A mathematical formalism is developed for RF/microwave design flows using category theory as a means for creating a formal foundation for their analytical and empirical, rather than anecdotal, study. A simplified model of design flow is developed using categories to capture the essential structure and dynamics of the schematic, layout, and behavior of both in terms of electrical phenomena. Specifically, schematics and layouts are defined as separate monoidal cateogries and the behaviors is defined as sets of name-value pairs. Functions, or morphisms, within and between these categories are explored for interconnect design and analysis in particular, and show typical EDA features such as forward- and back-annotation, portioning, simulation, and analysis. Traditional microwave and analog design are separately investigated using this formalism and found to be in agreement with expected outcomes. A new design flow and supporting EDA tool are identified based on the mathematical implications of the category theory models and associated design flow constraints of top-down design and design closure.