Interfacial microstructure and the kinetics of interfacial reaction in diffusion couples between Sn-Pb solder and Cu/Ni/Pd metallization

The interfacial microstructure and the kinetics of interfacial reaction bet ween eutectic Sn-Pb solder and electroplated Ni/Pd on a Cu substrate have b een studied by scanning, transmission and analytical electron microscopies. Besides PdSn4 and Ni3Sn4, small grains of Ni3Sn2 with a hexagonal structur e are also observed after long-time aging of the diffusion couples at 125 d egrees C. The presence of intermetallic phases is correlated with the diffu sion paths in the calculated Fd-Pb-Sn and Ni-Pb-Sn isothermal sections. The growth kinetics of the Ni,Sn, scallops in the submicrometer length scale w as analyzed with an Arrhenius type of equation. The thickening kinetics yie lds a time exponent n=3.1 and an apparent activation energy (Q(h)) of 25,75 0 J/mol, while the radial growth kinetics data yield a time exponent m appr oximate to 6.6 and an apparent activation energy (Q(d)) of 15,300 J/mol. Th e radial size distributions (RSDs) of Ni,Sn, scallops were also quantified. The parameters describing RSDs are consistent with the theories of coarsen ing in two-phase systems containing a very high volume fraction of the seco nd phase. Selective etching of solder revealed the three-dimensional morpho logy of PdSn4 and Ni3Sn4, and also the dynamical phenomena, such as facetin g, competitive growth and coalescence of Ni,Sn, scallops during. interfacia l reaction, Non-parabolic growth kinetics is discussed in terms of the exis ting theories and characteristics of the evolving microstructure. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserv ed.