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26 DESIGN007 MAGAZINE I JULY 2025 either during the schematic creation stage or before actual layout begins. Industry best practice is to create and implement them prior to compo- nent placement and signal routing. Most of today's mid-to-high end EDA tools can address design con- straints either in the front end (the schematics) or in the back end (the PCB layout). Breaking Down the Basics of Design Constraints Net classes and constraint classes (the net groups/ categories): • Net classes are physical rules created by grouping or categorizing nets with similar physical characteristics. Once created, specific physical design rules are applied to them. • Constraint classes are electrical rules created to define electrical parameters. They enable efficient control over nets with similar electri- cal and timing properties. • Class-to-class rules define rules between groups of net classes and constraint classes. Layer Stackup This is often referred to as the foundation of the PCB: • Think of your PCB like a layer cake. You need a solid foundation to create the stackup. You also need to know what goes in each layer and how thick it should be. Containing and controlling the energy within the PCB is key. Make sure you: » Know the target realm of your design intent (e.g., digital, analog, mixed signal, RF, microwave, etc.). » Know what IPC performance class you're designing to (i.e., Class 1, 2, or 3). » Implement a balanced physical layer struc- ture, both in the X-Y axis and the Z-axis as well. This includes details such as layer count, material, Cu weight, standard through-hole, and/or HDI/UHDI technol- ogy requirements. » Know that impedance requirements will drive the construction of the PCB stackup as well as other constraints with the design. ▼ F i g u re 1 : E xa m p l e of a c o n st ra i nt s m a n a g e r.