Design Tips And Key Points For Efficient PCB Automatic Routing

Although the current EDA tools are very powerful, as PCB size requirements become smaller and the device density becomes higher and higher, PCB design is not difficult. How to achieve high PCB throughput and shorten design time? This article describes the design tips and key points of PCB planning, layout, and routing.

Nowadays PCB design is getting shorter and shorter, smaller and smaller board space, higher device density, 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 market launch, many manufacturers now prefer to use dedicated EDA tools to achieve PCB design. However, dedicated EDA tools do not produce the desired results, nor can they achieve 100% throughput, and they are messy, and usually take a lot of time to complete the rest of the work.

There are a lot of EDA tools available on the market, but they are similar except for the terms and function keys used. How to use these tools to better design the PCB? Careful analysis of the design and careful setup of the tool software before starting the wiring will make the design more compliant. Below are the general design procedures and procedures.

Determine the number of layers of the PCB

Board size and number of wiring layers need to be determined early in the design process. If the design requires the use of high-density ball grid array (BGA) components, the minimum number of routing layers required for wiring these devices must be considered. The number of wiring layers and the stack-up method directly affect the wiring and impedance of the printed wiring. The size of the board helps determine the stacking and line width to achieve the desired design.

For many years, people have always thought that the lower the number of board layers, 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 boards has been greatly reduced. At the beginning of the design, it is better to use more circuit layers and evenly distribute the copper to avoid a small number of signals not meeting the defined rules and space requirements at the end of the design, and thus forced to add new layers. Careful planning before design will reduce a lot of trouble in wiring.

Design rules and restrictions

The autorouting tool itself does not know what to do. To complete the routing task, the routing tool needs to work under the correct rules and constraints. Different signal lines have different wiring requirements, and all special required signal lines are classified, and different design classifications are also different. 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 interaction between the signal lines, and the limitations of the layers, which have a large impact on the performance of the routing tool. Careful consideration of design requirements is an important step in successful cabling.

Component layout

To optimize the assembly process, the Manufacturability Design (DFM) rules impose restrictions on component placement. If the assembly department allows components to move, the circuit can be properly optimized for easier automatic routing. The defined rules and constraints affect the layout design.

In the layout, you need to consider the routing channel and the via area, 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. By setting the wiring constraints and setting the layers of the signal lines, the routing tools can be imagined by the designer. That completes the wiring.

Fanout design

In the fan-out design phase, for the auto-wiring tool to connect component leads, each pin of the surface mount device should have at least one via so that the board can perform the inner layer when more connections are needed. Connectivity, online testing (ICT) and circuit reprocessing.

In order to make the automatic routing tool the most efficient, it is necessary to use the largest via size and the printed wiring as much as possible, and the interval is preferably set to 50 mils. Use a via type that maximizes the number of routing paths. When performing fan-out design, consider the online test of the circuit. Test fixtures can be expensive and are usually ordered when they are ready for full production. 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 at the beginning of the design, implemented in the later stage of the production process, and the type of via fanout is determined according to the wiring path and the circuit online test. The power supply and grounding also affect the wiring and fanout design. . To reduce the inductive reactance of the filter capacitor connection, the via should be placed as close as possible to the surface mount device pins. If necessary, manual routing can be used, which may affect the originally proposed routing path and may even cause you to re- Consider which via is used, so you must consider the relationship between via and pin inductance and set the priority of the via specification.

Manual routing and handling of critical signals

Although this article focuses on the problem of automatic routing, manual routing is an important process for printed circuit board design now and in the future. Manual routing helps the automated routing tool complete the routing work. By manually routing and fixing the selected network (net), a path that can be relied upon for automatic routing can be formed.

Regardless of the number of critical signals, these signals can be routed first, manually wired, or combined with an automated routing tool. Critical signals must usually be carefully designed to achieve the desired performance. After the wiring is completed, it is relatively easy to check the signal wiring by the relevant engineering personnel. This process is relatively easy. After the check is passed, the wires are fixed and the remaining signals are automatically routed.

Automatic wiring

Wiring of critical signals requires consideration of controlling some electrical parameters during routing, such as reducing distributed inductance and EMC, and wiring for other signals. All EDA vendors offer 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 wiring, the quality of the automatic wiring can be guaranteed to a certain extent.

General rules should be used to automatically route signals. By setting constraints and disabling the routing area to define the layers used for a given signal and the number of vias used, the routing tool can be automatically routed according to the engineer's design philosophy. If there are no restrictions on the number of layers and the number of vias used in the automatic routing tool, each layer will be used for automatic routing and many vias will be created.

After setting the constraints and the rules created by the application, the automatic routing will achieve similar results as expected. Of course, some finishing work may be required, and other signals and network wiring space need to be ensured. After a part of the design is completed, fix it to prevent it from being affected by the rear wiring process.

The same steps are used to route the remaining signals. The number of wirings depends on the complexity of the circuit and the general rules you define. After each type of signal is completed, the constraints of the remaining network cabling are reduced. But what comes along is that many signal wiring requires manual intervention. Today's automated routing tools are very powerful and typically complete 100% wiring. However, when the automatic routing tool does not complete all signal routing, the remaining signals need to be manually routed.

The main points of automatic wiring design include: 1. Change the settings slightly, try multiple routings; 2. Keep the basic rules unchanged, try different wiring layers, different traces and spacing widths, and different line widths and types. Holes such as blind holes, buried holes, etc., to observe how these factors affect the design results; Let the routing tool handle the default network as needed; 4. The less important the signal, the more freedom the automated routing tool will route it.

Wiring of wiring

If the EDA tool you are using can list the routing length of the signal, check the data and you may find that some signal routing with very few constraints is very long. This problem is easier to handle, and manual editing can reduce signal routing length and reduce the number of vias. In the finishing process, you need to determine which wiring is reasonable and which wiring is unreasonable. As with manual wiring design, the automatic routing design can also be organized and edited during the inspection process.

Board appearance

Previous designs often pay attention to the visual effects of the board, which is different now. The automatically designed circuit board is no more aesthetically pleasing than the manual design, but it meets the specified requirements in terms of electronic characteristics, and the complete performance of the design is guaranteed.