The electronic packaging industry has been using electroless Ni͑P͒ / immersion Au as bonding pads for solder joints. Because of the persistence of the black pad defect, which is due to cracks in the pad surface, the industry is looking for a replacement of the Ni͑P͒ plating. Several Cu-based candidates have been suggested, but most of them will lead to the direct contact of solder with Cu in soldering. The fast reaction of solder with Cu, especially during solid state aging, may be a concern for the solder joint reliability if the package will be used in a high temperature environment and is highly stressed. In this work, the reaction of eutectic SnPb solder with electrodeposited laminate Cu is studied. Emphasis is given to the evolution of the microstructure in the interfacial region during solid state aging and its effect on solder joint reliability. A large number of Kirkendall voids were observed at the interface between Cu 3 Sn and Cu. The void formation resulted in weak bonding between solder and Cu and led to brittle fracture at the interface in the ball shear and pull tests. The experimental results indicate that a barrier for Cu diffusion may be needed between the solder and the type of Cu used in the test vehicle for the packages that will experience high temperature ͑Ͼ100°C͒ and high stress.

Constitutive models for SnAgCu solder alloys are of great interest at the present. Commonly, constitutive models that have been successfully used in the past for Sn-Pb solders are used to describe the behavior of SnAgCu solder alloys. Two issues in the modeling of lead-free solders demand careful attention: 1) Lead-free solders show significantly different creep strain evolution with time, stress and temperature, and the assumption of evolution to steady state creep nearly instantaneously may not be valid in SnAgCu alloys and 2) Models derived from bulk sample test data may not be reliable when predicting deformation behavior at the solder interconnection level for lead-free solders due to the differences in the inherent microstructures at these different scales. In addition, the building of valid constitutive models from test data derived from tests on solder joints must de-convolute the effects of joint geometry and its influence on stress heterogeneity. Such issues have often received insufficient attention in prior constitutive modeling efforts. In this study all of the above issues are addressed in developing constitutive models of Sn3.8Ag0.7Cu and Sn1.0Ag0.5Cu solder alloys, which represent the extremes of Ag composition that have been mooted at the present time. The results of monotonic testing are reported for strain rates ranging from 4.02E-6 to 2.40E-3 s 1 . The creep behavior at stress levels ranging from 7.8 to 52 MPa is also described. Both types of tests were performed at temperatures of 25 C, 75 C and 125 C. The popular Anand model and the classical time-hardening creep model are fit to the data, and the experimentally obtained model parameters are reported. The test data are compared against other reported data in the literature and conclusions are drawn on the plausible sources of error in the data reported in the prior literature.

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The interconversion relations for viscoelastic functions are derived with the consideration of the time-dependent bulk modulus, K(t), for both traditional and fractional Prony series representations of viscoelasticity. The application of these relations is to replace the fitting parameters of Young’s relaxation modulus, E(t), by the unknown parameters of K(t) and the known parameters of the shear relaxation modulus, G(t), and to fit the E(t) to the experimental data for obtaining the parameters of K(t). The fitting results show that only two experiments for measuring the viscoelastic functions of an isotropic material are not enough to determine the other viscoelastic functions. However, if we consider the relaxation rates of K(t) and G(t), we may conclude that the constant bulk modulus is a more reasonable assumption, and the corresponding Poisson’s ratio, ν(t), is a monotonic-increasing function.

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The electromigration behavior of the Cu/Au/SnAgCu/Cu combination was investigated under 103 A/cm2 of current stressing at ambient temperature. The Au layer, when it acts as a cathode, was consumed continuously, and no significant compound was found at the interface. Meanwhile, Cu6Sn5 was formed at the anodic Cu layer, and the thickness of the compound increased with increasing time. The Au atoms were found to be trapped in Cu6Sn5 within the solder matrix. The AuSn4 compound precipitated while attaching to Cu6Sn5 at the Cu6Sn5/solder interface. The thermomigration effect was found to be insignificant in this work as no obvious reaction occurred at the cathode/anode sides or in the solder matrix without current stressing.