Mechanism to improve the reliability of copper wire bonding with palladium-coating of the wire. (August 2019)

• Record Type: Journal Article
• Authors: Qin, Wentao; Anderson, Tom; Chang, George
• Is Part Of: Microelectronics and reliability. Volume 99(2019)
• Page Start: 239
• Page End: 244
• Publication Date: 2019-08
• Subjects: Reliability -- Corrosion resistance -- Copper wire bonding -- Palladium coating -- Packaging -- Semiconductor -- Electronics -- Phase stability; Electronic apparatus and appliances -- Reliability -- Periodicals; Miniature electronic equipment -- Periodicals; Appareils électroniques -- Fiabilité -- Périodiques; Équipement électronique miniaturisé -- Périodiques; Electronic apparatus and appliances -- Reliability; Miniature electronic equipment; Periodicals; 621.3815
• DOI: 10.1016/j.microrel.2019.04.010
• Languages: English
• ISSNs: 0026-2714

Abstract

Abstract: Pd-coating of Cu (PCC) bond wires has been widely employed to improve the reliability of Cu wire bonding technology for microelectronics. Description of the underlying mechanism, however, is lacking in literature. We provide an interpretation of how the improvement is induced. After an un-biased highly accelerated stress test (uHAST), the most Cu-rich intermetallics (IMC) were Cu3 Al2 under the bare Cu bond wire, and predominantly (CuPdx )Al under the PCC wire. The Pd-coating therefore had suppressed the formation of (CuPdx )3 Al2 . The suppression stemmed from the improvement of the thermodynamic stability of (CuPdx )Al, which also decreased the kinetics of phase transformation toward (CuPdx )3 Al2 . The cathode/anode area ratio of (CuPdx )Al is lower than that of Cu3 Al2 . The corrosion rate of (CuPdx )Al therefore becomes lower, and the reliability of the wire bonding increases. Ni behaves thermodynamically quite similar to Pd in the ternary system of Cu wire bonding, and therefore possesses the potential to similarly improve the corrosion resistance. Furthermore, the use of Al wires on Al alloy pads will implement a monometallic system across the bonding interface which can also enhance the corrosion resistance. The underlying correlation between the atomic ratio of (Inert Element)/(Active Element) of the IMC and the corrosion rate of the IMC revealed here is pertinent to the research in other fields including the corrosion of alloys/dealloying and nanoporosity, and the making of functional materials with nanoporosity for applications such as catalysis and advanced energy technologies. Graphical abstract: Unlabelled Image Highlights: The global semiconductor industry chose Pd-coating as a main stream method, the second alternative to Au wires to improve the reliability of Cu wire bonding. This choice was made based on hypotheses, experiments, and subsequently practice. A description of the mechanism, nonetheless, has been lacking. In this paper we provide an interpretation of the mechanism. We correlated the atomic ratio of (the more inert element)/(the more active element) of the IMC with the cathode/anode area ratio, and further, the corrosion rate of the IMC. We concluded that the corrosion rate of the IMC can be reduced by suppressing the formation of the IMC that is rich in the inert element (Cu3 Al2 in the paper). We identified the thermodynamic behavior of Ni in the Cu wire bonding system to be similar to that of Pd in the same system. As a result, the potential of reliability improvement by Ni-coating of the bond wire, which is cheaper than Pd-coating, is revealed.