Reactivity of No-Clean Flux Residues in Electronic Assemblies: A Systematic Study
Authors: Bruno Tolla, Ph.D., Denis Jean, Hemal Bhavsar, Yanrong Shi, Ph.D., Xiang Wei, Ph.D. Company: Kester Inc. Date Published: 9/27/2015
Abstract: The manufacturing process of electronic assemblies and semiconductor packages is considerably simplified by the adoption of no-clean fluxes. However, the transition from water soluble to no clean systems requires a thorough understanding of the reactivity of the unwashed flux residues in the final assembly. Case studies of in-field reliability failures are too application-specific in essence to shed some light on the complex physicochemical interactions at play. In this paper, we report a systematic study of various reliability failure modes associated with a chemically homogeneous series of fluxes tested in model conditions. Leveraging traditional reliability testing standards as well as in-house protocols using customdesigned boards, we demonstrated how the interplay between chemical packages, processing conditions and endusage environment is a far-more decisive parameter than the “Halogen-Free” label on a soldering flux. Electrochemical migration, corrosion and other chemically-driven failure modes were analyzed in terms of the chemical equilibria at play between the flux and its environment. From this perspective, progresses in the fundamental understanding of the reactivity of flux residues, combined with an end-user driven testing program of these processing by-products can significantly mitigate the risks associated with the use of noclean fluxes.