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Real-time wetting dynamics and interfacial chemistry in low-melting 57Bi–42Sn–1Ag solder paste on Ni–Au

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1 Author(s)
Bozack, M.J. ; Department of Physics, Center for Advanced Vehicle Electronics (CAVE), Auburn University, Auburn, Alabama 36849

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We report the observation of real-time, in situ, wetting and spreading dynamics for 57Bi–42Sn–1Ag solder paste on Ni–Au surfaces during melting in a scanning electron microscope. The 57Bi–42Sn–1Ag is a low melting (139 °C) Pb-free eutectic alloy currently under consideration by automobile manufacturers for use in instrument displays. We find that, while there is excellent wetting of 57Bi–42Sn–1Ag solder paste on Ni–Au, there is almost no spreading. A large amount of Bi segregates to the surface of 57Bi–42Sn–1Ag solder balls during the sintering process. At melting, excessive flux outgassing and pooling are observed, several melted solder balls float on top of the flux, and substantial elemental segregation occurs during the first minutes of wetting. Neither Ni nor Au fully intermixes throughout the alloy at the interface within seconds of wetting. Bi does not move outward with the expanding alloy front. This combination of detrimental effects forms voids in the solder paste, contributes to low reliability of solder joints, and complicates the materials science at the solder-substrate interface as shown by Auger electron spectroscopy. Reliability work in progress (3000 cycles) shows that 57Bi–42Sn–1Ag on Ni–Au is less reliable than eutectic Sn–37Pb on Ni–Au for 2512 chip resistors cycled from -40 to 125 °C.

Published in:

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films  (Volume:22 ,  Issue: 6 )