Electromigration-induced failure of Ni/Sn3.0Ag0.5Cu/ENEPIG flip chip solder joint

The failure mechanisms of Ni/Sn3.0Ag0.5Cu/ENEPIG flip chip solder joint were investigated during EM at 150°C under a current density of 1×10
⁴
A/cm
²
. In as-soldered state, (Cu,Ni)
₆
Sn
₅
IMCs formed at the both Ni/solder and electroless Ni-P/solder interfaces, and the spalled (Cu
_0.58
Ni
_0.42
)
₆
Sn
₅
was observed in the solder at the chip side. After aging for 600 h, the interfacial IMCs at the both Ni/solder and electroless Ni-P/solder interfaces transformed from the initial (Cu,Ni)
₆
Sn
₅
into (Ni, Cu)
₃
Sn
₄
. The composition of spalled IMC was nearly unchanged. In the solder, (Au,Pd,Ni)Sn
₄
phases coarsened during aging. During EM, the failure mechanisms could be classified to two types. When electrons flowed from the chip to the PCB, at the chip side (the cathode) the current crowding effect induced the serious localized dissolutions of interfacial Cu-Ni-Sn IMC, Ni UBM and Cu trace at the electron-entry corner. The solder joint would fail due to that the complete dissolution of Cu trace induced an open circuit at the electron-entry point. While when electrons flowed from the PCB to the chip, at the PCB side (the cathode) EM induced the consumption of electroless Ni-P to form Ni
₃
P and Ni
₂
SnP. After the complete consumption of electroless Ni-P, the poor attachment between Ni
₃
P and Cu pad induced a crack across the cathode interface, and then the failure of the solder joint occurred. Furthermore, during EM (Au,Pd,Ni)Sn
₄
phase tended to deposit at the anode interface and in the solder near the anode interface.