It is an extremely important part of PCB layout design outsourcing, which will directly affect the performance of PCB. In the process of layout design, different layout engineers have their own understanding of layout, but all layout engineers are consistent in how to improve the efficiency of wiring, which can not only save the development cycle of the project for customers, but also ensure the quality and cost to the maximum extent. The following is the general layout outsourcing design process and steps.
1. Determine the number of PCB layers
The size of circuit board and the number of wiring layers need to be determined in the early stage of layout design. If the design requires the use of high-density ball grid array (BGA) components, the minimum number of wiring layers required for wiring these devices must be considered. The number of wiring layers and stack-up mode will directly affect the wiring and impedance of printed lines. The size of the board is helpful to determine the lamination mode and printed line width, and achieve the desired design effect.
2. pcb design rules and restrictions
The automatic wiring tool itself doesn’t know what to do. To complete the routing task, the routing tools need to work under correct rules and restrictions. Different signal lines have different wiring requirements, so all signal lines with special requirements should be classified, and different design classifications are different. Every signal class should have priority. The higher the priority, the stricter the rules. Rules are related to the width of printed lines, the maximum number of vias, parallelism, the interaction between signal lines and the limitation of layers. These rules have great influence on the performance of routing tools. Careful consideration of design requirements is an important step for successful wiring.
3. Layout of circuit board components
In order to optimize the assembly process, design for manufacturability (DFM) rules will restrict the layout of components. If the assembly department allows the components to move, the circuit can be properly optimized to facilitate automatic wiring. The defined rules and constraints will affect the layout design.
4. Fan out design
In the fan-out design stage, in order to enable the automatic wiring tool to connect the component pins, each pin of the surface mount device should have at least one via hole, so that when more connections are needed, the circuit board can carry out internal layer connection, on-line test (ICT) and circuit reprocessing.
In order to achieve the highest efficiency of automatic routing tools, the largest via size and printed lines must be used as much as possible, and the interval of 50mil is ideal. Use the via type to maximize the number of routing paths. When designing the fan-out, the on-line test of the circuit should be considered. Test fixtures can be expensive, and they are usually ordered when they are about to be put into full production. It is too late to consider adding nodes to achieve 100% testability at this time.
5. Manual wiring and processing of key signals
Although this paper mainly discusses the problem of automatic wiring, manual wiring is an important process of PCB design now and in the future. Manual wiring is helpful for automatic wiring tools to complete the wiring work. Regardless of the number of key signals, these signals can be wired first, manually or combined with automatic wiring tools. The key signals usually have to be carefully designed to achieve the desired performance. After the wiring is completed, it is relatively easy for relevant engineers to check the wiring of these signals. After passing the inspection, fix these wires, and then start to automatically route the remaining signals.
6. layout design automatic wiring
For the wiring of key signals, it is necessary to consider controlling some electrical parameters, such as reducing distributed inductance and EMC, etc., and the wiring of other signals is similar. All EDA vendors will provide a way to control these parameters. The quality of automatic wiring can be guaranteed to a certain extent after knowing what input parameters of automatic wiring tools have and their influence on wiring.
7, the appearance of the circuit board
The previous designs often paid attention to the visual effects of circuit boards, but now they are different. The automatically designed circuit board is not as beautiful as the manually designed circuit board, but it can meet the specified requirements in electronic characteristics, and the complete performance of the design is guaranteed.
For layout engineers, whether the technology is strong or not should not be judged only by the number of layers and speed. Only when the number of devices and signal speed are equal, can the PCB board with smaller area, fewer layers and lower cost be designed, and can guarantee good performance and beauty, which is the master.
