PCB Technical - PCB design for efficient automatic routing

Today, this article introduces the design techniques and key points of PCB planning, layout and wiring.

1. Determine the number of layers of the PCB

For many years, people have always thought that the lower the number of PCB layers, the lower the cost, but there are many other factors that affect the manufacturing cost of the circuit board. In recent years, the cost difference between multilayer boards has been greatly reduced. It is best to use more circuit layers and evenly distribute the copper at the beginning of the design, so as to avoid discovering that a small number of signals do not meet the defined rules and space requirements until the end of the design, so that new layers are forced to be added. Careful planning before designing will reduce a lot of troubles in wiring.

2. Design rules and restrictions

The automatic routing tool itself does not know what to do. In order to complete the wiring task, the wiring tool needs to work under the correct rules and restrictions. Different signal lines have different wiring requirements. All signal lines with special requirements must be classified, and different design classifications are different. 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, the degree of parallelism, the mutual influence between the signal lines, and the limitation of layers. 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 the components

To optimize the assembly process, design for manufacturability (DFM) rules impose restrictions on 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 PCB layout design.

The routing channel and via area need to be considered during layout. These paths and areas are obvious to the designer, but the automatic routing tool only considers one signal at a time. By setting routing constraints and setting the layer of the signal line, the routing tool can be as the designer imagined Complete the wiring like that.

4. Fan-out design

In the fan-out design stage, to enable automatic routing tools to connect component pins, 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 layered Connection, online testing (ICT) 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 ideally set to 50mil. Use the via type that maximizes the number of routing paths. When carrying out fan-out design, it is necessary to consider the problem of circuit online testing. Test fixtures can be expensive, and they are usually ordered when they are about to go into full production. If only then consider adding nodes to achieve 100% testability, it would be too late.

After careful consideration and prediction, the design of circuit online test can be carried out at the early stage of the design and realized in the later stage of the production process. The type of via fan-out is determined according to the wiring path and circuit online test. The power supply and grounding will also affect the wiring and fan-out design. . In order to reduce the inductive reactance generated by the connection line of the filter capacitor, the vias should be as close as possible to the pins of the surface mount device, and manual wiring can be used if necessary. This may affect the originally envisaged wiring path, and may even cause you to re- Consider which type of via to use, so the relationship between via and pin inductance must be considered and the priority of via specifications must be set.

5. Manual wiring and processing of key signals

Although this article mainly discusses automatic wiring, 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), a path that can be used for automatic routing can be formed

Regardless of the number of key signals, these signals should be routed first, either manually or in combination with automatic routing tools. Critical signals usually have to pass careful circuit design to achieve the desired performance. After the wiring is completed, the relevant engineering personnel will check the signal wiring. This process is relatively easy. After the inspection is passed, fix these lines, and then start to automatically route the remaining signals

6. Automatic wiring

The wiring of key signals needs to consider controlling some electrical parameters during wiring, such as reducing distributed inductance and EMC, etc. The wiring of other signals is similar. All EDA vendors will provide a way to control these parameters. After understanding the input parameters of the automatic wiring tool and the influence of the input parameters on the wiring, the quality of the automatic wiring can be guaranteed to a certain extent.

General rules should be used for automatic routing of 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. If the number of layers used by the automatic routing tool and the number of vias are not limited, each layer will be used during automatic routing, and many vias will be generated.

After setting the constraints and applying the rules created, the automatic routing will achieve results similar to expectations. Of course, some sorting work may be required, and space for other signal and network wiring needs to be ensured. After a part of the design is completed, fix it to prevent it from being affected by the subsequent wiring process.

Use the same steps to route the remaining signals. The number of wiring depends on the complexity of the circuit and the number of general rules you define. After each type of signal is completed, the constraints of the remaining network wiring will be reduced. But with it comes a lot of signal wiring that requires manual intervention. 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, the remaining signals need to be manually wired.

7. The design points of automatic wiring include:

1. Change the settings slightly and try a variety of route wiring;

2. Keep the basic rules unchanged, try different wiring layers, different printed lines and spacing widths, different line widths, and different types of vias such as blind holes, buried holes, etc., and observe how these factors affect the design results;

3. Let the wiring tool process those default networks as needed;

4. The less important the signal is, the more freedom the automatic wiring tool has for its wiring.

8. Arrangement of wiring

If the EDA tool software you are using can list the signal wiring length, check these data, you may find that some signal wiring lengths with few constraints are very long. This problem is relatively easy to deal with, and the signal wiring length can be shortened and the number of vias can be reduced by manual editing. In the finishing process, you need to determine which wiring is reasonable and which wiring is unreasonable. Like manual wiring design, automatic wiring design can also be sorted and edited during the inspection process.

9. The appearance of the circuit board

In the past,PCB design often paid attention to the visual effect of PCB, but now it is different. The automatically designed circuit board is not as beautiful as the manual design, but the electronic characteristics can meet the specified requirements, and the complete performance of the design is guaranteed.