At present, with the PCB size requirements getting smaller and smaller, the device density requirements are getting higher and higher, and the difficulty is gradually increasing. How to reduce design time while maintaining quality? Engineers need to have strong technical knowledge, as well as master some design skills.
1. determine the number of PCB layers
The size of the board and the number of wiring layers need to be determined at the beginning of the design. The number of wiring layers and the STack-up mode will directly affect the wiring and impedance of the printed line.
The size of the board helps determine the stacking pattern and the width of the printed line to achieve the desired design effect. At present, the cost difference between multi-layer boards is very small, and more circuit layers are used at the beginning of the design and the coating copper is evenly distributed.
2. Design rules and restrictions
To successfully complete the wiring task, the wiring tool needs to work under the correct rules and restrictions. To classify all signal lines with special requirements, each signal class should have a priority, and the higher the priority, the stricter the rules.
The rules involve the width of the printed line, the number of holes, the parallelism, the interaction between the signal lines and the layer limits, and these rules have a great impact on the performance of the wiring tool. Careful consideration of design requirements is an important step in successful wiring.
3. the layout of components
Design for Manufacturability (DFM) rules restrict component layout during assembly. If the assembly department allows the components to move, the circuit can be properly optimized for automatic routing.
For example, for the layout of power cables:
(1) The power decoupling circuit should be designed near the relevant circuits, rather than placed in the power supply part, otherwise it will affect the bypass effect, and will flow pulsating current on the power line and ground wire, causing interference;
(2) For the power supply direction inside the circuit, power supply should be taken from the final stage to the forward stage, and the power filter capacitor of this part is arranged near the final stage;
(3) For some main current channels, such as to disconnect or measure the current during debugging and detection, the current notch should be arranged on the printed wire during layout.
In addition, pay attention to the layout of the regulated power supply, as far as possible arranged on a separate printed board. When the power supply and circuit are combined with the printed board, in the layout, the mixed layout of the regulated power supply and circuit components should be avoided or the power supply and circuit are combined with the ground wire.
Because this wiring is not only easy to cause interference, but also can not disconnect the load during maintenance, then only part of the printed wire can be cut, 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 through-hole so that the board can perform inner connections, in-line testing, and circuit reprocessing when more connections are needed.
5. manual wiring and key signal processing
Manual wiring is an important process of PCB design now and in the future, and the use of manual wiring helps automatic wiring tools to complete the wiring work.
By manually routing and fixing the selected network (net), a path can be formed for automatic routing.
First of all, the key signal is routed, either manually or in combination with automatic routing tools.
After the wiring is completed, the signal wiring is checked by the relevant engineering and technical personnel, and after the check is passed, the lines are fixed, and then the rest of the signals are automatically routed. Because of the existence of impedance in the ground wire, it will bring common impedance interference to the circuit.
6. automatic wiring
For the wiring of key signals, it is necessary to consider the control of some electrical parameters during wiring, such as reducing the distributed inductance, etc., after understanding what input parameters of automatic wiring tools and the impact of input parameters on wiring, the quality of automatic wiring can be guaranteed to a certain extent. General rules should be applied in the automatic routing of signals.
By setting limits and forbidden wiring areas to limit the layers used by a given signal and the number of holes used, the wiring tool can automatically route according to the engineer's design ideas.
After the constraints are set and the rules created are applied, the automatic wiring will achieve similar results as expected, and after a part of the design is completed, it is fixed to prevent the influence of the later wiring process.
The number of routing times depends on the complexity of the circuit and the number of general rules defined. Today's automatic wiring tools are very powerful and can usually complete 100% of the wiring. However, when the automatic routing tool does not complete all signal routing, it is necessary to manually route the remaining signals.
7. the arrangement of wiring
Some signals with few constraints, the length of the wiring is very long, then you can first determine which wiring is reasonable, which wiring is not reasonable, and then manually edit to shorten the length of the signal wiring and reduce the number of holes.
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