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 for electronic components, and a provider of electrical connections for electronic components. Because it is made using electronic printing, it is called a "printed" circuit board.
With the smaller and smaller PCB size requirements and higher device density requirements, the difficulty of PCB design is also increasing. How to achieve a high PCB pass rate and shorten the design time? Then let's talk about the design skills of PCB planning, layout and wiring.
Careful analysis of the design and careful setting of the tool software should be made before starting the wiring, which will make the design more in line with the requirements.
1. Determine the number of layers of the PCB
Board size and wiring layers need to be determined early in the design. The number of wiring layers and the way of stacking (Stack-up) will directly affect the wiring and impedance of the printed lines.
The size of the board helps determine the stackup and trace width to achieve the desired design effect. At present, the cost difference between multi-layer boards is very small, and it is better to use more circuit layers and distribute copper evenly when starting the design.
2. Anti-T8 is not only P but profit
To successfully complete the routing task, the routing tool needs to work within the correct rules and constraints. To classify all signal lines with special requirements, each signal class should have a priority, the higher the priority, the stricter the rules.
Rules related to trace width, maximum number of vias, parallelism, interaction between signal lines, and layer constraints have a great 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 component movement, the circuit can be properly optimized and more
Facilitates automatic routing.
The defined rules and constraints affect the layout design. The automatic routing tool will only consider one signal at a time. By setting the routing constraints and setting the layer of the routing signal line, the routing tool can complete the routing as the designer imagined. For example, for the layout of the power cord:
In the PCB layout, the power decoupling circuit should be designed near the relevant circuits instead of placed in the power supply part, otherwise it will not only affect the bypass effect, but also flow through the power line and ground line.
Pulsating currents, causing crosstalk;
For the power supply direction inside the circuit, power should be supplied from the final stage to the previous stage, and the power filter capacitor of this part should be arranged near the final stage;
For some main current channels, if the current is to be disconnected or measured during debugging and testing, current gaps should be arranged on the printed wires during layout.
In addition, pay attention to the layout of the regulated power supply, and arrange it on a separate printed board as much as possible. When the power supply and the circuit share the printed board, in the layout, it should avoid the mixed arrangement of the regulated power supply and the circuit components or make the power supply and the circuit share the ground wire. 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 at that time, thus damaging the printed board.
4. Fan-out design
During the fan-out design phase, each pin of the surface mount device should have at least one via so that the board can be used for interlayer connections, in-circuit testing, and circuit reprocessing when more connections are required.
In order to maximize the efficiency of the automatic routing tool, it is necessary to use the largest via size and printed line possible, and the spacing is set to 50mil, which is ideal. To adopt the via type that maximizes the number of wiring paths, after careful consideration and prediction, the design of circuit online testing can be carried out in the early stage of design and realized in the later stage of the production process. The via fan-out type is determined according to the routing path and the circuit in-circuit test, and the power supply and ground will also affect the routing and fan-out design.
5. Manual wiring and key signal processing
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 wiring and fixing the selecte net (net), a path that can be used for automatic wiring 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, these lines will be fixed, and then the automatic wiring of the remaining signals will start. Due to the existence of impedance in the ground wire, it will bring common impedance interference to the circuit.
Therefore, it is not allowed to connect the points with grounding symbols arbitrarily during wiring, 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 occur.
Therefore, for high-frequency circuits, the PCB layout should be as compact as possible to keep the printed wires as short as possible. There are also mutual inductance and capacitance between the printed wires. When the operating frequency is high, it will cause interference to other parts, which is called parasitic coupling interference. The suppression methods that can be adopted are:
Keep signal traces between stages as short as possible;
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 adjacent two panels should be vertical or crossed, not parallel;
When the signal wires are to be laid 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 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 impact 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 restrictions and forbidden routing areas to limit the layers used by a given signal and the number of vias used, the routing tool can automatically route according to the engineer's design ideas. After setting the constraints and applying the created rules, the automatic routing will achieve a result similar to the expectation. After a part of the design is completed, it will be fixed to prevent it from being affected by the subsequent routing process.
The number of routings depends on the complexity of the circuit and the number of general rules defined. The current automatic routing tools are very powerful,
