Design skills and key points of realizing efficient automatic wiring of PCB

Although the current EDA tools are very powerful, with the smaller and smaller PCB size requirements and higher device density, the difficulty of PCB design is not small. How to achieve high PCB throughput and shorten design time? This paper introduces the design skills and key points of PCB planning, layout and wiring. Now the time of PCB design is getting shorter and shorter, the smaller and smaller circuit board space, the higher and higher device density, extremely harsh layout rules and large-size components make the work of designers more difficult. In order to solve the design difficulties and speed up the listing of products, many manufacturers tend to use special EDA tools to realize PCB design. However, special EDA tools can not produce ideal results, nor can they achieve 100% deployment rate, and they are very messy. It usually takes a lot of time to complete the rest of the work.

There are many popular EDA tools and software on the market, but they are similar except that the terms used and the position of function keys are different. How to use these tools to better realize PCB design? Careful analysis of the design and careful setting of the tool software before wiring will make the design more in line with the requirements. The following is the general design process and steps.

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. If the design requires the use of high-density ball grid array (BGA) components, the minimum number of wiring layers required for the wiring of these devices must be considered. The number of wiring layers and stack up mode will directly affect the wiring and impedance of printed wiring. The size of the board is helpful to determine the stacking mode and printed line width to achieve the desired design effect.

For many years, people always think that the fewer layers of circuit board, the lower the cost, but there are many other factors affecting the manufacturing cost of circuit board. In recent years, the cost difference between multilayer boards has been greatly reduced. At the beginning of the design, it is best to use more circuit layers and evenly distribute the copper coating, so as to avoid finding that a small number of signals do not meet the defined rules and space requirements near the end of the design, so as to be forced to add new layers. Careful planning before design will reduce a lot of trouble in wiring.

2. Design rules and limitations

The auto routing tool itself does not know what to do. In order to complete the routing task, the routing tool needs to work under the correct rules and restrictions. Different signal lines have different wiring requirements. All signal lines with special requirements shall be classified, and different design classifications are also different. 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 order to optimize the assembly process, 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.

Routing channel and via area shall be considered during layout. These paths and areas are obvious to designers, but the automatic wiring tool will only consider one signal at a time. By setting wiring constraints and setting the layer of deployable signal lines, the wiring tool can complete the wiring as envisaged by the designer.

4. Fan out design

In the fan out design stage, in order to enable the automatic wiring tool to connect the component pins, each pin of the surface mount device should have at least one via, so that the circuit board can carry out inner connection, on-line test (ICT) and circuit reprocessing when more connections are required.

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. The problem of circuit on-line test should be considered in fan out design. Test fixtures can be expensive and are usually ordered only when they are about to be put into full production. It is too late to consider adding nodes to achieve 100% testability.

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 can be 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. In order to reduce the inductive reactance generated by the connecting line of the filter capacitor, the via should be as close to the pin of the surface mount device as possible. If necessary, manual wiring can be used, which may affect the originally envisaged wiring path and even cause you to reconsider which via to use. Therefore, the relationship between the via and the pin inductive reactance must be considered and the priority of the via specification must be set.

5. Manual wiring and key signal processing

Although this paper mainly discusses the problem of automatic wiring, manual wiring is an important process of printed circuit board design now and in the future. The use of manual wiring helps the automatic wiring tool to complete the wiring work. As shown in Fig. 2a and Fig. 2B, a path that can be used for automatic wiring can be formed by manually wiring and fixing the selected network (net).

Regardless of the number of key signals, these signals can be wired first, either manually or in combination with automatic wiring tools. Key signals usually have to be carefully designed to achieve the desired performance. After the wiring is completed, the relevant engineers will check these signal wiring. This process is much easier. After passing the inspection, fix these wires, and then start the automatic wiring of other signals.

6. Automatic routing

For the wiring of key signals, it is necessary to control some electrical parameters during wiring, such as reducing distributed inductance and EMC. It is also similar for the wiring of other signals. All EDA manufacturers will provide a method to control these parameters. After understanding the input parameters of the automatic routing tool and the influence of the input parameters on the routing, the quality of automatic routing 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. If the number of layers and vias used by the automatic routing tool is 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 wiring will achieve similar results as expected. Of course, some sorting work may be needed, and the space for other signal and network wiring needs to be ensured. After part of the design is completed, fix it to prevent it from being affected by the later wiring process.

Use the same procedure 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 other network wiring will be reduced. However, many signal wiring needs manual intervention. 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. The design points of automatic wiring include:

7.1 slightly change the setting and try multiple path wiring;

7.2 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 and buried holes, and observe the impact of these factors on the design results;

7.3 let the wiring tool handle those default networks as needed;

7.4 the less important the signal is, the greater the freedom of the automatic wiring tool to its wiring.

8. Arrangement of wiring

If the EDA tool software you use can list the signal wiring length and check these data, you may find that the signal wiring length with few constraints is very long. This problem is easy to deal with. Manual editing can shorten the length of signal wiring and reduce the number of vias. In the sorting process, you need to judge which wiring is reasonable and which wiring is unreasonable. Like manual routing design, automatic routing design can also be sorted and edited during inspection.

9. Appearance of circuit board

The previous design often paid attention to the visual effect of the circuit board, but now it is different. The circuit board designed automatically is not more beautiful than that designed manually, but it can meet the specified requirements in terms of electronic characteristics, and the complete performance of the design can be guaranteed.

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