Ultra-Short-Reach Interconnects for Die-to-Die Links: Global Bandwidth Demands in Microcosm

Bandwidth density metrics are emerging as essential criteria for high performance serial links in addition to energy efficiency. These include bandwidth per bump, per wire, and per millimeter of cross-sectional bus width as well as per-unit chip area. In comparison to conventional board level chip-to-chip interconnects over a PCB, USR interfaces between copackaged dies increase density with relatively modest frequency-dependent losses and, thus, low power consumption. To achieve the highest possible bandwidth density over USR links, traditional assumptions about high performance electrical interconnects such as termination impedances must be reexamined. For example, single ended signaling can offer comparable signal integrity and higher bandwidth density than differential signaling, particularly if there is a pitch constraint on the die-to-die traces. Moreover, termination of the USR links has a significant impact on their performance, so a detailed analysis of the terminating impedances is required to optimize link performance.