Flexible Connectors and PCB Segmentation for Signaling and Power Delivery in Wafer-Scale Systems

摘要

The Silicon-Interconnect Fabric (Si-IF) is a wafer-scale platform in which bare dies are heterogeneously bonded to copper pillars using a thermal compression bonding (TCB) process. Electrical characteristics of interconnects in Si-IF are comparable to standard back-end wiring levels. A method of off-wafer communication via a high-bandwidth, low loss, high signal integrity connector can complement the Si-IF's excellent electrical characteristics for communication between the Si-IF and a PCB or another Si-IF. For standard packaged dies, signals and power are often delivered through ball grid arrays (BGAs) in which the package is soldered to a PCB. For wafer-scale systems, however, the large area of such a BGA would be prohibitively unreliable due to the coefficient of thermal expansion (CTE) mismatch between silicon and the PCB. Additionally, high inductance limits I/O data rates, particularly problematic in signaling through wire bonds. In this work we propose solutions for high-speed signal delivery and power delivery using the FlexTrate™ platform. For signal delivery we propose a flexible, high-speed connector which we call FlexCon. FlexCon consists of copper wiring over a grounded SU-8 dielectric on a highly flexible poly-dimethyl siloxane (PDMS) substrate. FlexCon is bonded via solder at either end, with one end at the edge of the Si-IF and the other bonded to a PCB or another Si-IF. Signals are transmitted through FlexCon differentially at up to 32 GHz per channel. Thermal stresses at the solder joints are minimal due to the low Young's modulus of the PDMS substrate. For power delivery, we propose the Compliant Power Platform. Here, the PCB onto which the Si-IF is mounted is segmented into smaller PCBs, embedded within PDMS, and interconnected using the FlexTrate™ process. The small area of the PCB segments mitigates thermal stresses due to CTE mismatch, enabling power delivery to the Si-IF using standard BGAs connected to through-wafer vias (TWVs) while significantly reducing stress in solder joints. Here, we present an electrical characterization of the FlexCon and an analysis of the proposed Compliant Power Platform.