PCB (Printed Circuit Board), also known as a printed circuit board or printed wiring board in Chinese, is an important electronic component. It serves as a support for electronic components and provides electrical connections between them. Because it is made using electronic printing techniques, it is called a "printed" circuit board.

　　As PCB size requirements become smaller and component density requirements become higher, PCB design is becoming increasingly difficult. To achieve high PCB routing rates and shorten design time, let's discuss design techniques for PCB planning, placement, and routing. Before starting routing, careful analysis of the design and careful setup of the software tools are necessary to ensure the design meets requirements.

　 　1. Determine the number of PCB layers

　　The circuit board size and the number of routing layers need to be determined at the beginning of the design. The number of routing layers and the stack-up method directly affect the routing of printed lines and impedance. The size of the board also helps determine the stack-up method and printed line width to achieve the desired design effect. Currently, the cost difference between multilayer boards is small; using more circuit layers and evenly distributing copper is recommended at the start of the design.

　 　2. Design rules and constraints

　　To successfully complete the routing task, the routing tool needs to work under the correct rules and constraints. Signal lines with special requirements should be categorized, each with a priority level. Higher priority signals require stricter rules. Rules involve printed line width, the number of vias, parallelism, mutual influence between signal lines, and layer limitations. These rules significantly impact the performance of the routing tool. Carefully considering design requirements is a crucial step for successful routing.

　 　3. Component placement

　　In advanced optimized assembly, Design for Manufacturing (DFM) rules will restrict component placement. If the assembly department allows component movement, the circuit can be appropriately optimized for easier autorouting. Therefore, the defined rules and constraints will affect the layout design. Autorouting tools only consider one signal at a time. By setting routing constraints and specifying the layers for routable signal lines, the routing tool can complete the routing as the designer intends. For example, for power supply line placement:

　　① In PCB layout, the power decoupling circuit should be designed near the relevant circuits, not in the power supply section. Otherwise, it will affect the bypass effect and cause ripple current to flow through the power line and ground line, resulting in crosstalk;

　　② For the internal power supply path of the circuit, power should be supplied from the last stage to the previous stage, and the power filter capacitor for this part should be arranged near the last stage;

　　③ For some major current channels, such as those that need to be disconnected or measured during debugging and testing, current gaps should be arranged on the printed conductors during layout.

　　In addition, when placing voltage regulators, try to arrange them on separate printed circuit boards. When the power supply and circuit share a printed circuit board, avoid mixing the voltage regulator and circuit components in the layout, or sharing the ground line between the power supply and circuit. This wiring is not only prone to interference but also makes it impossible to disconnect the load during maintenance, requiring cutting parts of the printed conductors, thus damaging the printed circuit board.

　 　4. Fan-out design

　　In the fan-out design stage, each pin of a surface mount device should have at least one via to allow for inner layer connections, in-circuit testing, and circuit rework when more connections are needed. To improve the efficiency of the autorouting tool, use larger via sizes and printed lines whenever possible. A spacing of 50mil is ideal. Use via types with a large number of routing paths. After careful consideration and prediction, in-circuit testing design can be done at the beginning of the design and implemented in the later stages of production. Determine the via fan-out type based on routing paths and in-circuit testing. Power and ground will also affect routing and fan-out design.

　　 5. Manual routing and critical signal handling

　　Manual routing is and will continue to be a crucial step in printed circuit board design. Manual routing helps the autorouting tool complete the routing work. By manually routing and fixing selected nets, paths can be created for the autorouting tool to follow.

　　First, route the critical signals, either manually or using a combination of manual and autorouting tools. After routing, relevant engineering and technical personnel should check these signal routings. Once the check is passed, these lines are fixed, and then autorouting of the remaining signals begins. Due to the impedance in the ground line, common impedance interference will be introduced to the circuit. Therefore, points with ground symbols should not be connected arbitrarily during routing, as this may cause harmful coupling and affect circuit operation. At higher frequencies, the inductive reactance of the conductor will be several orders of magnitude larger than the conductor's resistance. Even a small high-frequency current flowing through the conductor will produce a certain high-frequency voltage drop. Therefore, for high-frequency circuits, the PCB layout should be as compact as possible, making the printed conductors as short as possible. There is also mutual inductance and capacitance between printed conductors. When the operating frequency is high, it will interfere with other parts, which is called parasitic coupling interference. Suppression methods include:

　　① Minimize the signal trace length between stages;

　　② Arrange the stages in signal order to avoid signal lines crossing each other;

　　③ Conductors on adjacent boards should be perpendicular or intersecting, not parallel;

　　④ When parallel signal conductors are required on the board, keep them at a certain distance or separate them with ground lines and power lines for shielding.

　　6. Autorouting

　　Routing critical signals requires controlling certain electrical parameters during routing, such as minimizing distributed inductance. After understanding the input parameters of the autorouting tool and their impact on routing, the quality of autorouting can be ensured to a certain extent. General rules should be used when autorouting signals. By setting constraints and prohibited routing areas to limit the layers and the number of vias used for a given signal, the routing tool can autoroute according to the engineer's design ideas. After setting the constraints and applying the created rules, the autorouting will achieve results close to expectations. After completing a portion of the design, fix it to prevent it from being affected by subsequent routing processes. The number of routing iterations depends on the complexity of the circuit and the number of general rules defined. Current autorouting tools are very powerful and can usually complete 100% of the routing. However, if the autorouting tool does not complete all signal routing, the remaining signals need to be manually routed.

　　 7. Routing cleanup

　　For signals with few constraints and long routing lengths, determine which routings are reasonable and which are not, and then manually edit to shorten signal routing length and reduce the number of vias.