Electronic Design - PCB design skills for efficient automatic routing

Although the EDA tools are now powerful, as the PCB size requirements are getting smaller and smaller, the device density is getting higher and higher, and the difficulty of PCB design is increasing. How to achieve a high PCB layout rate and shorten the design time? This article will introduce the design techniques of PCB design planning, layout, and wiring. Nowadays, the time required for PCB design 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 work of designers 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 on the market, but they are all the same except for the different terms used and the position of the function keys. How do use these tools to better realize the PCB design? A careful analysis of the design and careful settings of the tool software before starting the wiring will make the design more in line with the requirements. The following are 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.

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 components

In order to optimize the assembly process, design for manufacturability (DFM) rules will restrict 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.

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. The design of the circuit online test can be carried out in 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

Regardless of the number of key signals, these signals should be routed first, either manually or in combination with automatic routing tools. Critical signals usually have to pass careful circuit design to achieve the desired performance. After the wiring is completed, the relevant engineering personnel will check the signal wiring. This process is relatively easy. After passing the inspection, fix these lines, and then start automatic wiring of the remaining signals.

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 will 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.

7. The design points of PCB automatic routing include:

7.1 Change the settings slightly, 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 vias, buried vias, etc., and observe how these factors affect the design results;

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

7.4 The less important the signal, the greater the freedom of the automatic wiring tool for its wiring

8. Wiring arrangement

The signal wiring length can be shortened and the number of vias can be reduced by manual editing. In the finishing process, you need to determine which wiring is reasonable and which wiring is unreasonable. Like manual wiring design, automatic wiring design can also be sorted and edited during the inspection process.

9. PCB appearance

The previous design often paid attention to the visual effect of the PCB 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.