Timothy A. Gosselin

Ag 3 Sn plate formation in the solidification of near ternary eutectic Sn–Ag–Cu alloys

Near-ternary eutectic Sn–Ag–Cu alloys are leading candidates for Pb-free solders. These alloys have three solid phases: β–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 β–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 microstructure of Sn in near-eutectic Sn–Ag–Cu alloy solder joints and its role in thermomechanical fatigue

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.

Interfacial reaction studies on lead (Pb)-free solder alloys

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.

Evaluation of thermal fatigue life and failure mechanisms of Sn-Ag-Cu solder joints with reduced Ag contents

The electronic industry is making substantial progress toward a full transition to Pb-free soldering in the near future. At present, the leading candidate Pb-free solders are near-ternary eutectic Sn-Ag-Cu alloys. The electronic industry has begun to study both the processing behaviors and the thermomechanical fatigue properties of these alloys in detail in order to understand their applicability in context of current electronic card reliability requirements. In recent publications, the solidification behavior of the near-ternary eutectic Sn-Ag-Cu alloys has been reported in terms of the formation of large Ag/sub 3/Sn plates and their effects on mechanical properties of Pb-free solder joints. It was also demonstrated that reducing Ag content in the near-ternary eutectic Sn-Ag-Cu alloys was very effective in controlling the formation of large Ag/sub 3/Sn plates and thereby reducing the reliability risk factor of solder joints. In this study, thermal fatigue behavior of CBGA (ceramic ball grid array) solder joints was investigated in terms of Ag content, cooling rate, and thermal cycling conditions. Extensive failure analysis was conducted with thermal-cycled solder joints to understand the failure mechanisms operating during the accelerated thermal cycling (ATC) tests.

Interfacial reactions, microstructure and mechanical properties of Pb-free solder joints in PBGA laminates

Sn-based alloys have been developed as Pb-free solder candidates to replace the Pb-containing solders used in microelectronic applications. However, their high Sn content and high melting point often cause excessive interfacial reactions, namely, dissolution of surface finish layers and concomitant formation of intermetallic compounds at the soldering interface. These interfacial reactions can therefore influence the microstructure and mechanical properties of the solder joints and eventually their reliability. The choice of a proper surface finish layer in printed circuit boards is an important issue in successfully introducing the Sn-based, Pb-free solders. The effects of surface finish layers and multiple reflows on the BGA solder joints have been investigated. A Pb-free solder alloy, Sn-Ag-Cu has been employed as the solder ball material. Five types of surface finish on opposite sides of the BGA balls, have been investigated. Intermetallic compound formation was measured as a function of reflow cycle. The effects of the interfacial reactions on the microstructure and mechanical properties of the solder joints were also investigated as a function of surface finish and reflow cycle.