Effect of Flexion on Performance of a High Bandwidth Flexible Connector with Low-Loss Dielectric

Systems-on-wafer (SoWs) has been a topic of increasing interest in advanced packaging research due to its applications in high-end systems such as high performance computing, machine learning, and artificial intelligence. Recent advances have been shown by industry, such as TSMC’s InFO_SoW, as well as academia, such as the Silicon Interconnect Fabric (Si-IF) from the UCLA Center for Heterogeneous Integration and Performance Scaling (CHIPS). Enabling large-scale practical implementation of these technologies requires advances in thermal management, power delivery, and inter-wafer and off-wafer signal transmission, for example from the SoW to another SoW or a PCB. For the latter point, we have previously shown the high frequency electrical characteristics of FlexCon, a flexible, high bandwidth connector cable platform for signal transmission. For a SoW on a 300mm wafer, FlexCon can provide >200 Tbps aggregate I/O bandwidth. However, dielectric loss in FlexCon limited its reach to a few cm, beyond which signal integrity becomes problematic.
Here we demonstrate the implementation of a low-loss polyimide dielectric in FlexCon. The polyimide replaces lossy SU-8, providing an insertion loss improvement of >20% for frequencies >30 GHz. Electrical characterization shows an insertion loss of < 0.25dB/mm at 40GHz. We further demonstrate the bonding of FlexCon to a silicon test vehicle using a conventional solder reflow process.
High flexibility of the connector enables signal transmission between media in which the pads at either end are closer to one another than the length of the FlexCon or out of plane with one another. For example, a silicon wafer and PCB are assembled in a fixture where the PCB sits slightly higher than the wafer or too close to the silicon wafer. In these scenarios where we wish to interconnect pads at the PCB and wafer edges, FlexCon can bend to accommodate any non-idealities in the physical geometry of the assembly. Because the bent geometry as well as repeated bending of FlexCon may affect the RF performance of FlexCon, we also provide the electrical characteristics as a function of bending radius and the number of bending cycles at various radii.