Effect of nickel metallization thickness on microstructure evolution and mechanical properties in Sn3.0Ag0.5Cu/Au/Ni/Cu solder joints

The effect of nickel metallization thickness on microstructure evolution and mechanical properties in Sn3.0Ag0.5Cu/Au/Ni/Cu solder joints was investigated here. First, a single-layer Cu–Ni–Au–Ag–Sn IMC formed at the interface after reflow and then differentiated into double layers during solid-state aging in both 3-μm nickel metallization (S1) and 6-μm nickel metallization (S2) samples. The interfacial IMC which closes to solder was recorded as (Cu, Au, Ni)₆Sn₅ and the other which closes to Ni metallization was recorded as (Cu, Ni, Au)₆Sn₅. The (Cu, Au, Ni)₆Sn₅ IMC kept growing in S1 during solid-state aging, whereas it experienced a negative growth among 500 h to 1000 h in S2. The growth trend of (Cu, Ni, Au)₆Sn₅ is contrary to (Cu, Au, Ni)₆Sn₅ and it decreased firstly then increased in both S1 and S2. Second, S1 exhibits higher shear force and lower frequency of interface fracture than S2, due to a thinner (Cu, Ni, Au)₆Sn₅ layer in S1. (Cu, Ni, Au)₆Sn₅ is more critical than (Cu, Au, Ni)₆Sn₅ on the failure process of solder joint. The negative growth of (Cu, Ni, Au)₆Sn₅ corresponds to the rising of shear forces and the interface of (Cu, Ni, Au)₆Sn₅/Ni is the weakest position in solder joints. Finally, the fracture behavior of AuSn₄ was investigated in detail. AuSn₄ with lamellar shaped dispersed in solder joint after reflow. It connected with the interfacial IMC and this structure can transfer fracture from inside of the solder ball to the interface. It resulted in decreasing of shear force and interfacial fracture. After 250 h of solid-state aging, AuSn₄ became crumb shaped dispersing in solder joint and did not transfer fracture.