How to quickly determine the number of layers, wiring and layout of the PCB?
As PCB size requirements become smaller and smaller, device density requirements become higher and higher, and PCB design becomes more difficult. How to achieve a high PCB layout rate and shorten the design time, then we will talk about the design skills of PCB planning, layout and wiring.
Before starting the wiring, the design should be carefully analyzed and the tool software should be carefully set, which will make the design more in line with the requirements.
1. Determine the number of layers of the PCB
The size of the circuit 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 method will directly affect the wiring and impedance of the printed lines.
The size of the board helps determine the stacking method 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 it is better to use more circuit layers and evenly distribute the copper when designing.
2. Design rules and restrictions
To successfully complete the wiring task, wiring tools need to work under the correct rules and restrictions. To classify all signal lines with special requirements, each signal class should have a priority. The higher the priority, the stricter the rules.
The rules involve the width of the printed lines, the maximum number of vias, parallelism, the mutual influence between signal lines, and layer restrictions. These rules have a great influence on the performance of the wiring tool. Careful consideration of design requirements is an important step for successful wiring.
3. Layout of components
In the optimal assembly process, design for manufacturability (DFM) rules will restrict component layout. If the assembly department allows the components to move, the circuit can be appropriately optimized to make automatic wiring easier.
The defined rules and constraints will affect the layout design. The automatic wiring tool only considers one signal at a time. By setting the wiring constraints and setting the layer of the signal line, the wiring tool can complete the wiring as the designer imagined.
For example, for the layout of the power cord:
①In 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 affect the bypass effect, and pulsating current will flow on the power line and ground line, causing interference;
②For 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;
③For some main current channels, such as disconnecting or measuring current during debugging and testing, current gaps should be arranged on the printed wires during layout.
In addition, it should be noted that the regulated power supply should be arranged on a separate printed circuit board as much as possible during the layout. When the power supply and the circuit share a printed circuit board, in the layout, it is necessary to avoid the mixed layout of the stabilized power supply and the circuit components or to make the power supply and the circuit share the ground wire. Because this kind of wiring is not only easy to produce interference, but also unable to disconnect the load during maintenance, only part of the printed wires can be cut at that time, thus damaging the printed board.
4. Fan-out design
In the fan-out design stage, each pin of the surface mount device should have at least one via, so that when more connections are needed, the circuit board can perform internal connection, online testing, and circuit reprocessing.
In order to maximize the efficiency of the automatic routing tool, the largest via size and printed line must be used as much as possible, and the interval is set to 50mil. It is necessary to adopt the via type that maximizes the number of wiring paths. After careful consideration and prediction, the design of the circuit online test can be carried out at the early stage of the design and realized at the later stage of the production process. Determine the via fan-out type according to the wiring path and circuit online testing. Power and ground will also affect the wiring and fan-out design.
5. Manual wiring and processing of key signals
Manual wiring is an important process of printed circuit board design now and in the future. Using manual wiring helps automatic wiring tools to complete the wiring work. By manually routing and fixing the selected network (net), a path that can be used for automatic routing can be formed.
The key signals are wired first, either manually or combined with automatic wiring tools. After the wiring is completed, the relevant engineering and technical personnel will check the signal wiring. After the inspection is passed, the wires will be fixed, and then the remaining signals will be automatically wired. Due to the existence of impedance in the ground wire, it will bring common impedance interference to the circuit.
Therefore, do not randomly connect any points with grounding symbols during wiring, which may produce harmful coupling and affect the operation of the circuit. At higher frequencies, the inductance of the wire will be several orders of magnitude larger 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 arranged as compactly as possible and the printed wires should be as short as possible. There are mutual inductance and capacitance between the printed wires. When the working frequency is large, it will cause interference to other parts, which is called parasitic coupling interference.
The suppression methods that can be taken are:
① Try to shorten the signal wiring between all levels;
②Arrange all levels of circuits in the order of signals to avoid crossing over each level of signal lines;
③The wires of two adjacent panels should be perpendicular or cross, not parallel;
④ When 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 wiring
For the wiring of key signals, you need to consider controlling some electrical parameters during wiring, such as reducing distributed inductance, etc. After understanding what input parameters the automatic wiring tool has and the influence of input parameters on the wiring, the quality of the automatic wiring can be obtained to a certain extent Guarantee. General rules should be used when automatically routing signals.
By setting restriction conditions and prohibiting wiring areas to limit the layers used by a given signal and the number of vias used, the wiring tool can automatically route the wires according to the engineer’s design ideas. After setting the constraints and applying the created rules, the automatic routing will achieve results similar to the expected results. 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 wiring 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 wiring tool has not completed all signal wiring, it is necessary to manually route the remaining signals.
7. Wiring arrangement
For some signals with few constraints, the wiring length is very long. At this time, you can first determine which wiring is reasonable and which wiring is unreasonable, and then manually edit to shorten the signal wiring length and reduce the number of vias.