Solder Size Effect on Early Stage Interfacial Intermetallic Compound Evolution in Wetting Reaction of Sn3.0Ag0.5Cu/ENEPIG Joints

Solder size effect on early stage interfacial intermetallic compound (IMC) evolution in wetting reaction between Sn-3.0Ag-0.5Cu solder balls and electroless nickel electroless palladium immersion gold (ENEPIG) pads at 250 °C was investigated. The interfacial IMCs transformed from initial needle- and rod-type (Cu,Ni)6Sn5 to dodecahedron-type (Cu,Ni)6Sn5 and then to needle-type (Ni,Cu)3Sn4 at the early interfacial reaction stage. Moreover, these IMC transformations occurred earlier in the smaller solder joints, where the decreasing rate of Cu concentration was faster due to the Cu consumption by the formation of interfacial (Cu,Ni)6Sn5. On thermodynamics, the decrease of Cu concentration in liquid solder changed the phase equilibrium at the interface and thus resulted in the evolution of interfacial IMCs; on kinetics, larger solder joints had sufficient Cu flux toward the interface to feed the (Cu,Ni)6Sn5 growth in contrast to smaller solder joints, thus resulted in the delayed IMC transformation and the formation of larger dodecahedron-type (Cu,Ni)6Sn5 grains. In smaller solders, no spalling but the consumption of (Cu,Ni)6Sn5 grains by the formation of (Ni,Cu)3Sn4 grains occurred where smaller discrete (Cu,Ni)6Sn5 grains formed at the interface.