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Interfacial Degradation of Copper Wire Bond due to Growth of Cu9Al4 phase
Keywords: Interfacial Degradation, Copper Wire Bond, Wire Bonding
Copper (Cu) wire bonds have become the dominant wire material used in microelectronic packages, replacing gold (Au) in a majority of applications. Cost savings has been the key factor driving the transition to Cu wire bonding, although there are other advantages to Cu such as better electrical and thermal conductivity, slower intermetallic compound (IMC) formation and reduced wire sweep during transfer molding. Automotive, industrial and aerospace market segments, however, are still reluctant to adopt Cu wire bonded products due to inadequate understanding and modeling of reliability risks. Bond pad – ball bond interfacial failure is one of the risks for Cu wirebond on aluminum (Al) pad, due to interfacial IMC formation of CuAl2 and Cu9Al4 phases. Literature indicates failure in Cu-Al system typically occurs at the interface between Cu9Al4 and Cu. Another concern is that in shear testing of fresh wirebonds, very different behavior is seen when comparing Au bonds and Cu bonds on Al pads. The Au ball tends to shear apart but Cu does not, causing the pad Al to shear apart instead. Exposing Cu wirebonds to high temperature causes the detrimental IMC phases to nucleate and grow in thickness. IMC growth kinetics are initially interface controlled as the CuAl2 phase grows, then becomes diffusion controlled as thickness increases. At a critical CuAl2 thickness, diffusion through this IMC is slower than the interfacial reaction time, and Cu9Al4 begins to nucleate between CuAl2 and the Cu interface, which will then lead to bond failure. This presentation reviews a model for this phenomenon, and elaborates on the methods to find out important factors related to wire bonding that affect model accuracy.
Stevan Hunter,
ON Semiconductor
Phoenix, AZ

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