Electroplated Sn-Au structures for fabricating fluxless flip-chip Sn-rich solder joints

A fluxless bonding process in hydrogen environment based on newly introduced Sn-Au electroplated multilayer that is highly Sn-rich is presented. Electroplating method is an economical alternative to vacuum deposition method in many soldering applications that require thicker solder joints. Non-eutectic Sn-rich Sn-Au multilayer design with 94 at. % Sn and 6 at. % Au are employed. Microstructure and phase formation of electroplated Sn-Au thin films are investigated using X-ray diffraction method (XRD), Scanning Electron Microscope (SEM), and Energy Dispersive X-ray Spectroscopy (EDX). The joints produced are also examined using these techniques. It is found that the small Sn-Au intermetallic compound (AuSn₄) grains are uniformly distributed in Sri matrix. Some voids are identified by a Scanning Acoustic Microscope (SAM). The remelting temperature of the joints ranges in 217 ∼ 222°C. Highly Sn-rich Sn-Au solder bumps are fabricated by plating through thickness photoresist pattern to develop fluxless flip-chip bonding technique. Thicker bumps (over 50 μm) have been produced. This new fluxless bonding process is the first fluxless bonding technology that was successfully achieved by electroplating within author's knowledge.