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978-3-8439-3717-7, Reihe Mikrosystemtechnik
Improving the reliability of high density interconnects in hybrid assemblies of active microimplants
196 Seiten, Dissertation Albert-Ludwigs-Universität Freiburg im Breisgau (2018), Softcover, B5
Active neural implants with high channel-count need robust and reliable functional assemblies for sustained operation in physiological environment in order to be classified as viable fully implantable systems. A critical interface exists at the interconnection sites between the electrode array and the implant electronics, particularly in hybrid assemblies (e.g. retinal implants) where electrodes and electronics are on different substrates. The interconnects in such assemblies cannot be hermetically sealed with existing assembly techniques. A reliable protection against the physiological environment would be essential for delivering high insulation resistance and low difusibility of salt ions.
This dissertation reports on the spring-designs of contact pads on polyimide arrays for high-density interconnection to rigid substrates in combination with a laser structured silicone gasket underfill as insulation for hybrid active implantable systems. The spring interconnects overcome the uniform vertical interconnections gap introduced by the gasket and relieves the propagation of bonding-induced extrinsic stresses to the bulk polyimide. For sustained adhesion of silicone gasket to the polyimide array, a sacrificial layer based fabrication process was devised for the deposition of dual-sided graded interfaces on polyimide substrate as adhesion promoter coatings for silicone.
The gasket underfill maintained high insulation during accelerated aging at 60°C and over 1 billion current pulses of ±1 mA in phosphate buffered saline (PBS) solution with Microflex bonded spring contacts as well as with conventional Microflex pads. The insulation impedance upto 4.5 years of projected age at 37°C exhibited highly capacitive nature with a magnitude higher than 10 MΩ for all sample sets. The Microflex bonded spring contacts maintained their continuity with almost constant contact resistances against current pulses and MIL 883 standard conformal test conditions of vibration, thermal cycling, humidity and high temperature storage. The contact and insulation results suggested that the spring interconnects successfully resisted the translation of extrinsic stresses to the bulk polyimide while maintaining self-intactness under test conditions that were largely surpassing the physiological environment.