Fracture Mechanisms in Sn-Ag-Cu Solder Micro-Bumps for 3D Microelectronic Packages

Solder micro-bumps, which serve as interconnects between stacked dies in 3D electronic packages, are typically very thin (25μm or less) and contain a high proportion of intermetallic compounds (IMCs). This makes micro-bumps brittle, and prone to fracture, particularly under drop conditions. In this paper, the fracture mechanics and mechanisms of SAC305 solder micro-bumps attached to Cu metallizations are studied as functions of the proportion of IMC in the joint. The effects of IMC content, IMC composition (i.e., the relative amounts of Cu₃Sn and Cu₆Sn₅), strain rate, and mode mixity on both fracture toughness and fracture mechanism were investigated. The fracture mechanisms were studied via quantitative fractography. It was found that fracture toughness decreases dramatically (∼80%) with increase in IMC content (from ∼22% to 100%IMC). In all samples, the majority of the fracture surface comprised IMC/Sn interface failure, which constitutes the predominant fracture mechanism. With increasing aging, the proportion of ductile fracture of Sn decreases, fracture through Cu₆Sn₅ cleavage increases, and more Cu₃Sn/Cu₆Sn₅ interface failure is observed. In general, bulk fracture properties of Cu₃Sn plays little role in the fracture process. Thin joints also frequently display an alternating crack path between the two solder-substrate interfaces with the crack transitioning from one interface to the other, through the intervening Sn. Such crack transition between two surfaces increases with increase in IMC content, as long as there is intervening Sn, and may be utilized to improve the fracture toughness of thin joints. Finally, joint fracture toughness decreased with increase in both strain rate and mode mixity.