Constraint Definition
With the physical modeling of the component substrate complete, the next phase is to set up constraints.
Constraints fall into two categories: physical and electrical. Physical constraints are driven by manufacturing guidelines, though electrical rules may impact the physical rules. For example, an electrical rule may be a 50-ohm line impedance that translates into a 4-mil trace width. Electrical rules are engineering-driven and are imposed to ensure signal quality and overall component performance.
Physical Constraints
Physical constraints can be further broken down into two categories: Physical (Line and Via sizes) and Spacing (Line, Via, Pad, and shape spacings). Again, most of the physical rules are driven by manufacturing specifications, though some latitude may be given depending on the foundry.
You can also assign physical constraints to specific nets or groups of nets (net classes). Net classes allow you to specify different line widths, via sizes, and element-to-element spacing to the entire group or to a specific area layer of the component substrate.
Electrical Constraints
Electrical constraints can be divided into two categories that are commonly lumped together: delay and distortion (D&D). Delay refers to the interconnect delays introduced by the physical layout, typically in terms of nanoseconds (ns). Distortion refers to sources of noise caused by the physical layout, such as undershoot or crosstalk. Distortion is measured in millivolts (mV).
You should divide signals in the layout into unique net classes based on performance requirements, for example, clocks and buses. Each constraint set would have its own noise budget and, therefore, corresponding distortion (overshoot, undershoot, crosstalk, and so on) constraints. For each net class, you also define timing constraints, such as delay and matched delay. In addition to defining electrical constraints, you should define any thermal constraints.
Template Files for Constraints
The effort involved in setting up all of the required constraints in a design may seem cumbersome and time consuming. Technology files allow you to dump out an ASCII representation of all defined constraints, which you can then import into other designs.
- Net classes
- Physical constraints
- Electrical constraints
- Stackup Information
- Design size, units, and origin
- Special attributes and properties created within the design
Although every design has some unique requirements, template files can minimize the constraint definition effort by allowing you to modify an existing data file rather than starting a new file.
Related Topics
Die-to-Component I/O Net Assignment
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