Organization is the key to design efficiency, and the lack of organization will reduce the efficiency of PCB design.
Take design rules as an example: when starting a new project, many PCB design engineers tend to redesign their designs according to their own design rules, instead of relying on an appropriate, organized and systematic rule method.
When the time factor is crucial, this approach will greatly reduce the progress of the project. This article will discuss the rules and methods that can improve the design efficiency of multilayer circuit boards.
Consistency and library management
It is a common practice to re-create PCB design rules for each ongoing new project. This may be because the design department has no proper process to reuse the rules, or simply because PCB design engineers don’t want to reuse these rules.
Whatever the reason, this indicates that the design workflow needs to be updated to improve the design efficiency and consistency. Without improvement and promotion, the current workflow may take a lot of extra time and effort, and inconsistency will lead to inheritance problems.
The design rules of each multilayer circuit board are different. Usually, we expect PCB design engineers to fully understand the rules and be able to enter the same value in each design. However, even if their memory is correct, there is always the possibility of inputting wrong values.
Lack of consistency may eventually lead to the multi-layer circuit board elements in the whole system being controlled by different rules. If different circuit boards are used on the same manufacturing panel, this may be a bigger problem. If the structure of one layer is different between different designs, it may lead to manufacturing delay, poor structure or redesign.
In order to simplify the process of creating design rules for multilayer circuit boards, it is best to save and manage these rules for future use. The place used to store and manage rules is called “library”, which includes schematic symbols, simulation models, design constraints, PCB footprints and STEP models. A library can be as simple as a central file directory for storing data, or it can be a complex system composed of multiple directory locations and links.
No matter how the CAD library is set up, the design rule set can be saved in the same way. In order to successfully manage the “library” of design rules, there should be three basic practices in the system:
1. Location: Establish a central location on the server that all users can access. Save frequently used design rules and other types of data on personal computers for future use. This can restrict the access of others, and the saved files can be changed or deleted more easily.
2. Naming Convention: Name the saved file in a way that is easier for others to recognize. Date, description and job number are all good choices for file names. The naming method “joes _ really _ cool _ design _ rules.txt” is not recommended.
3. Restricting editing permission: it is best to let design team members not edit their files in this location after saving them in this location. Usually, it will be very frustrating to suddenly find that the design rules of six-layer circuit boards have changed after the last use. Design team members can copy these rules to a new file, make changes at any time, and then name and save them with a new name.
When the design data storage system is in place, it will be used to save files.
Multi-layer PCB Constraint, Testing and System-level Integrity
Even if only one circuit board is available, signal integrity and power integrity are still the key factors to be carefully considered in the design. Compared with today’s multi-layer circuit board system, the considerations of single circuit board are simpler: simpler power supply, fewer interference components and shorter signal transmission distance.
1. System-level signal integrity and electromagnetic interference (EMI)
The best way to ensure proper signal integrity (SI) of multilayer circuit boards is to have a reliable and accurate signal integrity analysis and simulation tool, so as to work together with the design in every step of the process.
The interaction of high-speed and low-speed signals will definitely cause signal integrity problems. In addition, the possibility of crosstalk with all active components is greater. Therefore, due to the interaction between high-speed signals, the design of multilayer circuit boards almost always faces greater risk of electromagnetic interference.
By using effective tools, or enhancing their functions with our knowledge, separating analog and digital signals, adopting intelligent wiring (meaning that there is no right-angle wiring in the circuit) and keeping the signal and return current tightly coupled, the electromagnetic interference problem can be significantly reduced.
2. Partition design and multi-layer circuit board cost analysis
Physical grouping of components based on purpose and function can make many factors achieve higher security. This process is called PCB zoning, and subsystems can be regarded as a group of components in multilayer circuit board design.
Proper PCB zoning can simplify operation and improve cost-effectiveness, such as:
Modularization, or integrating standardized components into a larger variety of product designs.
Circuit board spacing and actual installation of enclosure circuit
Make a cheaper and smaller circuit board connected to the main board, instead of the accessories in the expensive multilayer circuit board laminated structure.
In the design of multilayer circuit board, analog and digital circuits are separated to reduce electromagnetic interference.
3.PCB constraint manager and multi-layer circuit board auxiliary equipment
Although many constraint managers have been equipped with the impedance, laminated structure, network and plane selection required to complete the work, an excellent constraint manager will also consider the component list and parameters, which will make the multilayer circuit board design easier to manage, especially when dealing with thousands of independent components on each circuit board.
The multilayer circuit board constraint manager can manage advanced network classes, set cooling requirements, quickly adjust cooling specifications under constraint conditions, and handle signal conditions.
Multilayer circuit board system architecture and design tools
For the integrity of the system, it is very important to manage the connectors in the multi-layer circuit board system.
Although we may have made proper plans for everything, when determining the connector suitable for design, it is essential to determine how the connector is handled in the enclosure environment, whether the circuit board is designed to be vertically stacked or horizontally layered, and potential corrosion (environment, current, electrolysis, etc.).
During the whole layout stage, connectors should be carefully managed. If they have not been fully considered, you may need to pay more attention to design rules and constraint managers. When we re-route for a long time and don’t have enough energy to make the final check (such as checking the impedance value of vias), design rule check (DRC) can make us clearly see the violations and solve the problems quickly, instead of worrying about them.
When design rules are first applied to PCB design tools, its operation is not simple. Users often have to browse multiple menus and fill out different forms. Usually, the methods of saving information in these systems are not reliable, or even if they are saved, they are only saved in an unformatted text file. However, the times are developing.
Today’s PCB design systems usually have advanced design rules and constraints, which can be accessed by spreadsheet-type utilities. In addition, these rules and constraints are usually associated with other functions in the design system. For example, the impedance calculator can directly input values into rules and constraints without manually inputting data.