Digital systems are packaged in a hierarchy of chips, carriers, circuit boards, chassis, and cabinets. Signals are transported on batch-fabricated thin-film and thick-film wires on chips, carriers, and boards, and over cables between boards and chassis. The physical characteristics of the wires at each level determine their electrical properties, their cost, and the maximum signal density that can be supported.

At the lowest level, electronic components (mostly field-effect transistors) are fabricated on the surface of silicon integrated circuit chips and interconnected by layers of thin-film metal wiring. One or more of these chips are mounted in a package that connects their pads to package pins. Packages are mounted on a circuit board that provides low-impedance power and signal return planes and interconnects the package pins with thick-film metal layers. Circuit boards in turn may be connected to one another by other circuit boards (backplanes or mother boards) or by cables. To share mechanical support, several circuit boards are usually packaged together in a chassis, and several chassis may be housed in a cabinet or rack.

At each level of the hierarchy, from chips to cables, a set of design rules governs the physical size and spacing of wires and connections. The geometry that arises from these design rules constrains available wire density and determines the electrical properties of the resulting wires. In this chapter we discuss available packaging technology at each level of the hierarchy and survey current design rules.