The interfacial bilayer Cu6Sn5 formed in a Sn-Ag-Cu flip-chip solder joint incorporating Au/Pd metallization during solid-state aging

A solid-state reaction between the Sn-Ag-Cu solder and Ni metallization resulted in the formation of interfacial bilayer Cu6Sn5-based intermetallic compounds (IMCs) in a solder joint incorporating Au/Pd metallization. The layer near the Ni metallization was identified as (Cu,Ni,Au)(6)Sn-5 containing 20.3 at.% of Ni and 1.7 at.% of Au, and the layer near the solder matrix was identified as (Cu,Au,Ni,Pd)(6)Sn-5 containing 5.7 at.% of Au, 1.2 at.% of Ni, and 1.0 at.% of Pd. The electron diffraction analysis with high resolution transmission electron microscopy further characterized the interfacial bilayer IMCs as having the same hexagonal crystal structure with different crystal orientations. The kinetics study revealed that the (Cu,Ni,Au)(6)Sn-5 in the interfacial bilayer reaction products was formed during the initial reflow process, while the (Cu,Au,Ni,Pd)(6)Sn-5 layer was formed during subsequent solid-state aging. Predominant growth of the (Cu,Au,Ni,Pd)(6)Sn-5 layer was observed during solid-state aging. In contrast, the growth of the initially formed (Cu,Ni,Au)(6)Sn-5 layer was suppressed. The growth of the (Cu,Au,Ni,Pd)(6)Sn-5 layer was governed by the dissolution of the (Au,Pd)Sn-4 IMC that occurred during solid-state aging. The complete dissolution of (Au,Pd)Sn-4 resulted in a reactant-limited chemical reaction from the conversion of (Au,Pd)Sn-4 to (Cu,Au,Ni,Pd)(6)Sn-5. The development of the interfacial bilayer IMCs in the solder joint incorporating Au/Pd metallization suppressed excessive IMC growth and unfavorable phase transformation during long-term solid-state aging. The detailed mechanism of the formation of the interfacial bilayer Cu6Sn5-based IMCs was investigated in this paper.