An Implantable Medical Device for Transcorneal Electrical Stimulation: Packaging Structure, Process Flow, and Toxicology Test

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, and 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. The surfaces of the gold stud bumps are in general nonuniform. Coining is an essential pretreatment of compressing the gold stud bumps against a flat solid surface to obtain much smoother and larger surfaces. This pretreatment helps to form high-strength joints between the gold stud bumps and the PCB pads. In the packaging process, an 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 then 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. Finally, dummy samples were made to test the biocompatibility of the device. The test results show that the packaging process can produce a biocompatible and well-sealed device.