Although the current EDA tools are very powerful, but with the increasingly smaller size requirements of PCB, the device density is higher and higher, the difficulty of PCB design is not small. How to achieve high PCB pass rate and shorten the design time? This paper introduces the design skills and key points of PCB planning, layout and wiring. Now the time of PCB design is shorter and shorter, the circuit board space is smaller and smaller, the device density is higher and higher, the extremely harsh layout rules and large-size components make the designer’s work more difficult. In order to solve the design difficulties and speed up the product launch, many manufacturers now tend to use special EDA tools to achieve PCB design. But the special EDA tools can’t produce ideal results, and can’t achieve 100% of the pass rate, and it’s very messy. It usually takes a lot of time to complete the rest of the work.
There are many popular EDA tools in the market, but they are almost the same except for the different terms and function keys. How to use these tools to better realize PCB design? Before the start of wiring, careful analysis of the design and careful setting of the tool software 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 the circuit board and the number of wiring layers need to be determined at the beginning of the design. If the design requires the use of high-density ball grid array (BGA) components, we must consider the minimum number of wiring layers required for these devices. The number of routing layers and the stack up mode will directly affect the routing and impedance of printed wires. The size of the board is helpful to determine the stacking mode and the width of the printed line, so as to achieve the desired design effect.
For many years, people always think that the lower the number of layers of circuit board, the lower the cost. But there are many other factors that affect 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 better to use more circuit layers and make the copper evenly distributed, so as to avoid that a small number of signals do not meet the defined rules and space requirements at the end of the design, so that new layers are forced to be added. Careful planning before design will reduce a lot of trouble in wiring.
2. Design rules and limitations
The automatic routing tool itself doesn’t know what to do. In order to complete the routing task, the routing tools need to work under the correct rules and constraints. Different signal lines have different wiring requirements, so it is necessary to classify all signal lines with special requirements, and different design classifications are also different. Every signal class should have priority. The higher the priority, the more strict the rules are. 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 routing tools. Careful consideration of design requirements is an important step in successful wiring.
3. Layout of components
In order to optimize the assembly process, design for manufacturability (DFM) rules restrict the layout of components. If the assembly department allows the components to move, the circuit can be properly optimized to facilitate automatic routing. The rules and constraints defined affect the layout design.
Routing channel and via area should be considered in layout. These paths and areas are obvious to designers, but the automatic routing tool only considers one signal at a time. By setting the routing constraints and setting the layer of signal lines, the routing tool can complete the routing as the designer envisions.
4. Fan out design
In the fan out design stage, each pin of SMD should have at least one via to enable the automatic wiring tool to connect the module pins, so that the circuit board can carry out inner layer connection, on-line test (ICT) and circuit reprocessing when more connections are needed.
In order to maximize the efficiency of automatic routing tools, we must use the largest via size and printed wire as far as possible, and the spacing of 50 Mil is ideal. It is necessary to adopt the type of through hole which can maximize the number of routing diameter. In the design of fan out, the problem of on-line test should be considered. Test fixtures can be expensive, and they are usually ordered when they are about to be put into full production. It’s 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 at the early stage of the design and realized at the later stage of the production process. According to the wiring path and circuit on-line test, the type of via fan out can be determined, and the power supply and grounding will also affect the wiring and fan out design. In order to reduce the inductive reactance generated by the filter capacitor connecting line, the via should be as close as possible to the pins of the surface mount device. If necessary, manual wiring can be used, which may affect the original wiring path, and even lead you to reconsider which via to use. Therefore, the relationship between the via and the inductive reactance of the pin 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 PCB design now and in the future. Manual routing is helpful for automatic routing tools to complete the routing work. As shown in Fig. 2a and Fig. 2B, by manually routing and fixing the selected net, a path that can be used for automatic routing can be formed.
Regardless of the number of key signals, these signals can be wired first, either manually or in combination with automatic routing tools. Key signals usually have to be carefully designed to achieve the desired performance. After the wiring is completed, it is much easier for relevant engineers to check the signal wiring. After passing the inspection, fix these wires, and then start the automatic wiring of the remaining signals.
6. Automatic routing
For the wiring of key signals, it is necessary to control some electrical parameters, such as reducing the distributed inductance and EMC, etc. for the wiring of other signals, it is also similar. All EDA manufacturers will provide a way 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 should be used to route signals automatically. By setting restrictions and forbidden routing area to limit the number of layers and vias used for a given signal, the routing tool can automatically route according to the design idea of the engineer. If there is no limit on the number of layers and vias used by the automatic routing tool, each layer will be used in automatic routing, and many vias will be generated.
After setting the constraints and applying the created rules, the automatic routing will achieve similar results as expected. Of course, some sorting work may need to be done, and the space for other signals and network routing needs to be ensured. After a part of the design is completed, it is fixed to prevent the influence of the back wiring process.
Use the same procedure to route the remaining signals. The number of routes 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 routing will be reduced. But then a lot of signal wiring needs manual intervention. Now the automatic routing tool is very powerful, usually can complete 100% of the routing. However, when the automatic routing tool does not complete all the signal routing, it is necessary to manually route the remaining signals.
7. The design points of automatic routing include:
7.1 slightly change the settings and try to use multiple paths for wiring; 7.2 keep the basic rules unchanged, try different wiring layers, different printed lines and interval widths, different line widths, different types of vias such as blind holes and buried holes, and observe the influence of these factors on the design results; 7.3 let the wiring tools process the default networks according to the needs; 7.4 the less important the signal is, the more important the signal is, The more freedom the automatic routing tool has for its routing.
8. Arrangement of wiring
If the EDA tool software you use can list the signal wiring length and check the data, you may find that the signal wiring length with few constraints is very long. This problem is easy to deal with, by manual editing can shorten the signal wiring length and reduce the number of vias. In the process of sorting out, you need to determine which wiring is reasonable and which is unreasonable. Just like the manual routing design, the automatic routing design can also be sorted and edited during the inspection process.
9. Appearance of circuit board
Previous designs used to pay attention to the visual effects of circuit boards, but now they are different. The circuit board of automatic design is not more beautiful than that of manual design, but it can meet the specified requirements in electronic characteristics, and the complete performance of the design is guaranteed.