Pros and cons of using (or not using) constraints.
A correct-by-construction methodology may appear to be a time-consuming planning task, but time taken during the layout planning can save hours or days on a schedule by preventing design mistakes that can cause product failures or hours of design rework. Proper constraint definition can make a PCB design much easier and eliminate the opportunity for errors.
When proper constraints are not used, it becomes impossible to get it right the first time. Some factors that will cause issues during design are:
Designers are continually pushed to reduce time to market. Using proper planning of constraints can:
In the past (and unfortunately some designers today stick to old methods), rules were used sparingly – if at all – to route designs. Many designers still use a grid-based routing system to avoid spacing errors on their designs. Using a grid, for example, of 5 mils permits the design to be routed easily with 5 mil lines and spaces. This works if all the design requirements are for 5 mil lines, but designs of the present seldom have such simple requirements.
Another commonly used method is to design and route with DRC (design rules checking) set to disabled, allowing a designer to quickly throw etch onto a design without worry of errors. This is because the designer plans to catch and correct errors at the end of the design. That’s an irresponsible approach, as it can cost hours to days of rework, all at a time when the design should be going to fabrication.
New tools and methods are very different. Modern PCB design tools are constraint-driven, and the layout tools are shape-based, relying on properly planned constraints to drive trace routing using gridless methods. A common spreadsheet-based constraints database that is updated, whether the designer is in the schematic or the layout tool, allows both engineer and PCB designer to collaborate on constraints throughout the design. Online DRC capability allows the designer to quickly lay down etch and have the system keep errors from occurring on-the-fly, thus making end-of-design cleanup a minimal effort, if needed at all. These more-effective methods allow the designer to focus on the design, and not the errors that need to be checked at late stages in the design (FIGURE 1).
Constraint planning. The Human Interface is a big part of the problems that can creep into a design. We work on removing that issue by creating constraints at the planning phase of the design to help the designer work smoothly through the design without the need to check rules along the way. This is why this stage is so important, and even though it may add some additional preparation time before even one component or trace is laid, it may be the greatest increase in productivity during a design.
Set this process in motion at the earliest stages of the design. Remember: rules can exist in a design even before the board outline is drawn. In fact, some of the early constraints may have been entered into the schematic by the engineer.
In a perfect world, an engineer will provide a statement of work with all the requirements they think are critical to the design (FIGURE 2). This will help drive constraint planning and entry.
Constraint entry. Let’s look at some steps to take when entering constraints. Using this process, the designer can quickly build a set of constraints that can be trusted, and then focus on the design process itself.
This is an overview, of course, but once the preparation of in-depth constraints for a design has been mastered, work will be seamless from beginning to end of the layout process without the need to check if the design meets the requirements of the engineering team.