PCB Tech - Seven steps to determine PCB layout and routing

Seven steps to determine PCB layout and routing

First, let's look at what PCB (PrintedCircuitBoard) is. The Chinese name is printed circuit board, also known as printed circuit board, printed circuit board. It is an important electronic component, the support of electronic components, and the electrical connection of electronic components. provider. Because it is made by electronic printing, it is called a "printed" circuit board. As PCB size requirements are getting smaller and smaller, device density requirements are getting higher and higher, and PCB design is becoming more and more difficult. How to achieve a high PCB layout rate and shorten the design time, here, the author talks 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 to 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 at the beginning of the design.

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 large number of vias, the parallelism, the mutual influence between the signal lines, and the 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, the layout of components
In the process of optimizing the assembly process, the design for manufacturability (DFM) rules impose restrictions on the component layout. If the assembly department allows the components to move, the circuit can be appropriately optimized, which is more convenient for automatic wiring. 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 line: 1. The power decoupling circuit should be designed near the relevant circuits in the PCB layout, and not placed in the power section, otherwise it will affect the bypass effect and will flow through the power line and the ground line. The pulsating current causes interference; 2. 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; 3. For some main current channels, such as debugging and The current should be disconnected or measured during the detection process, and 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 should be avoided that the stabilized power supply and the circuit components are mixed or the power supply and the circuit share the ground wire.
Because this kind of wiring is not only prone to interference, but also the load cannot be disconnected during maintenance. At that time, only part of the printed wires can be cut, 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 be internally connected, online testing, and circuit reprocessing.
In order to make the automatic routing tool more efficient, it is necessary to use as large a via size and printed lines as possible, and the interval is ideally set to 50mil. Use a via type that makes the number of routing paths larger. After careful consideration and prediction, the design of the circuit online test can be carried out at the initial 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 test. The power supply and grounding 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. The use of manual wiring helps automatic wiring tools to complete the wiring work.
By manually routing and fixing the selected network (net), it is possible to form a path that can be used for automatic routing.
First, route the key signals, either manually or in combination 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 connect any points with a grounding symbol at will during wiring, which may cause harmful coupling and affect the operation of the circuit.
When the frequency is higher, 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 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 adopted are: 1. Shorten the signal wiring between all levels as much as possible; 2. Arrange the circuits of all levels according to the order of the signals to avoid crossing the signal lines of each level; 3. The wires of two adjacent panels should be vertical or crossed. 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. After understanding the input parameters of the automatic wiring tool 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 adopted when automatically routing signals. By setting restrictions 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 expectations. 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
Some signals with few constraints have very long wiring lengths. 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.

The above is the ten steps of PCB layout, have you learned it?