Now PCB design time is getting shorter, smaller and smaller board space, increasing device density, extremely harsh layout rules and large size components make the designer's work more difficult. In order to solve the difficulties in the design and speed up the listing of products, many manufacturers tend to use EDA tools to achieve the design of PCB. But the special EDA tools and can not produce the desired results, can reach 100% completion rate, and is very chaotic, usually need to spend a lot of time to complete the remaining work.
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 initial design stage. If the design requires the use of high density ball grid array (BGA) components, it is necessary to consider the minimum number of wiring layers required for these devices. The number of wiring layers and the lamination (stack-up) mode will directly affect the wiring and impedance of printed wiring. The size of the board helps to determine the stacking mode and the width of the printed wire to achieve the desired design results.
In recent years, the cost difference between multilayer board has been greatly reduced. At the beginning of design is best to use more circuit layer and the copper distribution, in order to avoid the design at the end only found a small amount of signal does not conform to the defined rules and space requirements, to add a new layer. Careful planning prior to design will reduce the number of problems in wiring.
2, design rules and limitations
The wiring tool itself doesn't know what to do. In order to complete the routing task, the wiring tools need to work under the correct rules and constraints. Different signal lines have different wiring requirements, to all the special requirements of the signal line classification, different design classification is not the same. Each signal class should have priority, the higher the priority, the more stringent rules. The rules include the width of the printed circuit, the maximum number of vias, the parallelism, the mutual influence between the signal lines and the limitations of the layers. Careful consideration of design requirements is an important step in successful wiring.
3, the layout of components
In order to optimize the assembly process, the design of manufacturability (DFM) rules can restrict the assembly layout. If the assembly allows the component to move, the circuit can be properly optimized for easier wiring. The defined rules and constraints will affect the layout design.
The routing path (routing channel) and the vias are considered in the layout. These areas are obviously on the path and design staff, but automatic routing tools consider only one signal at a time, by setting the routing constraints and setting can be distributed signal line layer, can make routing tools like designers envisioned by complete wiring.
4, automatic wiring
The wiring of the critical signal needs to be taken into account when controlling some electrical parameters, such as reducing the distributed inductance and EMC, etc.. All EDA vendors will provide a way to control these parameters. After knowing the input parameters 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 for automatic routing of signals. By setting the limiting conditions and forbidding the wiring area to limit the number of vias used by the given signal and the number of vias used, the routing tool can automatically route in accordance with the design idea of the engineer. If there is no restriction on the number of layers and the number of holes used in the automatic routing tool, automatic wiring will be used for each layer, and a lot of vias will be generated.
After setting up the constraint conditions and the application of the rules, automatic routing and expected similar results will be achieved, of course, may also need some sort of work, but also need to ensure that other signals and network cabling space. After the completion of part of the design, it will be fixed to prevent the impact of the wiring process behind.
5, wiring arrangement
If you use the EDA tool software to list the wiring length of the signal, check the data, you may find a few restrictions on the signal wiring is very long. This problem is relatively easy to handle, by manual editing can shorten the signal wiring length and reduce the number of holes. In the process of sorting out, you need to determine which wiring is reasonable, which is unreasonable. As with manual wiring design, automatic wiring design can also be collated and edited during the inspection process.