A polymer-metal two step sealing concept for hermetic neural implant packages

In this paper, we introduce a technique for double-sealed ceramic packages for the long-term protection of implanted electronics against body fluids. A sequential sealing procedure consisting of a first step, during which the package is sealed with epoxy, protecting the implant electronics from aggressive flux fumes. These result from the application of the actual moisture barrier which is a metal seal applied in a second step by soft soldering. Epoxy sealing is carried out in helium atmosphere for later fine leak testing. The solder seal is applied on the laboratory bench. After the first sealing step, a satisfactory barrier for moisture is already achieved with values for helium leakage of usually L_He = 6·10^-8 mbar 1 s^-1. After solder sealing, a very low leakage rate of L_He ≤ 1·10^-12 mbar 1 s^-1 was found, which was the lower detection limit of the measurement setup, suggesting excellent hermeticity and hence moisture barrier. Presuming an implant package volume of V ≥ 0.5 cm³, the time to reach a critical humidity of p = 5000 ppm _H2O inside the package will be longer than any anticipated average life of human patients.