Bonding and Packaging Advancements to the Boston Retinal Prosthesis

Abstract

Purpose: : This work is related to the efforts of the Boston Retinal Implant Project to develop a sub-retinal prosthesis to restore vision to the blind. This poster focuses on high density packaging advancements for the Boston prosthesis and bonding processes for attaching flexible stimulating electrode arrays to the exterior of the packages.

Methods: : Custom SiC-encapsulated polyimide arrays with over 200 Au micro-contacts, each 100 microns in diameter, were microfabricated and the contacts electroplated to a thickness of 25 microns. Cofired ceramic structures with Pt signal feedthroughs were also fabricated to match this contact pattern and brazed to miniature titanium housings. The flexible stimulating arrays were joined to the feedthrough assemblies using a Kulicke & Soffa model 4522 bonder using a substrate heater set at 150 degrees C. Thermo-compression joints were made using a Small Precision Tools SPT 7645A-TI-0040-S-M tip and a laser targeting mechanism.

Results: : Bond shear tests to the retinal implant assemblies were performed using a XYZTec Condor 70 shear force tester. The thermo-compression bonds had an average shear strength of 50 grams force, indicating good quality bonding; the bonds were stronger than the joints to the polyimide matrix in which the stimulating electrodes and metal traces were embedded. Au to Au joining required less force and time than Au to Pt bonding. Consequently, the Pt feedthroughs were over-coated with gold to facilitate the bonding process. Optimization of the profiles of the electroplated contact pads and the sintered bumps on the electrode arrays and feedthrough discs respectively has improved the overall bondability of the assemblies. Polyimide mesh-reinforced contact pads, for example, were found to improve the integrity of the electrode array-feedthrough connections after bonding by preventing the bonded joints from separating from their host substrates.

Conclusions: : The feasibility of high-density bonding of microfabricated electrode arrays to hermetic signal feedthrough arrays having 200+ channels has been demonstrated. Such structures will form a key component in the next-generation Boston retinal prosthesis.

Keywords: retinal degenerations: hereditary • age-related macular degeneration • retinitis