Abstract: 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.

Key words: Size effect, Lead-free solder, Interfacial reaction, Intermetallic compound, Sn-3.0Ag-0.5Cu, Electroless nickel electroless palladium immersion gold (ENEPIG) pad