Design skills and key points of PCB automatic routing

Although the current EDA tools are very powerful, but with the PCB size requirements are getting smaller and higher, the difficulty of PCB design is not small. How to achieve high PCB penetration rate and shorten the design time? This article introduces the design tips and key points of PCB planning, layout and wiring. Nowadays PCB design time is getting 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, speed up the product market, now many manufacturers tend to use EDA tools to achieve PCB design. But EDA tools don't produce the desired results, don't get 100% penetration, and are messy and often take a lot of time to complete the rest of the work.

Now the market popular EDA tools a lot of software, but in addition to the use of terminology and function key location are 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 tools and software before 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 PCB layers

The size of the circuit board and the number of wiring layers need to be determined at the beginning of the design. If the design calls for the use of high-density spherical grid array (BGA) components, the number of wiring layers required to route 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 helps determine the pattern of layering and the width of the printed line to achieve the desired design effect.

For years, it was thought that the fewer layers a board had, the cheaper it would be, but there were many other factors that affected the cost of making a board. In recent years, the cost differential between multilayers has decreased considerably. At the beginning of the design, more circuit layers should be used and the copper applied should be evenly distributed, so as to avoid being forced to add new layers when a few signals are found near the end of the design that do not conform to the defined rules and space requirements. 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. To complete wiring tasks, wiring tools need to work within the correct rules and constraints. Different signal lines have different wiring requirements, to all the special requirements of the signal line classification, different design classification is not the same. Each signal class should have a priority, and the higher the priority, the stricter the rules. The rules involve the width of printed lines, the number of holes, parallelism, the interaction between signal lines and the limits of layers. 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, design for manufacturability (DFM) rules impose restrictions on component layout. If the assembly department allows the components to move, the circuit can be optimized for automatic wiring. The rules and constraints you define affect the layout design.

The wiring path (roung channel) and hole area should be considered in the layout. These paths and areas are obvious to the designer, but the automatic wiring tool takes into account only one signal at a time. By setting routing constraints and setting layers of deployable signal lines, the wiring tool can perform the wiring as envisioned by the designer.

4 fan out design

In the fan-out design phase, to enable component pins to be connected by the automatic wiring tool, each pin of the surface-mount device should have at least one through-hole so that the board can be internally connected, in-line tested (ICT), and reprocessed when additional connections are required.

In order to make the automatic wiring tool efficient, it is necessary to use as large as possible through the hole size and printed line, the interval set to 50mil is ideal. Use the type of hole that allows the number of routing paths to be large. The problem of online circuit testing should be considered in the design of fan - out. Test fixtures can be expensive and are usually ordered when they are about to go into 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 at the initial stage of design and implemented later in the production process. The type of through-hole fan-out can be determined according to the wiring path and online circuit testing. The power supply and grounding will also affect the wiring and fan-out design. In order to reduce the inductive reactance generated by the connection line of the filter capacitor, the pass hole should be as close as possible to the pin of the surface-mounted device. If necessary, manual wiring can be used, which may affect the original thought of the wiring path, or even cause you to reconsider which kind of pass hole to use. Therefore, the relationship between the pass hole and the pin inductive reactance must be considered and the priority of the pass 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 and will be an important process in PCB design. Manual wiring is helpful for automatic wiring tools to complete the wiring work. As shown in Figure 2a and 2b, by manually routing and fixing the selected network (net), a path can be formed for automatic routing.

Regardless of the number of key signals, these signals are routed first, either manually or in combination with automatic wiring tools. Critical signals usually have to be carefully designed to achieve the desired performance. After the wiring is completed, it is much easier for the engineer concerned to check the signal wiring. After the inspection is passed, the wires are secured and the rest of the signals are automatically routed.