PCB (Printed Circuit Board), the Chinese name is printed circuit board, also known as printed circuit board, printed circuit board, is an important electronic component, a support body for electronic components, and a provider of electrical connections for electronic components. Because it is made by electronic printing, it is called a "printed" circuit board.
As the PCB size requirements become smaller and higher, the device density requirements become higher and higher, and the PCB design becomes more and more difficult. How to achieve high PCB routing rate and shorten design time? Then let's talk about the design skills of PCB planning, layout and routing.
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 of the PCB
Board size and routing layers need to be determined early in the design. The number of wiring layers and the stack-up 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. At present, the cost difference between multilayer boards is small, and it is better to use more circuit layers and evenly distribute the copper at the beginning of the design.
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, maximum 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 optimal assembly, 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 at a time. By setting the constraints of routing and setting the layers that can route the signal lines, the routing tool can complete the routing as the designer envisioned. For example, for the layout of the power lines:
In the PCB layout, the power supply decoupling circuit should be designed near the relevant circuits, 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 supply line and ground line, causing harassment;
For the power supply direction inside the circuit, 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;
For some major current channels, such as to disconnect or measure 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 board.
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 maximize the efficiency of the automatic routing tool, be sure to use the largest possible via size and trace, with an ideal spacing of 50mil. To use the type of via that maximizes the number of routing paths, 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 key signals
Manual routing is and will be an important process in printed circuit board design, and manual routing helps automatic routing tools to complete the routing work. By manually routing and fixing selected nets, paths that can be followed for automatic routing can be formed.
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 layout should be arranged as compactly as possible to keep the traces 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:
Minimize the signal traces between all levels;
Arrange the circuits at all levels in the order of the signals to avoid the signal lines at all levels crossing each other;
The wires of the two adjacent panels should be vertical or cross, not parallel;
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. Automatic wiring
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, routing tools can automatically route the engineer's design. After setting up the constraints and applying the created rules, auto-routing will achieve similar results as expected, and after a portion of the design is complete, it is fixed to prevent it from being affected by subsequent routing processes.
The number of traces depends on the complexity of the circuit and how many general rules are defined. Today's autorouting tools are very powerful.
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