Intermetallic compound formation and morphology evolution in the 95Pb5Sn flip-chip solder joint with Ti/Cu/Ni under bump metallization during reflow soldering

Abstract

Intermetallic compound formation and morphology evolution in the 95Pb5Sn flip-chip solder joint with the Ti/Cu/Ni under bump metallization (UBM) during 350°C reflow for durations ranging from 50 sec to 1440 min were investigated. A thin intermetallic layer of only 0.4 µm thickness was formed at the 95Pb5Sn/UBM interface after reflow for 5 min. When the reflow was extended to 20 min, the intermetallic layer grew thicker and the phase identification revealed the intermetallic layer comprised two phases, (Ni,Cu)3Sn2 and (Ni,Cu)3Sn4. The detection of the Cu content in the intermetallic compounds indicated that the Cu atoms had diffused through the Ni layer and took part in the intermetallic compound formation. With increasing reflow time, the (Ni,Cu)3Sn4 phase grew at a faster rate than that of the (Ni,Cu)3Sn2 phase. Meanwhile, irregular growth of the (Ni,Cu)3Sn4 phase was observed and voids formed at the (Ni,Cu)3Sn2/Ni interface. After reflow for 60 min, the (Ni,Cu)3Sn2 phase disappeared and the (Ni,Cu)3Sn4 phase spalled off the NI layer in the form of a continuous layer. The gap between the (Ni,Cu)3Sn4 layer and the Ni layer was filled with lead. A possible mechanism for the growth, disappearance, and spalling of the intermetallic compounds at the 95Pb5Sn/UBM interface was proposed.