Cu aggregation behavior on interfacial reaction of Sn-3.0Ag-0.5Cu/ENIG solder joints.

• The direct evidence of Cu aggregation from ENIG substrate to interface is given. • Contribution of solder and substrate to Cu aggregation at the interface was investigated; • Drive force of Cu aggregation in thermodynamic view is clarified. Cu aggregation of Sn-3.0Ag-0.5Cu /electroless nickel-immersion gold (ENIG) solder joints generally affect the formation of intermetallic compounds (IMCs) at the interfacial region. Herein, the interfacial microstructure of Sn-3.0Ag-0.5Cu/ENIG solder joint is compared with the joints obtained with Cu-free solder or substrate to reveal the Cu contributor and its metallurgical behavior. Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) were used to observe the IMCs and elemental distribution, respectively. The Gibbs free energy of the Sn-Ni-Cu system was calculated by JMatPro software. The results showed that Cu had significant aggregation in the interface. The solder is the major contributor to Cu aggregation in the interface, while the ENIG substrate also makes a minor contribution. The Cu 6 Sn 5 is then formed at the interface because of its lowest Gibbs free energy in the system. These results are of significance for subsequently controlling the interfacial microstructure and performance of joints. [ABSTRACT FROM AUTHOR]