Study on short time interfacial reactions between Sn-3.0Ag-0.5Cu solder balls and ENEPIG pads

Short time interfacial reactions between Sn-3Ag-0.5Cu solder balls and ENEPIG pads at 250 °C were investigated. Transitions in morphology and type of the interfacial intermetallic compounds (IMCs) with reaction time were revealed. It was found that the transition rate of the interfacial IMCs in smaller solder joints was faster than that of larger ones. For 200 μm solder joint, dodecahedron type (Cu,Ni)₆Sn₅ grains together with brick type (Ni,Cu)₃Sn₄ particles first formed at the interface. Then (Ni,Cu)₃Sn₄ grew up rapidly with (Cu,Ni)₆Sn₅ particles submerged and even disappeared in the weed shape (Ni,Cu)₃Sn₄. For 300 μm solder joint, hexagonal prism (Cu,Ni)₆Sn₅ particles with pointed tips firstly formed at the interface. Then the particles quickly transformed to polyhedron type IMC within the following few seconds. Thereafter, with the interfacial reaction kept going, (Ni,Cu)₃Sn₄ needles grew up from the interspaces of (Cu,Ni)₆Sn₅ particles. It is convinced that the change of Cu concentration in the solder balls is the main reason causing the IMCs transformations.