Guidelines governing the need for low-impedance drivers for MCM's

High-speed, large-area integrated circuits (ICs) require packages with high-density, small cross-sectional interconnections. This trend toward more resistive interconnections is particularly clear for advanced thin-film multichip modules (MCMs). To obtain high clock speeds with these ICs and these packages requires a combined design of drivers, lines, and loads. By extending traditional impedance-matched design to a generalization that adjusts load, line, and driver for overshoot-controlled performance, maximum line lengths and driver impedances are found beyond which underdamped (i.e., rapid) response is not possible. These limits are provided for on-chip, MCM, and printed-circuit-board (PCB) interconnections. For line lengths typical of advanced thin-film MCM interconnections, an underdamped circuit response requires a driver with impedance significantly less than the characteristic impedance of a lossless interconnection line of the same geometry. As a result, drivers are required with impedances below the normal impedance-matched condition. Alternatively, wiring cross-sections can be increased with a corresponding decrease in wiring density or an increase in the number of wiring layers.<
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