Important PCB Design Principles Engineers Should Keep in Mind
Shortening PCB design cycles has become a regular issue. Designers are also facing drastic changes in circuit board technology, such as faster processing speeds and increasingly complex IC packages, which add complexity to PCB design, which is the simplest part of the design process. Therefore, in order to improve the efficiency of the design process, constraint management tools have become increasingly indispensable.
Although constraint management tools can be found in some CAD tool packages available on the market, many designers fail to fully utilize the full potential of these tools. The key to utilizing constraint management tools is to understand the capabilities and characteristics of the software used and how to integrate it into the overall design flow. Effective use of constraints and rules in design can shorten design cycles, improve the scalability of current or under-development technologies, and ease the trouble of reusing designs in future designs.
The concept of constraint management is very simple to understand: just set up constraints and rules, and then hand over the design to layout. And, in fact, sometimes, it is indeed that straightforward to implement. But once engineers enter the constraint and rule establishment stage in the design environment, they will find that not everything is as clear as they thought.
It is important to start by exploring the capabilities and functions available. The following are some important principles that designers should keep in mind.
DO
- Understand what is relevant to your design and how information flows from one stage of the design process to the next. It is good practice to assign differential signals. Ideally, you want to assign differential signals as part of a top-down approach; therefore, you want to ensure that the above requirements can be implemented in the schematic or constraint management tool and then sent to layout.
- Properly archive information to ensure that it can be used in future designs. Establishing a system that can store proven implementations and share them among design team members will save time and help shorten the overall design cycle of the project. Many CAD systems are structured so that design data will be stored in a design folder that contains all the information including constraints and templates used in the design. Smartly manage your data in project folders so that specific data can be easily retrieved when it is needed again in a future design.
- Take advantage of the flexibility of constraint management tools. Such software can pass constraints forward or backward at any point in the design cycle, generate reports, and record changes. All of this reduces rework and helps maintain design integrity.
- Make sure the tool you choose has the ability to draw topology for analysis, set excitations, and create unique board stackups for single or combined segments.
It is more efficient to make the constraint management system the center of the application than to use data from manually calibrated values in spreadsheets and then try to pass this data back and forth between schematics and layout tools.
Figure: Constraint management can be done at different stages of the PCB design process
Don’t
- Over-constrain nets. It makes signals difficult to route. In extreme cases, especially when additional layers need to be added, design time can be multiplied to overcome corresponding routing issues such as timing or spacing requirements.
- Set constraints in the middle of the design process. It is important to do this at the beginning of the task. If you choose to set constraints for signals at a later stage, you may have to reroute the PCB, which will waste a lot of time. Before placement and routing, relevant strategies should be planned and handled to ensure that this does not happen.
- Forget about critical nets. If you use a constraint management system to set constraints, it is wise to route the critical nets before routing the entire PCB – especially if you set length or delay constraints for signals. Otherwise, when a fixed-length strategy is implemented for a signal, there is no room to lengthen it.
- Use a constraint management tool that does not interact with the routing tool and does not have a reporting system that can pull in data. Setting constraints and rules is of course all well and good, but if you have to manually query each assigned constraint, it defeats the purpose of using a constraint management tool.
- Apply constraints that are inconsistent with design rules. Designers route PCBs based on meeting manufacturing guidelines. If constraints are not managed and used in conjunction with design guidelines such as proper signal spacing for better etching results or good layer stacking to minimize crosstalk, problems will arise. Some tools allow you to freely apply any constraints to critical nets in your design; others apply your design rules at any stage of the design cycle, so they are enforced. Either way, when you apply constraints, make sure you do not violate the design rules.
