Method for improving layout efficiency in PCB design

writer: G April 17, 2018

1. Determine the number of PCB layers

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

Over the years, people have always believed that the lower the number of circuit board layers, the lower the cost, but there are many other factors that affect the manufacturing cost of the circuit board. In recent years, the cost difference between multi-layer boards has been greatly reduced. It is preferable to use more circuit layers and evenly distribute the copper when starting the design to avoid finding that a small amount of signals do not meet the defined rules and space requirements at the end of the design and are forced to add new layers. Careful planning before design will reduce many of the hassles in the cabling.

2. Design rules and restrictions

The automatic routing tool itself does not know what to do. To complete the wiring tasks, the wiring tools need to work under the correct rules and restrictions. Different signal lines have different wiring requirements. All special signal lines must be classified. Different design categories are different. Each signal class should have priority. The higher the priority, the stricter the rules. The rules relate to the width of the traces, the maximum number of vias, the degree of parallelism, the mutual influence between the signal lines, and the limits of the layers. These rules have a great influence on the performance of the routing tool. Serious consideration of design requirements is an important step in successful cabling.

3. The layout of the components

In order to optimize the assembly process, the Design for Manufacturing (DFM) rules impose restrictions on the layout of components. If the assembly department allows components to move, the circuit can be properly optimized for easier autowiring. The defined rules and constraints affect the layout design.

The routing and via areas need to be considered during layout. These paths and areas are obvious to the designer, but the autorouter will only consider one signal at a time. By setting the routing constraints and setting the layers of the signal lines, the routing tools can be as designers imagine. Complete the wiring in that way.

4. Fan out design

During the fan-out design stage, the auto-wiring tool must be able to connect the component pins. Each surface-mount device pin should have at least one via hole so that the circuit board can make inner layers when more connections are needed. Connection, in-circuit test (ICT) and circuit reprocessing.

In order to maximize the efficiency of the auto-wiring tool, it is imperative to use the largest via size and traces as much as possible, with an interval setting of 50 mils ideal. Use the type of via that maximizes the number of routing paths. When conducting the fan-out design, consider online circuit test issues. Test fixtures can be expensive and are usually ordered just before full production. It is too late to consider adding nodes to achieve 100% testability.

After careful consideration and prediction, the design of the circuit on-line test can be carried out in the early stage of the design and implemented later in the production process. The type of via fanout can be determined according to the wiring path and the circuit online test. Power supply and grounding can also affect the wiring and fan-out design. . To reduce the inductive reactance of the filter capacitor connection, the vias should be placed as close as possible to the pins of the surface-mount device. If necessary, manual routing can be used. This may affect the originally conceived wiring path and may even cause you to Consider which via is used, so the relationship between the via and the pin inductance must be considered and the priority of the via specification must be set.

5. Manual wiring and key signal processing

Although this article mainly discusses the issue of auto-wiring, manual cabling is an important process for PCB design now and in the future. Using manual routing helps the automatic routing tool to complete the wiring work. Regardless of the number of critical signals, these signals are first routed, manually routed or combined with automated routing tools. The critical signal must usually pass the careful circuit design to achieve the desired performance. After the wiring is completed, the relevant engineering personnel will check these signal wirings. This process is relatively easy. After the inspection passes, the wires are fixed and then the other signals are automatically routed.

6. Automatic routing

The wiring of key signals needs to consider the control of some electrical parameters during wiring, such as reducing the distributed inductance and EMC, 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 routing tool and the influence of the input parameters on the wiring, the quality of the automatic wiring can be guaranteed to some extent.

General rules should be used to automatically route signals. By setting restrictions and inhibiting the wiring area to define the layer used for a given signal and the number of vias used, the routing tool can automatically route according to the engineer's design philosophy. If there is no limit to the number of layers and the number of holes to be used for the automatic routing tool, each layer will be used for automatic routing and many vias will be produced.

After setting up the constraints and applying the rules created, the automated cabling will achieve similar results as expected. Of course, some sort of finishing work may also be needed, while also ensuring the space for other signals and network cabling. After a part of the design is completed, fix it to prevent it from being affected by the wiring process behind it.

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

7. The design points of the automatic wiring in the PCB copy board include:

7.1 slightly change the settings, try a variety of routing routes;

7.2 keep the basic rules unchanged, try different wiring layers, different printed lines and spacing width and different line width, different types of vias such as blind holes, buried holes, etc., observe these factors have any effect on the design results;

7.3 Let the routing tools handle the default networks as needed;

7.4 The less important the signal, the greater the degree of freedom of the automatic routing tool to route it.

8. PCB wiring finishing

If you are using EDA tool software that can list the length of the traces of the signal, and check this data, you may find that the length of the signal wiring with a few constraints is very long. This problem is easier to deal with. Manual editing can shorten the signal wiring length and reduce the number of vias. During the sorting process, you need to determine which cabling is reasonable and which cabling is unreasonable. As with manual layout design, automatic layout design can also be checked