Wiring is an extremely important part of PCB design and will directly affect the performance of the PCB. In the PCB design process, different layout engineers have their own understanding of the pcb layout, but all the layout engineers are consistent in how to improve the efficiency of the wiring, which not only saves the project development cycle for the customer, but also maximizes Guarantee quality and cost. The following describes the PCB design process and steps to improve wiring efficiency.
1, 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.
2, 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 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 wiring.
3, the layout of the 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 easier automated routing. The defined rules and constraints affect the layout design.
4, fanout design
In the fan-out design phase, for the auto-wiring 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 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 usually ordered when they are ready for full production. It is too late to consider adding nodes to achieve 100% testability.
5, manual wiring and key signal processing
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 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.
6, automatic wiring
Wiring of critical signals requires consideration of controlling some electrical parameters during wiring, such as reducing distributed inductance and EMC, etc., and wiring for other signals is similar. All EDA vendors offer 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.
7, the appearance of the board
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.
For the layout engineer, the technology is not good, it should not be judged only from the number of layers and speed. Only when the conditions such as the number of components and the signal speed are equal, the smaller the area, the fewer the number of layers, and the lower the cost. The PCB board is designed to be good and can guarantee good performance and aesthetics. This is the master.