Thin Film Metallization Stacks Serve as Reliable Conductors on Ceramic-based Substrates for Active Implants

Abstract

Hermetic and non-hermetic packages of active implantable medical devices are often fabricated of ceramics like alumina. Screen printed PtAu paste is the state of the art metallization for functional structures. Due to solid state and liquid diffusion of Au at thermal exposure, solder times are limited; otherwise metal structures tend to delaminate. Moreover, it was shown that PtAu with solder fails after 37.4 years. We established a thin film metallization on alumina process to overcome these disadvantages. The metallization consists of sputtered platinum with an underlying adhesion layer made of tungsten-titanium to increase the adhesion strength to the alumina substrate. We avoided using gold in this work due to its high diffusion tendency. Instead, the materials in use provide relatively low diffusion properties, which potentially increases the long term mechanical performance and usability during assembly and packaging.Utilizing the Design of Experiment (DoE) methodology, we derived an optimal Pt thickness of 500 nm with 43 nm of WTi as adhesion promoting layer. After accelerated aging at 150 °C, corresponding to 125 years at body temperature (37 °C), the contact pad adhesion strength was with 32.75 MPa ± 7.08 MPa still significantly higher than the safety limit of 17 MPa, following the recommendations for a robust screen-printing metallization process. Moreover, soldering times of up to 120 s did not influence the adhesive strength. The new process reduced the minimum track distance to 50% of screen printing values and is capable of rapid prototyping. It helps to make the assembly process independent of the manufacturing person, to increase the yield of device fabrication and -most important in implantable device manufacturing-to make it more robust and thereby more safe for the patient.Competing Interest StatementThe authors have declared no competing interest.View Full Text