PDMS Gasket Underfill for Long-Term Insulation of High-Density Interconnections in Active Implantable Medical Devices

This work presents reliability investigations of silicone gasket as solid underfill for interconnection interfaces in hybrid implant systems with high channel count flexible electrode arrays and hermetically packed electronics. The gasket is fabricated by laser structuring thin sheet of silicone rubber. The surface activation of silicone sheet ensures mechanical bonds with the mating surfaces thereby improving the mechanical stability of the assembly and the insulation of the interconnects. The gasket samples with $10 \times 10$ openings for interconnect pads, each with diameter of $270 \mu \mathrm {m}$ and a center to center pitch size of $490 \mu \mathrm {m}$, were sandwiched between a polyimide array and a metallized ceramic substrate. The gasket maintained high insulation impedance of $15 \pm 0.30 \mathrm {M}\Omega $ between the adjacent interconnects with markedly capacitive behavior (phase angle, $- 89 ^{\circ})$ after 17 weeks in soaked conditions under accelerated aging at $60 ^{\circ}\mathrm {C}$. The gasket also survived electrical stresses and sustained high impedance $(10.93 \mathrm {M}\Omega $ with phase angle of $- 88 ^{\circ})$ when subjected to constant 3 VDC for 100 days.