SnAg Microbumps for MEMS-Based 3-D Stacks

Fine pitch 100-mum electroplated SnAg microbump interconnection technology is presented and discussed for use in microelectromechanical systems (MEMS) based 3-D stacks. Electrochemical deposition (ECD) of copper top side metallization (TSM) is compared to performance of nickel-copper TSM on substrate chips. Nickel-copper was selected for under bump metallization (UBM) of die chips. Lead free Sn3.5%Ag was deposited on the die chip UBM and chip-to-wafer bonded by a standard SnAg reflow process in inert atmosphere. An automotive application module was selected as target application for investigating reliability and failure mechanism of the interconnection technology. Bonded units have been investigated by mechanical and physical analysis, visual inspection and electrical resistance measurements after assembly and subsequent environmental stress test including thermal cycling, elevated temperature, humidity, and high current. The fine pitch lead free microbumps displayed promising capability for 3-D stacking of silicon devices. Excellent performance under thermal cycling with no global thermal mismatch was demonstrated. The microbump interconnection technology proved to be tolerant to high temperature and extensive current exposure. TSM consumption and intermetallic compound (IMC) formation were less evident for the nickel-copper TSMs compared to those entirely made of copper. Only minor Kirkendall porosity was observed at the solder alloy interfaces in the present study.