1. Determine the number of layers of PCB
Circuit board size and wiring layer number need to be determined in the early design.If the design requires the use of high-density spherical grid array (BGA) components, the minimum number of layers required for wiring these components must be considered.The number of wiring layers and stack-up mode will directly affect the wiring and impedance of the printed line.The size of the board helps to determine the lamination mode and print line width to achieve the desired design effect.
For years, it was thought that fewer layers would be cheaper, but there are many other factors that affect the cost of making a circuit board.In recent years, the cost difference between multiple layers has been greatly reduced.It is better to start the design with more circuit layers and uniform distribution of the copper coating, so as to avoid finding a small number of signals near the end of the design that do not meet the defined rules and space requirements, thus forcing the addition of new layers.Careful planning before design will reduce the hassle of wiring.
2. Design rules and limitations
The automatic wiring tool itself doesn't know what to do.To complete the wiring task, the wiring 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. Different design classification is also different.Each signal class should have a priority, and the higher the priority, the stricter the rules.Rules relate to print line width, maximum number of passes, parallelism, mutual influence between signal lines and layer limitations, and these rules have a great impact on the performance of wiring tools.Careful consideration of design requirements is an important step in successful wiring.
3. Component layout
To optimize the assembly process, manufacturability design (DFM) rules place limitations on the component layout.If the assembly department allows the component to move, the circuit can be optimized to facilitate automatic wiring.The rules and constraints defined affect the layout design.
Routing channel and through-hole region should be considered when layout, as shown in figure 1.These paths and areas are obvious to the designer, but the automated wiring tools consider only one signal at a time, and by setting the wiring constraints and setting the layer on which the signal can be laid, the wiring tools can be wired as the designer envisioned.
4. Fan out design
In the fan out design phase, to enable automatic wiring tools to connect the component pins, each pin of the surface-mounted device should have at least one via hole so that the circuit board can be interlocked, online testing (ICT) and circuit reprocessing when more connections are required.
In order to maximize the efficiency of automatic wiring tools, it is necessary to use the maximum hole size and printing line as much as possible, and the spacing is set to 50mil.Use the type of crossing that maximizes the number of routing paths.When designing the fan out, we should take the circuit testing into account.Test fixtures can be expensive and are usually ordered shortly before they go into full production, and it is too late to consider adding nodes to achieve 100% testability.
After careful consideration and prediction, the design of online circuit testing can be carried out in the early stage of design and implemented in the later stage of production. The type of through-hole fan out can be determined according to the wiring path and the online test of circuit. Power supply and grounding will also affect the wiring and fan out design.To reduce the filter capacitor impedance of cable, via should as far as possible near the surface-mount device pins, manual wiring can be used when necessary, it may affect the originally envisaged wiring path, even may cause you to reconsider using what kind of a hole, must therefore be considered the relationship between impedance and hole and pin set via the specifications of the priority.