The electronic packaging industry has been using electroless Ni͑P͒ / immersion Au as bonding pads for solder joints. Because of the persistence of the black pad defect, which is due to cracks in the pad surface, the industry is looking for a replacement of the Ni͑P͒ plating. Several Cu-based candidates have been suggested, but most of them will lead to the direct contact of solder with Cu in soldering. The fast reaction of solder with Cu, especially during solid state aging, may be a concern for the solder joint reliability if the package will be used in a high temperature environment and is highly stressed. In this work, the reaction of eutectic SnPb solder with electrodeposited laminate Cu is studied. Emphasis is given to the evolution of the microstructure in the interfacial region during solid state aging and its effect on solder joint reliability. A large number of Kirkendall voids were observed at the interface between Cu 3 Sn and Cu. The void formation resulted in weak bonding between solder and Cu and led to brittle fracture at the interface in the ball shear and pull tests. The experimental results indicate that a barrier for Cu diffusion may be needed between the solder and the type of Cu used in the test vehicle for the packages that will experience high temperature ͑Ͼ100°C͒ and high stress.
Intermetallic compound (IMC) growth during solid-state aging at 125, 150, and 170 °C up to 1500 h for four solder alloys (eutectic SnPb, Sn–3.5Ag, Sn–3.8Ag–0.7Cu, and Sn–0.7Cu) on Cu under bump metallization was investigated. The samples were reflowed before aging. During the reflow, the solders were in the molten state and the formation of the IMC Cu6Sn5 in the cases of eutectic SnPb and Sn–3.5Ag had a round scallop-type morphology, but in Sn–0.7Cu and Sn–3.8Ag–0.7Cu the scallops of Cu6Sn5 were faceted. In solid-state aging, all these scallops changed to a layered-type morphology. In addition to the layered Cu6Sn5, the IMC Cu3Sn also grew as a layer and was as thick as the Cu6Sn5. The activation energy of intermetallic growth in solid-state aging is 0.94 eV for eutectic SnPb and about 1.05 eV for the Pb-free solders. The rate of intermetallic growth in solid-state aging is about 4 orders of magnitude slower than that during reflow. Ternary phase diagrams of Sn–Pb–Cu and Sn–Ag–Cu are used to discuss the reactions. These diagrams predict the first phase of IMC formation in the wetting reaction and the other phases formed in solid-state aging. Yet, the morphological change and the large difference in growth rates between the wetting reaction and solid-state aging cannot be predicted.
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