelligent. Automated wiring does not mean that all determined by tool, and there are many aspects that require human intervention. My personal experience is that network classification (class), routing priority settings, routing rules are the main factors affecting the routing rate and wiring effect.

Some people also call PCB smt design is the transition from abstract concepts to actual products. PCB design is actually a hybrid technology. Designers are required to have an understanding of circuit principles, electromagnetic fields, routing algorithms, production processing, testing, etc., so an excellent PCB engineer should pay attention to learning this knowledge.