Board size and the number of routing 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 routing these devices must be considered. The number of routing layers and the stack-up method directly affect the routing and impedance of the printed routing. 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 routing.
The automatic routing tool 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 routing requirements, and all special required signal lines are classified, and different design classifications are different. Each signal class should have a 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 routing.
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 easier automated routing. The defined rules and constraints affect the layout design.
Routing channels and via areas should be considered when laying out. These paths and areas are obvious to the designer, but the automatic routing tool only considers one signal at a time. By setting the routing constraints and setting the layers of the signal lines, the routing tools can complete the routing which is imagined by the designer.
In the fan-out design phase, for the auto-routing tool to connect the component pins, each pin of the surface mount device should have at least one via so that the board can perform the inner layer connectivity, online testing (ICT) and circuit reprocessing when more connections are needed.
In order to make the automatic routing tool the most efficient, it is necessary to use the largest via size and the printed routing 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, the online test of the circuit should be considered. Test fixtures can be expensive and 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 fan-out is determined according to the routing path and the circuit online test. The power supply and grounding also affect the routing and fan-out 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 reconsider which via is used, so you must consider the relationship between via and pin inductance and set the priority of the via specification.
Although this article focuses on automatic routing issues, 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.
Regardless of the number of critical signals, these signals can be routed first, manually routed, or combined with an automated routing tool. Critical signals must usually be carefully designed to achieve the desired performance. After the routing is completed, it is relatively easy to check the signal routing 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.
Routing of critical signals requires consideration of controlling some electrical parameters during routing, such as reducing distributed inductance and EMC, etc., and routing for other signals is similar. 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 routing, the quality of the automatic routing 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 for 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, as well as other signal and network routing space. After a part of the design is completed, fix it to prevent it from being affected by the routing process behind.
The same steps are used to route the remaining signals. The number of wires 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 routing requires manual intervention. Today's automated routing tools are very powerful and typically complete 100% routing. However, when the automatic routing tool does not complete all signal routing, the remaining signals need to be manually routed.
1) Change the settings slightly, try a variety of path routing.
2) Keep the basic rules unchanged, try different routing layers, different printed lines and spacing widths, and different line widths, different types of vias such as blind holes, buried holes, etc., to observe how these factors affect the design results.
3) Let the routing tools handle the default networks as needed.
4) The less important the signal, the greater the freedom of the automatic routing tool to route it.
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 routing is reasonable and which routing is unreasonable. As with manual routing design, the automatic routing design can also be organized and edited during the inspection process.
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.
