Compute and memory demands continue to rise, driven by recent developments in artificial intelligence and machine learning (AI/ML). As large language model (LLM) parameter counts scale beyond 1 trillion, similarly scalable computing systems are necessary to efficiently train models with respect to time and energy. High performance compute (HPC) systems implemented on wafer-scale heterogeneous integration packaging platforms, such as the Silicon Interconnect Fabric (Si-IF), provide a scalable path to meet memory and compute demands for future systems. However, several challenges must be addressed prior to practical implementation, such as power delivery, thermal management, and off-wafer signaling.
To address off-wafer signaling, we have developed FlexCon, a flexible connector cable platform enabling high bandwidth (≥240 Gbps/mm) signaling for wafer-scale systems. FlexCon consists of a highly flexible PDMS substrate on which 133.3 μm pitch differential microstrip channels are lithographically defined over a SU-8 dielectric. Connection to the periphery of a wafer-scale system is achieved via solder bumps, an integration method chosen to enable both wafer-wafer and wafer-PCB assemblies. With FlexCon, aggregate I/O bandwidth across a 300 mm Si-IF HPC system is >200 Tbps. However, due to the relatively lossy SU-8, channel reach is limited to ~25 mm for a targeted insertion loss of < 6dB.
In this work, we implement and demonstrate two methods of extending the channel reach: replacement of SU-8 with a low-loss dielectric of Parylene-N; and integration of an active buffer IC with equalization (Texas Instruments, SN75LVPE4410). The advantages of Parylene-N over SU-8 include lower loss tangent (4E-2 vs. < 6E-4), resulting in ~32% lower insertion loss at 32GHz, and lower dielectric constant (3.2 vs. 2.4), reducing latency as well as enabling a ~20% thickness reduction, thus improving flexibility. Furthermore, we investigate the reliability of FlexCon channels under cyclic mechanical bending and show that they are robust down to 2.5mm bending radius after 1000 bending cycles.
