Effect of Soldering Sequences on Cu-Ni interaction in flip-chip interconnects

The effect of sequential soldering and initial Cu concentration in solders on the interfacial reaction, especially the morphology, growth of IMC and Cu consumption, were investigated by using 300-μm (diameter) various composition of solder balls and substrates with a 250-μm (diameter) opening of OSP and ENIG pads. Two paths were employed. For path I, a solder ball was first joined to the Cu substrate and then to the Ni substrate. For path II, the sequence was the opposite. Main IMCs found at the interfaces on both sides were Cu
₆
Sn
₅
and (Cu,Ni)
₆
Sn
₅
for interconnects via path I, while for path II, IMCs on Ni side affected the interfacial reaction on Cu side with a novel (Cu,Ni)
₆
Sn
₅
bedded IMCs morphology. A mechanism was introduced to illustrate the phenomenon. In addition, the thickness of IMCs and Cu consumption depended on soldering sequences and initial Cu concentration in solders.