Techniques of PCB layout and processing defects

1. Determine the number of PCB layers

The board size and the number of wiring layers need to be determined early in the design. The number of wiring layers and the STACK-up method 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 lines to achieve the desired design effect. Currently, the cost difference between multi-layer boards is very small. It is better to use more circuit layers and evenly distribute copper during the initial design.

2. Design rules and restrictions

To successfully complete the wiring tasks, the wiring tools need to work under the correct rules and restrictions. To classify all special signal lines, each signal class should have priority. The higher the priority, the stricter the rules. The rules relate to the width of the trace, the maximum number of vias, the parallelism, the mutual influence between the signal lines, and the restrictions of the layers. These rules have a great influence on the performance of the routing tool.

Serious consideration of design requirements is an important step in successful cabling.

3. The layout of the components

In the optimization of the assembly process, the DFM rules impose restrictions on the layout of components. If the assembly department allows components to move, the circuit can be properly optimized for easier autowiring. 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 the wiring and setting the layers of the signal lines, the routing tool can complete the wiring as the designer imagined.

For example, for the layout of the power cord:

In the PCB layout, the power supply decoupling circuit should be designed near each related circuit, instead of being placed in the power supply, otherwise it will not only affect the bypass effect, but will also flow pulsating current on the power line and ground line, causing nuisance;

For the internal power supply of the circuit, power supply should be taken from the last stage to the front stage, and the power supply filter capacitor of this part should be arranged near the final stage;

For some major current channels, such as disconnecting or measuring current during debugging and testing, current gaps should be placed on the conductors during layout.

In addition, pay attention to the regulated power supply in the layout, as far as possible on a separate printed circuit board. When the power supply and the circuit are used together with the printed circuit board, in the layout, the regulated power supply and the circuit elements should be prevented from being mixedly laid or the power supply and the circuit should be combined with the ground wire.

Because this kind of wiring is not only easy to cause interference, at the same time, it is impossible to disconnect the load during the maintenance, and only part of the printed conductors can be cut at the time, 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 via, so that when more connections are required, the board can make inner-layer connections, in-circuit testing, and circuit reprocessing.

In order to maximize the efficiency of the auto-wiring tool, it is imperative to use the largest via size and traces as much as possible, with an interval setting of 50 mils ideal. Use the type of via that maximizes the number of routing paths. After careful consideration and prediction, the design of the circuit on-line test can be carried out in the early stage of the design and implemented later in the production process.

The type of via fanout is determined based on the routing path and the circuit's on-line test. Power and ground can also affect the layout and fanout design.

5. Manual wiring and key signal processing

Manual routing is an important process for PCB design now and in the future, and manual routing helps auto-wiring tools to complete the wiring work.

By manually wiring and fixing the selected network (net), it is possible to form a route that can be based on the automatic routing.

The key signals are first wired, manually routed or combined with automatic routing tools. After the wiring is completed, the relevant engineers and technicians will inspect these signal wirings. After the inspection passes, the wires are fixed and then the other signals are automatically routed.

Due to the presence of impedance in the ground line, it will cause common impedance interference to the circuit. Therefore, you must not randomly connect any points with ground symbols during wiring. This may cause harmful coupling and affect the operation of the circuit.

At higher frequencies, the inductance of the wire will be several orders of magnitude greater than the resistance of the wire itself. At this time, even if only a small high-frequency current flows on the wire, a certain high-frequency voltage drop will occur. Therefore, for high-frequency circuits, the PCB layout is as compact as possible, making the traces as short as possible.

There are mutual inductance and capacitance between the printed conductors. When the operating frequency is large, it will cause interference to other parts, which is called parasitic coupling interference. The following suppression methods can be taken:

Minimize signal traces at all levels;

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

The two adjacent panels must be perpendicular or cross-wired and must not be parallel.

When the signal conductors are to be laid in parallel in the board, these conductors should be separated by a certain distance as much as possible, or separated by a ground wire or a power cord, so as to achieve the purpose of shielding.

6. Automatic routing

The wiring of key signals needs to consider controlling some electrical parameters during wiring, such as reducing the distributed inductance. After understanding the input parameters of the automatic routing tool and the influence of the input parameters on the wiring, the quality of the automatic wiring can be obtained to some extent. Guarantee.

General rules should be used when auto-routing signals. By setting restrictions and inhibiting the wiring area to define the layer used for a given signal and the number of vias used, the routing tool can automatically route according to the engineer's design philosophy. After the constraints are set and the rules created by the application are applied, the automatic routing will be similar to the expected result. After a part of the design is completed, it is fixed to prevent it from being affected by the subsequent wiring 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 typically complete 100% of the wiring. However, when the automatic routing tool does not complete all signal wiring, the remaining signals need to be manually routed.

7. The arrangement of the routing

For signals with few constraints, the length of the wiring is very long. At this time, we can first determine which wirings are reasonable and which ones are unreasonable. Then we need to manually edit to shorten the signal wiring length and reduce the number of vias.