Although the current EDA tools are very powerful, as the PCB size requirements are getting smaller and smaller, the device density is getting higher and higher, the difficulty of PCB design is not small. How to achieve high PCB flux and shorten the design time? This paper introduces the design skills and key points of PCB planning, layout and routing.
Now PCB design time is getting shorter and shorter, smaller and smaller board space, higher and higher device density, extremely demanding layout rules and large-size components make the designer's work more difficult. In order to solve the design difficulties and speed up the market of products, many manufacturers now tend to use special EDA tools to achieve PCB design. However, dedicated EDA tools do not produce ideal results, do not achieve 100% distribution flux, and it is messy, and usually takes a lot of time to complete the rest of the work.
Now the market popular EDA tools a lot of software, but in addition to the use of terminology and function key location is not the same, how to use these tools to better achieve the design of PCB? Careful analysis of the design and careful setup of the tool software before starting the wiring will make the design more compliant. The following is the general design process and steps.
1. determine the number of PCB layers
The size of the 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, the minimum number of wiring layers required for the wiring of these devices must be considered. 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.
For many years, people have always thought that the fewer layers of the board the lower the cost, but there are many other factors that affect the manufacturing cost of the board. In recent years, the cost difference between multilayer panels has been greatly reduced. At the beginning of the design, it is best to use more circuit layers and make the coating copper evenly distributed, 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 restrictions
The automatic wiring 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, to classify all the special requirements of the signal line, different design classification is not the same. 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. Layout of components
To optimize the assembly process, design for Manufacturability (DFM) rules place restrictions on 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.
routing channels and through-hole areas should be considered in the layout, as shown in Figure 1. These paths and areas are obvious to the designer, but the automatic routing tool only considers one signal at a time, and by setting routing constraints and setting layers of deploable signal lines, the routing tool can be routed as the designer intends.
4. Fan out the design
In the fan-out design phase, to enable the automatic wiring tool to connect component pins, 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 (ICT), 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. In the fan-out design, the problem of circuit online testing should be considered. Test fixtures can be expensive and are usually ordered when they are about to go into full production, and it is too late to consider adding nodes to achieve 100% testability.
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, according to the wiring path and circuit online test to determine the through-hole fan-out type, 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 connection line, the hole should be as close as possible to the pin of the surface-mount device, and manual wiring can be used if necessary, which may have an impact on the originally envisioned wiring path, and may even cause you to reconsider which hole to use, so you must consider the relationship between the hole and the pin inductive reactance and set the priority of the hole specification.
5. manual wiring and key signal processing
Although this article focuses on automatic routing, manual routing is and will be an important process in PCB design. The use of manual wiring helps the automatic wiring tool to complete the wiring work. By manually routing and fixing the selected network (net), a path can be formed for automatic routing.
Regardless of the number of key signals, these signals are routed first, either manually or in combination with automatic routing tools. Critical signals usually have to pass through careful circuit design to achieve the desired performance. After the wiring is completed, the relevant engineering personnel to check these signal wiring, this process is relatively easy. After the inspection is passed, these lines are fixed and the rest of the signal is automatically routed.
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., and the wiring of other signals is similar. All EDA vendors 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 to route signals automatically. 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. If there is no limit to the number of layers and holes used in the automatic wiring tool, each layer will be used in the automatic wiring process, and there will be many holes.
After setting the constraints and applying the rules created, the automatic routing will achieve similar results as expected, of course, there may be some finishing work, but also need to ensure that other signals and network wiring space. After part of the design is completed, it is fixed to prevent it from being affected by the wiring process behind.
Follow the same steps to route the remaining signals. The amount of wiring depends on the complexity of the circuit and how many general rules you define. After each type of signal is completed, the constraints on the rest of the network wiring are reduced. But with that 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 does not complete all signal wiring, it is necessary to manually route the remaining signals.
The design points of automatic wiring include:
a. Slightly change the Settings, try a variety of path routing;
b. Keep the basic rules unchanged, try different wiring layers, different printed lines and spacing widths, different line widths, different types of holes such as blind holes, buried holes, etc., to observe the impact of these factors on the design results;
c. Let the wiring tool process those default networks as needed;
d. The less important the signal, the more freedom the automatic routing tool has for its routing.
7. wiring arrangement
If the EDA tool software you use can list the length of the signal route, check the data, you may find that some of the signal route length with few constraints is very long. This problem is relatively easy to deal with, through manual editing can shorten the signal wiring length and reduce the number of holes. In the process of finishing, you need to judge which wiring is reasonable and which wiring is not reasonable. Like manual routing designs, automatic routing designs can also be collated and edited during inspection.
8. the appearance of the circuit board
The previous design often paid attention to the visual effect of the circuit board, and now it is different. The automatically designed circuit board is not as beautiful as the manual design, but it can meet the specified requirements in terms of electronic characteristics, and the complete performance of the design is guaranteed.
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