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.