Fluxless packaging of an implantable medical device for transcorneal electrical stimulation

Transcorneal electrical stimulation (TcES) is one of the potential therapies to prevent retinal degeneration. This study reports on an implantable medical device for TcES. The regular TcES components, such as stimulator chips, metal wires, electrodes and printed circuit boards (PCBs), are all packaged into the device. The device has a thin-film shape and is soft enough to bend, thus it is placed beneath the cornea without exposure to the outside. The stimulator chip, with the function of generating precise stimulation current to prevent retinal degeneration, is flip bonded onto a flexible PCB using gold studs and thermosonic bonding process. Thermosonic bonding eliminates the use of flux, which is not biocompatible with in-vivo implants. The surfaces of the gold stud bumps are in general non-uniform. Coining is an essential pretreatment of compressing the gold stud bumps against a flat solid surface to obtain much smoother & larger surfaces. This pretreatment helps form high-strength joints between the gold stud bumps and the PCB pads. In the packaging process, underfill is applied in the gaps between the chips and the flexible PCB to enhance the reliability of the gold joints. A compression molding step is subsequently conducted to cover all internal components with a silicone material. The two electrodes, which are made of gold studs, can be exposed by grinding. A coining step is followed to ensure the flatness of the two electrodes.