Reliability of nickel flip chip bumps with a tin–silver encapsulation on a copper/tin–silver substrate during the bonding process

This study focused on the feasibility of using Ni flip chip bumps with a Sn–2.5Ag (wt.%) solder encapsulation. The interfacial reaction and die shear property of the Ni flip chip bump with a Sn–Ag solder cap bonded on the electroplated Cu/Sn–Ag substrate were investigated with increasing bonding time. After bonding for 1 s (CuxNi1−x)6Sn5 and Cu6Sn5 intermetallic compound (IMC) layers were formed at the upper and lower interfaces, respectively, with the former IMC being the predominant phase during bonding. The transformation of the solder cap into the (CuxNi1−x)6Sn5 IMC depleted the solder after bonding for 30 s, and then the Ni concentration in the IMC gradually decreased with increasing bonding time. The shear property peaked after 30 s, and then decreased with increasing bonding time. The fractures occurred at the solder/Cu6Sn5 interface and inside the (Cux Ni1−x)6Sn5 IMC after bonding for 1 s and 30 s, respectively, after which the fracture location shifted toward the (NixCu1−x)3Sn4/(Cux Ni1−x)6Sn5 interface with increasing bonding time.