Realize the design of efficient automatic PCB routing

Now PCB design time is getting shorter and shorter, smaller and smaller board space, higher and higher device density, extremely demanding layout rules and large-size components make the designer's job more difficult. In order to solve the design difficulties and speed up the market of products, many manufacturers now tend to use special EDA tools to achieve PCB design. However, dedicated EDA tools do not produce ideal results, do not achieve 100% distribution flux, and it is messy, and usually takes a lot of time to complete the rest of the work. Today, this article introduces the design tips and key points of PCB planning, layout and routing.

Now the market popular EDA tools a lot of software, but in addition to the use of terminology and function key location is not the same, how to use these tools to better achieve the design of PCB? Careful analysis of the design and careful setup of the tool software before starting the wiring will make the design more compliant. The following is the general design process and steps.

1, determine the number of PCB layers

The size of the board and the number of wiring layers need to 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 the wiring of these devices must be considered. The number of wiring layers and the stack-up mode will directly affect the wiring and impedance of the printed line. The size of the board helps determine the stacking pattern and the width of the printed line to achieve the desired design effect.

For many years, people have always believed that the fewer layers of the board the lower the cost, but there are many other factors that affect the manufacturing cost of the board. In recent years, the cost difference between multilayer panels has been greatly reduced. At the beginning of the design, it is best to use more circuit layers and make the coating copper evenly distributed, so as to avoid finding that a small number of signals do not meet the defined rules and space requirements near the end of the design, so as to be forced to add new layers. Careful planning before design will reduce a lot of trouble in wiring.

2. Design rules and restrictions

The automatic wiring tool itself does not know what to do. In order to complete the wiring task, the wiring tool needs to work under the correct rules and restrictions. Different signal lines have different wiring requirements, to classify all the special requirements of the signal line, different design classification is not the same. Each signal class should have a priority, and the higher the priority, the stricter the rules. Rules related to the width of the printed line, the maximum number of holes, parallelism, the interaction between the signal lines, and layer limitations, these rules have a great impact on the performance of the wiring tool. Careful consideration of design requirements is an important step in successful wiring.

3. Layout of 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 automatic routing. The rules and constraints you define affect the layout design.

routing channels and through-hole areas should be considered in the layout. These paths and areas are obvious to the designer, but the automatic routing tool only considers one signal at a time, and by setting routing constraints and setting layers of deploable signal lines, the routing tool can be routed as the designer intends.

4. Fan out the design

In the fan-out design phase, to enable the automatic wiring tool to connect component pins, each pin of the surface-mount device should have at least one through-hole so that the board can perform inner connections, in-line testing (ICT), and circuit reprocessing when more connections are needed.

In order to maximize the efficiency of the automatic wiring tool, it is necessary to use the largest hole size and printed line as possible, and the interval is set to 50mil. Use the through-hole type that maximizes the number of routing paths. In the fan-out design, the problem of circuit online testing should be considered. Test fixtures can be expensive and are usually ordered when they are about to 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 the circuit online test can be carried out in the early stage of design, and realized in the later stage of the production process, according to the wiring path and circuit online test to determine the through-hole fan-out type, power supply and grounding will also affect the wiring and fan-out design. In order to reduce the inductive reactance generated by the filter capacitor connection line, the hole should be as close as possible to the pin of the surface-mount device, and manual wiring can be used if necessary, which may have an impact on the originally envisioned wiring path, and may even cause you to reconsider which hole to use, so you must consider the relationship between the hole and the pin inductive reactance and set the priority of the hole specification.