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Near-ternary eutectic Sn-Ag-Cu alloys are leading candidates for Pb-free solders. These alloys have three solid phases: ␤-Sn, Ag 3 Sn, and Cu 6 Sn 5 . Starting from the fully liquid state in solidifying near-eutectic Sn-Ag-Cu alloys, the equilibrium eutectic transformation is kinetically inhibited. The Ag 3 Sn phase nucleates with minimal undercooling, but the ␤-Sn phase requires a typical undercooling of 15 to 30°C for nucleation. Because of this disparity in the required undercooling for nucleation, large, platelike Ag 3 Sn structures can grow rapidly within the liquid phase, before the final solidification of the solder joints. At lower cooling rates, the large Ag 3 Sn plates can subtend the entire cross section of solder joints and can significantly influence the mechanical deformation behavior of the solder joints under thermomechanical fatigue conditions. In this paper, it is demonstrated that the Ag 3 Sn plate formation can be inhibited, an important factor in assuring the reliability of solder joints composed of these alloys.The electronics industry will make substantial progress toward a full transition to Pb-free soldering technology in the near future. At present the leading candidate alloys are near-ternary eutectic Sn-Ag-Cu alloys. The ternary-eutectic composition is now thought to be close to the composition Sn-3.5Ag-0.9Cu 1,2 with a melting point of 217°C. The near-eutectic, commercially available alloys are exemplified by the Sn-3.8Ag-0.7Cu and Sn-4.0Ag-0.5Cu compositions. The electronics industry has begun to study both the processing behaviors and the thermomechanical fatigue properties of these alloys in detail to understand their applicability in the context of current electronic card reliability requirements. 3 The near-eutectic ternary Sn-Ag-Cu alloys yield three phases upon solidification: ␤-Sn, Ag 3 Sn, and Cu 6 Sn 5 . Attempts to characterize the solidification behavior and define the ternary eutectic composition in the Sn-Ag-Cu system have been reported by several investigators. 1,2,4 It has been previously reported 5,6 that relatively large Ag 3 Sn plates can form during solidification of the nearternary eutectic Sn-Ag-Cu alloys. Using differential thermal analysis (DTA) methods, 1 it was reported that Ag 3 Sn plate nucleation and ensuing growth may occur with minimal undercooling. The ␤-Sn phase required significantly larger undercoolings to induce nucleation and bring about final solidification. 1,4 The physical requirement for large undercoolings to promote the solidification of Sn-bearing solders has been previously reported. 7,8 These findings on the nucleation behavior of Sn-based solder systems are similar to that of the present study. The results of our work are consistent with the nucleation of the Ag 3 Sn phase with minimal undercooling in the Sn-Ag-Cu system. Under manufacturing process conditions, solder joints, composed of Sn-3.8Ag-0.7Cu alloy, required 15 to 30°C undercooling for the nucleation of the ␤-Sn phase. This determination was made by implanting thermocouples in b...

During the solidification of solder joints composed of near-eutectic Sn–Ag–Cu alloys, the Sn phase grows rapidly with a dendritic growth morphology, characterized by copious branching. Notwithstanding the complicated Sn growth topology, the Sn phase demonstrates single crystallographic orientations over large regions. Typical solder ball grid array joints, 900 μm in diameter, are composed of 1 to perhaps 12 different Sn crystallographic domains (Sn grains). When such solder joints are submitted to cyclic thermomechanical strains, the solder joint fatigue process is characterized by the recrystallization of the Sn phase in the higher deformation regions with the production of a much smaller grain size. Grain boundary sliding and diffusion in these recrystallized regions then leads to extensive grain boundary damage and results in fatigue crack initiation and growth along the recrystallized Sn grain boundaries.

a b cNear-ternary eutectic Sn-Ag-Cu alloys are leading lead-free candidate solders for various applications. These alloys yield three phases upon solidifi cation: β-Sn, Ag 3 Sn, and Cu 6 Sn 5 . Large, plate-like, pro-eutectic Ag 3 Sn structures can grow rapidly within the liquid phase, potentially adversely affecting the mechanical behavior and reducing the fatigue life of solder joints. This article reports on the formation of such plates in Sn-Ag-Cu solder balls and joints and demonstrates how large Ag 3 Sn plate formation can be minimized.

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