PCB (printed circuit board), also known as printed circuit board and printed circuit board in Chinese, is an important electronic component, the support of electronic components and the provider of electrical connection of electronic components. Because it is made by electronic printing, it is called “printed” circuit board.
With the smaller and smaller requirements of PCB size and higher requirements of device density, PCB design is becoming more and more difficult.
How to realize the high distribution rate of PCB and shorten the design time, the author talks about the design skills of PCB planning, layout and wiring.
Before starting wiring, the design should be carefully analyzed and the tool software should be carefully set, which will make the design more meet the requirements.
1. Determine the number of layers of PCB
The size of circuit board and the number of wiring layers need to be determined at the early stage of design. The number of wiring layers and the stack up method will directly affect the wiring and impedance of printed lines. The size of the board is helpful to determine the stacking mode and printed line width to achieve the desired design effect. At present, the cost difference between multilayer boards is very small. At the beginning of design, it is best to use more circuit layers and make the copper coating evenly distributed.
2. Design rules and limitations
To successfully complete the routing task, the routing tool needs to work under the correct rules and restrictions. To classify all signal lines with special requirements, each signal class should have priority. The higher the priority, the stricter the rules. The rules involve the width of printed lines, the maximum number of vias, parallelism, the interaction between signal lines and the limitation of layers. These rules have a great impact on the performance of wiring tools.
Careful consideration of design requirements is an important step in successful cabling.
3. Layout of components
In the process of optimal assembly, design for manufacturability (DFM) rules will restrict the component layout. If the assembly department allows the components to move, the circuit can be properly optimized to facilitate automatic wiring. The defined rules and constraints affect the layout design. The automatic routing tool only considers one signal at a time. By setting the constraints of routing and the layer of deployable signal lines, the routing tool can complete the routing as envisaged by the designer.
For example, for the layout of the power line: ① in the PCB layout, the power decoupling circuit should be designed near each relevant circuit rather than placed in the power part, otherwise it will not only affect the bypass effect, but also flow pulsating current on the power line and ground wire, resulting in tampering; ② For the power supply direction inside the circuit, power shall be supplied from the last stage to the front stage, and the power filter capacitor of this part shall be arranged near the last stage; ③ For some main current channels, such as disconnecting or measuring current during commissioning and testing, current gaps shall be arranged on printed conductors during layout.
In addition, it should be noted that the regulated power supply should be arranged on a separate printed board as far as possible. When the power supply and circuit share the printed board, in the layout, it should be avoided to mix the regulated power supply and circuit elements, or make the power supply and circuit share the ground wire.
Because this kind of wiring is not only easy to cause interference, but also can not disconnect the load during maintenance. At that time, only some printed wires can be cut, thus damaging the printed board.
4. Fan out design
In the fan out design stage, each pin of surface mount device shall have at least one via, so that when more connections are required, the circuit board can carry out inner layer connection, on-line test and circuit reprocessing.
In order to maximize the efficiency of automatic wiring tools, the maximum via size and printed wire must be used as much as possible. It is ideal to set the interval to 50mil. The via type that maximizes the number of routing paths shall be used. After careful consideration and prediction, the design of circuit on-line test can be carried out in the early stage of design and realized in the later stage of production process.
The type of via fan out is determined according to the wiring path and circuit on-line test. The power supply and grounding will also affect the wiring and fan out design.
5. Manual wiring and key signal processing
Manual wiring is an important process of printed circuit board design now and in the future. The use of manual wiring is helpful for automatic wiring tools to complete the wiring work.
By manually wiring and fixing the selected network (net), the path that can be used for automatic wiring can be formed.
Firstly, the key signals can be wired manually or combined with automatic wiring tools. After the wiring is completed, relevant engineers and technicians shall check the signal wiring. After passing the inspection, fix these wires, and then start the automatic wiring of other signals.
Due to the existence of impedance in the ground wire, it will bring common impedance interference to the circuit. Therefore, the points with grounding symbols shall not be connected at will during wiring, which may produce harmful coupling and affect the operation of the circuit.
When the frequency is high, the inductive reactance of the conductor will be several orders of magnitude larger than the resistance of the conductor 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 as compact as possible to make the printed wires as short as possible.
There are mutual inductance and capacitance between printed wires. When the working frequency is large, it will interfere with other parts, which is called parasitic coupling interference. The suppression methods that can be adopted are: ① shorten the signal routing between levels as far as possible; ② Arrange circuits at all levels in the order of signals to avoid signal lines at all levels crossing each other; ③ The conductors of two adjacent panels shall be vertical or cross, not parallel; ④ When signal conductors are to be laid in parallel in the board, these conductors shall be separated by a certain distance as far as possible, or separated by ground wire and power line to achieve the purpose of shielding.
6. Automatic routing
For the wiring of key signals, it is necessary to control some electrical parameters during wiring, such as reducing distributed inductance. After understanding the 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 shall be adopted for automatic wiring of signals. By setting restrictions and prohibited wiring areas to limit the layers used for a given signal and the number of vias used, the wiring tool can automatically route according to the design idea of the engineer. After setting the constraints and applying the created rules, the automatic routing will achieve similar results as expected. After part of the design is completed, it will be fixed to prevent being affected by the later routing process.
The number of wiring depends on the complexity of the circuit and the number of general rules defined. Today’s automatic routing tools are very powerful and can usually complete 100% routing. However, when the automatic wiring tool does not complete all signal wiring, it is necessary to manually route the remaining signals.
7. Arrangement of wiring
For some signals with few constraints, the wiring length is very long. At this time, you can first judge 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.