PCB layout clever skills and process defect handling

PCB(PrintedCircuitBoard), the Chinese name is called printed circuit board, also known as printed circuit board, printed circuit board, is an important electronic component, is the support body of electronic components, is the electrical connection provider of electronic components. Because it is produced by electronic printing, it is called a "printed" circuit board.

As PCB size requirements become smaller and smaller, device density requirements become higher and higher, PCB design is becoming more and more difficult. How to achieve PCB high distribution flux and shorten the design time, in this author to talk about PCB planning, layout and routing design skills.

Before starting the wiring, the design should be carefully analyzed and the tool software should be carefully set up, which will make the design more consistent with the requirements.

1. Determine the number of layers of the PCB

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 it is best to use more circuit layers and make the copper evenly distributed at the beginning of the design.

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. Rules related to the width of the printed line, the maximum number of holes, parallelism, the interaction between the signal lines, and layer limitations, 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. Component layout

During optimal assembly, design for manufacturability (DFM) rules restrict component layout. If the assembly department allows the components to move, the circuit can be properly optimized for automatic routing. The rules and constraints you define affect the layout design. The automatic wiring tool only considers one signal at a time. By setting the constraints of the wiring and setting the layer of the deploable signal line, the wiring tool can complete the wiring as envisaged by the designer.

For example, for the layout of power cables:

(1) In the PCB layout, 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.

In order to maximize the efficiency of the automatic wiring tool, it is necessary to use the largest hole size and printed line as possible, and the interval is set to 50mil. Use the through-hole type that maximizes the number of routing paths. After careful consideration and prediction, the design of the circuit online test can be carried out in the early stage of design and realized in the later stage of the production process.

The type of through-hole fan-out is determined based on the wiring path and circuit online testing, and the power supply and grounding also affect the wiring and fan-out design.

5. Manual wiring and critical 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. Therefore, any point with a grounding symbol cannot be connected at will during wiring, which may produce harmful coupling and affect the work of the circuit.

At higher frequencies, the inductive reactance 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 be generated. Therefore, for high-frequency circuits, the PCB layout is as compact as possible, so that the printed wire is as short as possible.

There is mutual inductance and capacitance between printed wires, and when the operating frequency is large, it will interfere with other parts, which is called parasitic coupling interference. The suppression methods that can be taken are:

① Try to shorten the signal routing between all levels;

② Arrange all levels of circuits according to the order of signals to avoid each level of signal lines crossing each other;

③ The wires of the adjacent two panels should be vertical or cross, not parallel;

④ When the board is to be arranged in parallel signal wires, these wires should be spaced as far apart as possible, or separated from the ground line and the power line to achieve the purpose of shielding.

6. Automatic routing

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. Wiring arrangement

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.