Characterization technique in phase distribution during sample preparation for applications to solder joints and wire-bonding chips

Characterization of solder joint microstructure is essential in evaluation of microelectronic packaging reliability. Analysis techniques and sample preparation play important roles in the material characterization. This study demonstrates several novel approaches in the microstructure evaluation of solder joints. By utilizing precision etching and coating techniques, the interface of the IC packaging chip between the solder and under-bump metallization (UBM) could be revealed much more precisely and the intermetallic compounds were distinctly visible in details by field-emission scanning electron microscopy (FE-SEM). At the Sn-Pb/Ni/Cu interface, the intermetallic compounds defined as Ni₃Sn₄ and Cu₆Sn₅ by electron probe microanalysis (EPMA) were formed between the nickel and the solder. Meanwhile, when the Au-Al wire bonding assembly was etched by the ion beam technique, the interface between gold and aluminum as well as the intermetallic compound could be clearly identified. By adjusting the operating variables, such as etching energy, etching period, and incident angle between the ion beam and sample, the time for sample preparation was significantly reduced. For example, when the cross-section surface of the Sn-Pb solder specimen was perpendicular to the incident ion beam, the highest quality SEM micrographs were obtained at 2.5 keV with an etching time of 7 min. and at 4.5 keV with an etching time of 3 min.