PCB Technical - Design skills and key points of PCB planning, layout and wiring

Although the current EDA tools are very powerful, as the PCB size requirements are getting smaller and the device density is getting higher and higher, the difficulty of PCB design is not small. How to achieve a high PCB layout rate and shorten the design time? This article introduces the design skills and key points of PCB planning, layout and routing. Now PCB design time is getting shorter and shorter, smaller and smaller circuit board space, higher and higher device density, extremely demanding layout rules and large-size components make the designer's work more difficult. In order to solve the design difficulties and speed up the launch of products, many manufacturers now tend to use dedicated EDA tools to realize PCB design. However, dedicated EDA tools cannot produce ideal results, nor can they achieve a 100% deployment rate, and are very messy. It usually takes a lot of time to complete the remaining work.

There are many popular EDA tools and software on the market, but they are all the same except for the different terms and the positions of the function keys. How to use these tools to better realize the PCB design? Carry out a careful analysis of the design before starting the wiring and Careful setting of the tool software 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 the PCB

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 requires the use of high-density ball grid array (BGA) components, the minimum number of wiring layers required for wiring these devices must be considered. The number of wiring layers and the stack-up method will directly affect the wiring and impedance of the printed lines. The size of the board helps determine the stacking method and the width of the printed line to achieve the desired design effect.

For many years, people have always thought that the lower the number of layers of the circuit board, 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. It is best to use more circuit layers and evenly distribute the copper at the beginning of the design, so as to avoid discovering that a small number of signals do not meet the defined rules and space requirements until the end of the design, so that new layers are forced to be added. Careful planning before designing will reduce a lot of troubles in wiring.

2. Design rules and restrictions

The automatic routing 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. All signal lines with special requirements must be classified, and different design classifications are different. Each signal class should have a priority, the higher the priority, the stricter the rules. The rules involve the width of the printed lines, the maximum number of vias, the degree of parallelism, the mutual influence between the signal lines, and the limitation of layers. These rules have a great influence on the performance of the wiring tool. Careful consideration of design requirements is an important step for successful wiring.

3. The layout of the components

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 appropriately optimized, which is more convenient for automatic wiring. The defined rules and constraints will affect the layout design.

The routing channel and via area need to be considered during layout. These paths and areas are obvious to the designer, but the automatic routing tool will only consider one signal at a time. By setting routing constraints and setting the layer of the signal line, the routing tool can be as the designer imagined Complete the wiring like that.

4. Fan-out design

In the fan-out design stage, to enable automatic routing tools to connect component pins, each pin of the surface mount device should have at least one via, so that when more connections are needed, the circuit board can be internally layered Connection, online testing (ICT) and circuit reprocessing.

In order to maximize the efficiency of the automatic routing tool, the largest via size and printed line must be used as much as possible, and the interval is ideally set to 50mil. Use the via type that maximizes the number of routing paths. When carrying out fan-out design, it is necessary to consider the problem of circuit online testing. Test fixtures can be expensive, and they are usually ordered when they are about to go into full production. If only then consider adding nodes to achieve 100% testability, it would be too late.

After careful consideration and prediction, the design of circuit online test can be carried out at the early stage of the design and realized in the later stage of the production process. The type of via fan-out is determined according to the wiring path and circuit online test. 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 vias should be as close as possible to the pins of the surface mount device, and manual wiring can be used if necessary. This may affect the originally envisaged wiring path, and may even cause you to re- Consider which type of via to use, so the relationship between via and pin inductance must be considered and the priority of via specifications must be set.

5. Manual wiring and processing of key signals

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

6. Automatic wiring

The wiring of key signals needs to consider controlling some electrical parameters during wiring, such as reducing distributed inductance and EMC, etc. The wiring of other signals is similar. All EDA vendors provide a way to control these parameters. After understanding the input parameters of the automatic wiring tool and the influence of the input parameters on the wiring, the quality of the automatic wiring can be guaranteed to a certain extent.

General rules should be used for automatic routing of signals. By setting restrictions and prohibiting wiring areas to limit the layers used by a given signal and the number of vias used, the wiring tool can automatically route the wires according to the engineer's design ideas. If the number of layers used by the automatic routing tool and the number of vias are not limited, each layer will be used during automatic routing, and many vias will be generated.

7. The design points of automatic wiring include:

7.1 Slightly change the settings, try a variety of route wiring;

7.2 Keep the basic rules unchanged, try different wiring layers, different printed lines and spacing widths, different line widths, and different types of vias such as blind holes, buried holes, etc., and observe how these factors affect the design results;

7.3 Let the wiring tool process those default networks as needed;

7.4 The less important the signal, the greater the degree of freedom the automatic routing tool has for its routing.

8. The appearance of the circuit board

The previous PCB design often paid attention to the visual effect of the circuit board, but now it is different. The automatically designed circuit board is not as beautiful as the manual design, but the electronic characteristics can meet the specified requirements, and the complete performance of the design is guaranteed.