GUIDELINES GOVERNING THE NEED FOR LOW-IMPEDANCE DRIVERS FOR MCMS

High-speed, large-area integrated circuits (IC's) require packages wit h high-density, small cross-sectional interconnections. This trend tow ard more resistive interconnections is particularly clear for advanced thin-film multichip modules (MCM's). To obtain high clock speeds with these IC's 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-co ntrolled performance, maximum line lengths and driver impedances are f ound 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 driv er 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-match ed condition. Alternatively, wiring cross-sections can be increased wi th a corresponding decrease in wiring density or an increase in the nu mber of wiring layers.