We have examined the kinetics of the growth of Cu/Sn intermetallics at composite solderlCu substrate interfaces. The composite solders consist of a eutectic SnlPb matrix plus additions of Cu, Cu, Sn, , Cu,Sn or Ni particles. Samples were annealed a t 1 10 to 1 60 OC for up to 6 4 days. The interfaces were examined using scanning electron microscopy, x-ray energy dispersive spectroscopy, cross-sectional transmission electron microscopy, electron diffraction, and in situ TEM observations of intermetallic formation.The activation energies measured for Cu,Sn, and Cu,Sn formation with the eutectic solder alone are in good agreement with previously published values. The Cu-containing particles increase the activation energy for Cu,Sn, formation and reduce the activation energy for Cu,Sn.It is proposed that the particles: (1) act as Snsinks and (2) reduce the cross-sectional area of Sn diffusion.Both effects reduce the amount of Sn available at the interface for intermetallic formation.
The formation and growth of Cu6Sn5 and Cu3Sn at the interface of Sn-Pb solder/copper substrate are factors which affect the solderability and reliability of electronic solder joints. The addition of particles such as Ni to eutectic Sn-Pb solder drastically affects the activation energies of formation for both intermetallics. This study was performed to understand the mechanisms of intermetallic formation and the effects of Ni on intermetallic growth. Cu/Sn and Cu/Sn/Ni thin films were deposited by evaporation and observed in the TEM in real time using a hot stage. The diffusion of Sn through Cu6Sn5 and Cu3Sn followed by reaction with Cu must occur for intermetallic formation and growth to take place. Ni is an effective diffusion barrier which prevents Sn from diffusing into Cu.
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