The design of PCB high efficient automatic wiring

- Jan 09, 2020-

The design of PCB high efficient automatic wiring


    Nowadays, the time of PCB design is shorter and shorter, the space of PCB is smaller and smaller, the density of devices is higher and higher, the layout rules and large size components are extremely demanding, which make the work of designers more difficult. In order to solve the design difficulties and speed up the listing of products, many manufacturers now tend to use special EDA tools to achieve PCB design. But the special EDA tools can't produce the ideal results, can't reach 100% of the distribution rate, and it's very messy, usually it takes a lot of time to complete the rest of the work. Today, this paper introduces the design skills and key points of PCB planning, layout and wiring.


    There are many popular EDA tools and softwares on the market now, but they are the same except for the different terminology and function key positions. How to use these tools to better realize PCB design? Careful analysis of the design and careful setting of the tool software before the start of wiring will make the design more in line with the requirements. The following is the general design process and steps.  


1. Determine the number of layers of PCB


    The size of circuit board and the number of wiring layers need to be determined in the early stage of design. If the design requires the use of high-density ball grid array (BGA) components, the minimum number of 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 wiring. The size of the board is helpful to determine the lamination mode and the width of the printed wire, so as to achieve the desired design effect.  


     For many years, people always think that the lower the number of 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 multilayer boards has been greatly reduced. At the beginning of the design, it is better to use more circuit layers and make the copper coating evenly distributed, so as to avoid finding a small number of signals that do not meet the defined rules and space requirements near the end of the design, and thus forced to add a new layer. Careful planning before design will reduce a lot of trouble in wiring.


2. Design rules and restrictions


    The auto routing tool itself does not know what to do. In order to complete the routing task, the routing tool needs to work under the correct rules and restrictions. Different signal lines have different wiring requirements. All signal lines with special requirements should be classified, and different design classifications are also different. Every signal class should have priority. The higher the priority is, the stricter the rules are. The rules involve the width of printed wire, the maximum number of vias, parallelism, the interaction between signal wires and the limitation of layers. These rules have a great influence on the performance of wiring tools. Careful consideration of design requirements is an important step in successful cabling.


3. Layout of components


    In order to optimize the assembly process, DFM rules will limit the layout of components. If the assembly department allows the components to move, the circuit can be properly optimized for easier automatic wiring. The rules and constraints you define affect the layout.  


    Routing channel and via area should be considered in layout. These paths and areas are obvious to the designer, but the automatic routing tool only considers one signal at a time. By setting the routing constraints and setting the layers of the routable signal lines, the routing tool can complete the routing as the designer imagined.


4. Fan out design


    In the fan out design stage, to enable the automatic wiring tool to connect the component pins, each pin of the surface mount device should have at least one through hole, so that when more connections are needed, the circuit board can carry out inner layer connection, online test (ICT) and circuit reprocessing.  


    In order to maximize the efficiency of automatic wiring tools, it is necessary to use the largest through-hole size and printed wire as much as possible, and it is better to set the interval of 50mil. Use the type of vias that maximize the number of routing paths. In the design of fan out, the problem of circuit on-line test should be considered. Test fixtures can be expensive and are usually ordered just before they are ready for full production, so it's too late to consider adding nodes to achieve 100% testability.  


    After careful consideration and prediction, the design of circuit online test can be carried out in the early stage of design and later stage of production. According to the wiring path and circuit online test, the type of through-hole fan out can be determined. The power supply and grounding will also affect the design of wiring and fan out. In order to reduce the inductive reactance produced by the connecting wire of filter capacitor, the through-hole should be as close to the pin of surface mount device as possible, and manual wiring can be used if necessary, which may affect the original wiring path, and even cause you to reconsider which through-hole to use, so the relationship between the through-hole and the pin inductive reactance must be considered and the priority of the through-hole specification must be set.


5. Manual wiring and key signal processing


   Although this paper mainly discusses the problem of automatic wiring, manual wiring is an important process of PCB design now and in the future. The use of manual routing helps the automatic routing tool to complete the routing work. By manually routing and fixing the selected network (net), the path that can be used for automatic routing can be formed.  


   Regardless of the number of critical signals, these signals can be wired first, either manually or in combination with automatic routing tools. Key signals usually have to be carefully designed to achieve the desired performance. After the wiring is completed, the relevant engineering personnel will check

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