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...

Lead-Free Solder Research SummaryAs a result of extensive studies, nearternary-eutectic Sn-Ag-Cu (SAC) alloys have been identifi ed as the leading lead-free solder candidates to replace lead-bearing solders for ball-grid array module assembly. However, recent studies revealed several potential reliability risk factors associated with the alloy system. The formation of large Ag 3 Sn plates in solder joints, especially when solidifi ed at a relatively slow cooling rate, poses a reliability concern. In this study, the effect of adding a minor amount of zinc in SAC alloy was investigated. The minor zinc addition was shown to reduce the amount of undercooling during solidifi cation and thereby suppress the formation of large Ag 3 Sn plates. In addition, the zinc was found to cause changes in both the microstructure and interfacial reaction of the solder joint. The interaction of zinc with other alloying elements in the solder was also investigated for a better understanding of the role of zinc during solidifi cation of the nearternary-eutectic alloys.

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.

Recently, the research and development activities for replacing Pb-containing solders with Pb-free solders have been intensified due to both competitive market pressures and environmental issues. As a result of these activities, a few promising candidate solder alloys have been identified, mainly, Sn-based alloys. A key issue affecting the integrity and reliability of solder joints is the interfacial reactions between a molten solder and surface finishes in the solder joint structures. In this paper, a fundamental study of the interfacial reactions between several Pb-free candidate solders and surface finishes commonly used in printed-circuit cards is reported. The Pb-free solders investigated include Sn-3.5 Ag, Sn-3.8 Ag-0.7 Cu, and Sn-3.5 Ag-3.0 Bi. The surface finishes investigated include Cu, Au/Ni(P), Au/Pd/Ni(P), and Au/Ni (electroplated). The reaction kinetics of the dissolution of surface finishes and intermetallic compound growth have been measured as a function of reflow temperature and time. The intermetallic compounds formed during reflow reactions have been identified by SEM with energy dispersive x-ray spectroscopy.Index Terms-Dissolution, electroless Ni(P), interfacial reactions, intermetallics, morphology, Pb-free solders, Sn-3.5Ag, Sn-3.5Ag-3Bi, Sn-3.8Ag-0.7Cu.