System integration of the utah electrode array using a biocompatible flip chip under bump metallization scheme

The advent of micro and nanotechnologies along with integrated circuit technologies has led to many exciting solutions in medical field. One of the major applications of microsystems is microelectrodes interfacing neurons for large scale in vivo sensing, deep brain stimulation and recording. For biomedical microsystems, material selection is a challenge because biocompatibility has to be considered for implantable electronic devices. We are using flip chip bonding to integrate a signal processing IC to the Utah electrode array (UEA). Conventionally the flip chip process is used to bond a die to a substrate or interposer. In this work the electrical interconnects are made from the under bump metallization (UBM) on the UEA to the solder bumps on the IC. The UBM selection and reliability is one of the critical issues in the total reliability of a flip chip bumping and interconnection technology. The UBM was optimized to achieve improved interconnect strength, and its reliability was evaluated by conducting solder ball shear strength testing. The UBM reliability was tested with two solder metallurgies including AuSn and SnCu0.7. These solders are needed to allow two reflow processes to be used, an initial higher temperature (350°C) and a second lower temperature process (250°C).