PCB Blog - Ingenious skills of PCB board layout and processing of process defects

As PCB board size requirements are getting smaller and smaller, the device density requirements are getting higher and higher, and the PCB board design is becoming more and more difficult. How to achieve a high routing rate of the PCB board and shorten the design time, let's talk about the design skills of PCB board planning, layout and wiring. Careful analysis of the design and careful setup of the tool software should be done before starting routing, which will make the design more compliant.

1. Determine the number of layers on the PCB board
Board size and routing layers need to be determined early in the design. The number of wiring layers and the stacking method will directly affect the wiring and impedance of the traces. The size of the board helps determine the stack-up and trace width to achieve the desired design effect. The cost difference between multi-layer boards is currently small, and the design starts with more circuit layers and an even distribution of copper.

2. Design Rules and Limitations
To successfully complete routing tasks, routing tools need to work within the correct rules and constraints. To classify all signal lines with special requirements, each signal class should have a priority, and the higher the priority, the stricter the rules. Rules related to trace width, number of vias, parallelism, interaction between signal lines, and layer limitations have a large impact on the performance of routing tools. Careful consideration of design requirements is an important step in successful routing.

3. Layout of components
During the assembly process, design for manufacturability (DFM) rules impose constraints on component placement. If the assembly department allows the components to move, the circuit can be properly optimized for easier automatic routing. The rules and constraints you define affect the layout design. The automatic routing tool only considers one signal. By setting the constraints of the routing and setting the layers where the signal line can be routed, the routing tool can complete the routing as the designer envisioned.
For example, for the layout of the power lines:
1) In the PCB board layout, the power supply decoupling circuit should be designed near each related circuit, rather than placed in the power supply part, otherwise it will not only affect the bypass effect, but also flow pulsating current on the power line and ground line, causing harassment;
2) For the power supply direction inside the circuit, the power supply should be adopted from the last stage to the previous stage, and the power supply filter capacitor of this part should be arranged near the last stage;
3) For some main current channels, such as disconnecting or measuring current during debugging and testing, current gaps should be arranged on the printed conductors during layout. In addition, pay attention to the layout of the regulated power supply on a separate printed board as much as possible. When the power supply and the circuit share the printed board, in the layout, the mixed arrangement of the regulated power supply and the circuit components or the grounding of the power supply and the circuit should be avoided. Because this kind of wiring is not only prone to interference, but also cannot disconnect the load during maintenance, and only part of the printed wires can be cut, thereby damaging the printed circuit boards.

4. Fan-Out Design
During the fan-out design phase, surface mount devices should have at least one via per pin to allow the board to perform interlayer connections, in-circuit testing, and circuit reprocessing when more connections are required. In order to make the automatic routing tool efficient, be sure to use the size of the vias and traces as much as possible, and the interval is ideally set to 50mil. The type of via to use that makes the number of routing paths available. After careful consideration and prediction, the design of the circuit in-circuit test can be carried out in the early stage of design and realized in the later stage of the production process. The type of via fan-out is determined based on the routing path and circuit in-circuit testing. Power and grounding also affect routing and fan-out design.

5. Manual wiring and processing of critical signals
Manual routing is an important process in printed circuit board design now and in the future, and manual routing helps automatic routing tools to complete the routing work. By manually routing and fixing selected nets, a path can be formed that can be followed for automatic routing. Route critical signals first, either manually or in combination with automated routing tools. After the wiring is completed, the relevant engineering and technical personnel will check the wiring of these signals. After the inspection is passed, the wires will be fixed, and then the automatic wiring of the remaining signals will be started. Due to the existence of impedance in the ground wire, it will bring common impedance interference to the circuit. Therefore, when wiring, do not connect any points with ground symbols at will, which may cause harmful coupling and affect the operation of the circuit. At higher frequencies, the inductive reactance of the wire will be orders of magnitude greater than the resistance of the wire itself. At this time, even if only a small high-frequency current flows through the wire, a certain high-frequency voltage drop will be generated. Therefore, for high-frequency circuits, the PCB board layout should be arranged as compactly as possible, and the printed conductors should be as short as possible.
There are also mutual inductance and capacitance between the printed wires. When the operating frequency is large, it will cause interference to other parts, which is called parasitic coupling interference. Possible suppression methods are:
1) Try to shorten the signal wiring between all levels;
2) Arrange the circuits at all levels in the order of the signals to avoid the signal lines at all levels crossing each other;
3) The wires of the two adjacent panels should be vertical or cross, not parallel;
4) When the signal wires are to be arranged in parallel in the board, these wires should be separated by a certain distance as much as possible, or separated by ground wires and power wires to achieve the purpose of shielding.

6. Auto-routing
For the wiring of key signals, it is necessary to consider controlling some electrical parameters during wiring, such as reducing the distributed inductance, etc. After understanding the input parameters of the automatic wiring tool and the influence of the input parameters on the wiring, the quality of the automatic wiring can be obtained to a certain extent. ensure. General rules should be used when autorouting signals. By setting constraints and no-routing areas to limit the layers used for a given signal and the number of vias used, the routing tool can automatically route according to the engineer's design thinking. After setting up the constraints and applying the created rules, the auto-routing will achieve similar results as expected, and after a part of the design is complete, it is fixed to prevent it from being affected by the subsequent routing process. The number of traces depends on the complexity of the circuit and how many general rules are defined. Today's auto-routing tools are very powerful and usually do 100% of the routing. However, when the automatic routing tool does not complete the routing of all signals, the remaining signals need to be manually routed.

7. Wiring arrangement
For some signals with few constraints, the length of the wiring is very long. At this time, you can first judge which wiring is reasonable and which is unreasonable, and then shorten the signal wiring length and reduce the number of vias by manual editing on PCB board.