This work presents the concept of silicone gasket as solid underfill for insulation of high-density interconnections of the polyimide based electrodes array and hermetic electronics package in an active implant. Two conventional underfilling techniques i.e. the capillary flow fluids and no-flow pastes are widely used as underfills in microsystems. The former one utilizes viscous polymers like silicone or fluidic expoxies and work on the principle of capillary flow for underfilling the narrow gap. The no-flow underfilling is a pre-bonding process and uses expoxies or adhesive pastes with good adhesion properties, especially in aqueous environment if targeted for physiological conditions. However, the existing technologies suffer from limitations regarding long-term reliability. The prominent challenges are achieving long-term adhesion with the polymer array and hermetic package, voids formation due to flow limitations or solvent evaporation and leakage between interconnects in long-run due to contamination of bonding residues. Most of the epoxies for no-flow either contain ions featuring hygroscopic properties or are not biocompatible when featured with good adhesion in aqueous environment.
Silicone rubber being biocompatible is widely used as capillary-flow underfill and encapsulation material for implants and has established adhesion properties for some other materials e.g. glass. Introducing the voids-free pre-structured silicone sheet as underfill gasket between the hermetic package and the polymer array allows proper cleaning of the surfaces with solvents as well as oxygen plasma at each assembly step. A 20μm thick silicone sheet is fabricated by spin coating polydimethylsiloxane (PDMS) precursor on a carrier tape. Openings (200μm in diameter with 400μm pitch) for interconnection pads are structured with picosecond laser. The resulting gasket is surface-activated with 80W oxygen plasma and aligned on the metallized ceramic substrate (electronics package) under microscope. In next step, the top surface of the gasket and the mating-side of polyimide array (with interconnect-pads at backend) are cleaned with plasma and properly aligned and left under weight for firm mechanical bond. The interconnect pads on the polyimide array and electronics package are electrically bonded with Microflex. The assembly is finally encapsulated with silicone in a custom mold. The gaskets in assemblies sustained high insulation resistance in phosphate buffered saline solution (PBS) at 60 °C. Insulation resistance between adjacent interconnects changed from 15.7 ± 0.62 MΩ to 11.8 ± 0.14 MΩ (1kHz electrochemical measurements) after 11 weeks of accelerated aging. In contrast, the reference samples with PDMS flow-underfill and epoxy encapsulation shown a large drop during initial few weeks. The gasket also retained high insulation of 5.8 MΩ while subjected to 1800 billion pulses (±1mA) in PBS and above 10 MΩ when stored in 20 mM H2O2 solution in PBS at 60 °C (4 weeks).