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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.

During service, microcracks form inside solder joints, making microelectronic packages highly prone to failure on dropping. Hence, the fracture behavior of solder joints under drop conditions at high strain rates and under mixed-mode conditions is a critically important design consideration for robust joints. This study reports on the effects of joint processing and loading conditions on the microstructure and fracture response of Sn-3.8%Ag-0.7%Cu (SAC387) solder joints attached to Cu substrates. The impact of parameters which control the microstructure (reflow condition, aging) as well as loading conditions (strain rate and loading angle) are explicitly studied. A methodology based on the calculation of the critical energy release rate, G C , using compact mixed-mode (CMM) samples was developed to quantify the fracture toughness of the joints under conditions of adhesive (i.e., interface-related) fracture. In general, higher strain rate and increased mode-mixity resulted in decreased G C . G C also decreased with increasing dwell time at reflow temperature, which produced a thicker intermetallic layer at the solder-substrate interface. Softer solders, produced by slower cooling following reflow, or post-reflow aging, showed enhanced G C . The sensitivity of the fracture toughness to all of the aforementioned parameters reduced with an increase in the mode-mixity. Fracture mechanisms, elucidating the effects of the loading conditions and process parameters, are briefly highlighted.

Sn-rich solders have been shown to have superior mechanical properties when compared to the Pb-Sn system. Much work remains to be done in developing these materials for electronic packaging. In this paper, we report on the microstructure and mechanical properties of La-containing Sn-3.9Ag-0.7Cu alloys.The addition of small amounts of La (up to 0.5 wt.%) to Sn-Ag-Cu refined the microstructure by decreasing the length and spacing of the Sn dendrites and decreased the thickness of the Cu 6 Sn 5 intermetallic layer at the Cu/solder interface. As a result of the change in the microstructure, Sn-Ag-Cu alloys with La additions exhibited a small decrease in ultimate shear strength but significantly higher elongations compared with Sn-Ag-Cu. The influence of LaSn 3 intermetallics on microstructural refinement and damage evolution in these novel solders is discussed. Our results have profound implications for improving the mechanical shock resistance of Pb-free solders.

Microstructure plays a critical role in the mechanical behavior of Sn-rich solder alloys. A unified mechanistic understanding of creep in Sn-rich solder alloys, at various microstructural length scales, is missing. Part I of this study focused on microstructure characterization of Sn-rich solder alloys. Part II focuses on the creep behavior of bulk solder alloys and small solder joints comparable in size and geometry to those in electronic packages. Pure Sn, Sn-0.7Cu, Sn-3.5Ag, and Sn-3.9Ag-0.7Cu were used in these studies. At the bulk level, creep appears to be controlled by subgrain formation, whose size is controlled by Ag 3 Sn orCu 6 Sn 5 particles. At the smaller joint level, where the microstructures are much finer, creep in Ag-containing alloys was controlled by local climb and detachment along Ag 3 Sn particles. The threshold stress for creep is inversely proportional to the Ag 3 Sn interparticle spacing. At the joint level, the creep of Sn and Sn-Cu alloys was controlled by viscous flow at grain boundaries at low stress, and transition to dislocation climb at higher stress.