Wiring is a very important part of PCB design, which will directly affect the performance of PCB board. In the PCB design process, different to 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 for the customers, but also ensure the maximum quality and cost. The following are the general design processes and steps.
1. Determine the number of layers of PCB
The size of the circuit board and the number of wiring layers must be determined at the beginning of the design. If the design requires the use of high density ball grid array (BGA) components, the minimum number of wiring layers required for these devices must be considered. The number of wiring layers and stacking (stack-up) mode will directly affect the wiring and impedance of printed wiring. The size of the panel helps to determine the stacking mode and the width of the printed line and achieve the desired design effect.
2. Design rules and restrictions
The automatic wiring tool itself does not know what to do. To complete routing tasks, wiring tools need to work under the correct rules and constraints. Different signal lines have different wiring requirements, and all kinds of special 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 involve the width of the printed line, the maximum number of holes, the degree of parallelism, the interaction between the signal lines and the limit of the layer. These rules have a great influence on the performance of the wiring tool. Careful consideration of design requirements is an important step in successful wiring.
3. Layout of components
In order to optimize assembly process, the DFM rules restrict the layout of components. If the assembly department allows components to move, the circuit can be properly optimized to facilitate automatic wiring. The rules and constraints defined will affect layout design.
4. Fan out design
In the fan out design stage, in order to make the automatic wiring tool connect to the component pins, each pin of the surface mount device should have at least one over hole, so that the circuit board can carry out the inner connection, on-line test (ICT) and circuit reprocessing when more connections are needed.
In order to maximize the efficiency of automatic routing tools, it is necessary to use the maximum through hole size and printing line as far as possible, and the interval setting to 50mil is ideal. The type of through hole that will maximize the number of cloth paths is to be adopted. In the fan out design, we should take into account the circuit on-line testing problem. Test jigs are likely to be expensive and are usually ordered when full production is to be put in, and 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 article mainly discusses the problem of automatic wiring, manual wiring is an important process in the design of printed circuit boards at present and in the future. Manual wiring is helpful for automatic wiring tools to complete wiring work. Regardless of the number of key signals, wiring them first, manual wiring or automatic wiring. Key signals usually have to be carefully designed to achieve desired performance. After the wiring is completed, the signal is checked by the engineers concerned. This process is much easier. After checking, these lines are fixed, and then the remaining signals are automatically wired.
6. Automatic wiring
For critical signal wiring, some electrical parameters should be considered when wiring, such as reducing the distribution inductance and EMC, and so on. All EDA vendors will provide a way to control these parameters. After understanding the input parameters and the influence of input parameters on wiring, the quality of automatic routing can be assured to a certain extent.
7. The appearance of the circuit board
Previous designs often pay attention to the visual effects of printed circuit boards, but now they are different. The automatically designed circuit board is not more beautiful than manual design, but it can meet the specified requirements on the electronic characteristics, and the complete performance of the design is guaranteed.
For layout engineers, the technology is severe or not, it should not be judged only from the number of layers and speed. Only when the number of devices and the speed of the signal are equal, the smaller the area, the less the number of layers, the lower the cost of the PCB board, and the good performance and beauty.
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