MCM LGA Package with Optical I/O Passively Aligned to Dual Layer Polymer Waveguides in PCB

Abstract

Over the past 30 years, IBM has provided leadership in high density I/O density and count interconnects at both chip and package levels as has been necessary for processor chips in high end symmetrical multiple processor (SMP) servers. For example, IBM introduced multi-chip modules (MCMs) in the 1970's, thermal conduction modules (TCMs) in the 1980's, and advanced organic micro-via buildup-layer packages in the 1990's (Patel, 2005). Typically, MCMs are necessary to provide significant increases in bandwidth between chips on the module, compared to the alternative route of lower bandwidth resulting from chip-to-chip interconnects going through the printed circuit board (PCB) for single chip modules (SCMs). CMOS chip-to-package pad scaling gap has been growing compared to the package-to-printed circuit board (PCB) pad scaling, which is the reason for the I/O advantage of MCMs vs. SCMs. For example, today's mainstream IC-to-package flipchip bonding uses 0.1 mm square pads on 0.2 mm pitch, with 0.15 mm ramping up, while the package-to-PCB ball- or land grid array (BGA or LGA) pitch uses 1-mm pitch, an areal density as much as 64 times lower. However, even with the larger bandwidth advantage of MCMs, more complex chips such as multiprocessor cores and the need for higher bandwidth to memory cache require a relatively larger number of signal I/Os, as well as more power and ground I/Os. Faster I/O clocks further exacerbate the need for more package I/O by forcing the transition from previous single-ended I/Os to differential signal I/Os to satisfy the higher frequency bit error rate specifications on SMP buses. Our goal is to alleviate the I/O bottleneck at the packaging level in the most cost effective manner, while providing the lowest risk, most flexible development package. To this end, we present our work on an electrical LGA field replaceable package with optical components. These optical components enable larger I/O bandwidth density between the MCM and PCB than that allowed by a standard electrical package.