Study Of Morphological Evolution Of Cu₆Sn₅ Formed In Reaction Between Sn3.5Ag Solder And Cu

In this thesis, the relationship between the morphology of intermetallic compound Cu₆Sn₅ formed at the interface between Cu pad and Sn3.5Ag solder and reflow temperature, reflow time, the crystal orientation at the surface of copper substrate, aging time, and aging temperature is studied. The crystal orientation of copper substrate surface is changed by heating copper substrate at high temperature. The morphology and the composition of the interface compound are investigated by optical microscope, scanning electric microscope, energy dispersive spectrometer and X-ray diffraction, as well as the effects of reflow temperature, reflow time, aging temperature, and aging time on the morphology of intermetallic compound. The mechanism of the function of these parameters is analyzed.The result of reflow test of interfacial compounds Cu₆Sn₅ formed on the interface between copper substrate and Sn3.5Ag solder shows that at different temperature, Cu₆Sn₅ compound grows in the form of scallop-like grains. Then with the increase of reflow time: at 240oC, small flat surface grows on surface of scallop-like grains, but the small plane is not obvious elongated. Instead, it grows up together with the scallop-shaped part of a grain; at 240oC to 260oC, significant elongation of the small plane is observed; at 300oC, scallop-like grains are quickly growing into lodging prisms. With increasing time, the scallop-shaped bottom of a grain grows up apparently and the tip of prism-shaped part points into a vertical direction from a horizontal direction. Aging tests on the intermetallic compound formed at the interface between Cu and Sn3.5Ag show that with the increase of aging time, the interface compound gradually grows into polyhedron morphology. During the aging process, Ag3Sn particles coarsen significantly.The growth dynamics of the interfacial compounds shows that in the reflow process, with increasing reflow temperature, the Jackson factor value of closely spaced surface of the compound gradually increases and the type of the corresponding atomic surface is also changed from the rough-type interface to smooth interface, leading to the morphological change of Cu₆Sn₅ compound at the interface. During the aging process, because the interfacial energy between interface compound and solid solder has the tendency to lower level from higher one, the morphology of intermetallic grains changes from scallop-shaped or prism-shaped to polyhedral shape. Also, during the aging process, the compound growth mainly relies on the diffusion of copper atoms, leading to the planar transition of the compound layer.The morphological evolution of interfacial intermetallic compound is concluded through this thesis. The principle and mechanism of the morphological evolution can be explained by view of thermodynamics and crystallographic.