Why do we start with clock routing first in physical design?

Typically, most designs incorporate clocks, with a substantial number of sequential cells governed by these clock signals. The prioritization of routing is not solely determined by whether clocks or data are considered first; rather, it hinges on factors such as criticality, vulnerability to variations, operational frequency, load magnitude, and the distance required for signal propagation to all sinks. In synchronous designs, clocks serve as the primary control signals for sequential cells. Given the clock network's high load, operational frequency, and stringent requirements for minimal variation, clock routing is initiated before data signal routing. This approach allows for greater flexibility in utilizing various Non-Design Rule (NDR) options and specific metal layers to meet specific timing objectives. Furthermore, in certain instances, the routing of exceptionally critical signals may commence even before clock routing.