Microstructural evolution and mechanical behavior of line-type Ni/Sn3.0Ag0.5Cu/Ni interconnects with a small thickness during isothermal aging

The microstructural evolution and mechanical behavior of line-type Ni/Sn3.0Ag0.5Cu/Ni interconnects with a small thickness (or height) of 100 μm during isothermal aging were investigated. Microstructural analysis shows that both (Ni,Cu)₃Sn₄ and (Cu,Ni)₆Sn₅ IMC phases formed at the interface of the as-reflowed joints; however, after the aging treatment at 125 °C for 100 h or more, the interfacial (Cu,Ni)₆Sn₅ disappeared due to the limited supply of Cu in the small volume joints, and only (Ni,Cu)₃Sn₄ with both planar type near the Ni metallization and needle-like near the bulk solder existed at the interface of the thermally aged joints. Ag₃Sn particles finely dispersed in the bulk solder of the as-reflowed joints, and grew and coalesced during the subsequent aging treatment. The large Ag₃Sn particles may be main sites of stress concentration in the bulk solder and thus degrade the tensile strength of the aged solder joints. Results of DMA tensile test show that the tensile strength of the joints decreases rapidly after isothermal aging, but a prolonged aging time has slight influence on the strength of the joints. Fracture analysis indicates that both the as-reflowed and aged solder joints fractured in the bulk solder with ductile elongation and necking under tension, and the tensile strength of the solder joints is mainly determined by the bulk solder under the constraint effect of Ni metal substrate. Fracture happened after the formation, growth and coalescence of microvoids in the center of the solder joints.