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Intermetallic-layer formation and growth in Pb-free solder joints, during solder reflow or subsequent aging, has a significant effect on the thermal and mechanical behavior of solder joints. In this study, the influence of initial intermetallic morphology on growth rate, and kinetics were examined in a Sn-3.5Ag solder reflowed on Cu. The initial morphology of the intermetallic was tailered by cooling in water, air, or furnace conditions. Solder aging was conducted at 100°C, 140°C, and 175°C and aged for 0-1,000 h. Cooling rate, aging temperature, and aging time played an important role on microstructure evolution and growth kinetics of Cu 6 Sn 5 (η) and Cu 3 Sn (ε) intermetallic layers. Prior to aging, faster cooling rates resulted in a relatively planar Cu 6 Sn 5 layer, while a nodular Cu 6 Sn 5 morphology was present for slower cooling. Intermetallic-growth rate measurements after aging at various times, indicated a mixed growth mechanism of grain-boundary and bulk diffusion. These mechanisms are discussed in terms of the initial intermetallic thickness and morphology controlled by cooling rate, diffusion kinetics, and the competition between Cu 6 Sn 5 and Cu 3 Sn growth.

The effect of cooling rate on microstructure and creep behavior of bulk, eutectic Sn-3.5Ag solders was studied. The cooling rate is an important processing variable that significantly affects the microstructure of the solder and therefore determines its mechanical behavior. Controlled cooling rates were obtained by cooling specimens in different media: water, air, and furnace, which resulted in cooling rates of 24°C/s, 0.5°C/s, and 0.08°C/s, respectively. The cooling rate decreased the secondary dendrite arm size and the spacing of the Sn-rich phase, as well as the morphology of Ag 3 Sn. The Sn-dendrite arm size and spacing were smaller at fast cooling rates, while slower cooling rates yielded a nearly eutectic microstructure. The morphology of Ag 3 Sn also changed from relatively spherical, at faster cooling rates, to needlelike for slower cooling. The effect of cooling rate on creep behavior was studied at 25°C, 60°C, 95°C, and 120°C. Faster cooling rates were found to increase the creep strength of the solder due to the refinement of the solder microstructure. Stress exponents, n, indicated that dislocation climb was the controlling mechanism. Activation energies, for all cooling rates, indicated that the dominant diffusional mechanism corresponded to dislocation pipe diffusion of Sn. Grain boundary sliding (GBS) measurements were conducted, using both scanning electron microscopy (SEM) and atomic force microscopy (AFM). It was observed that GBS had a very small contribution to the total creep strain.

The mechanical behavior of Sn-rich solder/Cu joints is highly sensitive to processing variables such as solder reflow time, cooling rate, and subsequent thermal aging. In this article, we focus on the lap shear behavior of Sn-3.5Ag/Cu joints as a function of solder yield strength and intermetallic thickness. Experimental results showed that the shear strength of the solder joints is primarily controlled by the mechanical properties of the solder, and not the intermetallic thickness. The thickness of intermetallic, however, controlled the fracture mode of the solder joints. At intermetallic thicknesses greater than 20 m, brittle fracture between Cu 6 Sn 5 and Cu 3 Sn was the most common failure mechanism. Finite-element simulations were carried out to evaluate the effect of solder properties and of intermetallic thickness and morphology on lap shear behavior. The finite-element simulations corroborated the experimental findings, i.e., that increased solder strength results in increased joint strength. The simulations also showed that thicker intermetallics, especially of nodular morphology, yielded higher local plastic shear strain and work hardening rate.