Dissolution and interface reactions between the 95.5Sn-3.9Ag-0.6Cu, 99.3Sn-0.7Cu, and 63Sn-37Pb solders on silver base m
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I. INTRODUCTION
SOLDERING is used to join a variety of base materials besides the traditional copper (Cu) and Cu-based alloys. The development of a lead (Pb)-free soldering technology, whether for structural or electronics applications, must address those alternative materials that include silver (Ag). Soldering to bulk Ag and Ag-based alloys is required in the assembly of relay and other switching components. Silver coatings, including the new immersion Ag coatings, are used in numerous electronic components.[1,2] Whether Ag serves as a base material or as a finish, an understanding of its dissolution by molten solder and the associated metallurgical reactions that occur at the solder/Ag interface is critical for establishing assembly processes for a wide range of products. Several literature studies have examined the dissolution of base metals and alloys by molten solders. By and large, those investigations addressed the more commonly used Cu and Ni alloys. There have been a few studies of Ag dissolution. Bader examined Ag dissolution by molten Sn-Pb solder.[3] Klein-Wassink combined Bader’s data with his own results to describe the dissolution rates of Ag and several other metals in molten 60Sn-40Pb solder.[4] Assuming a linear time dependence, Klein-Wassink calculated apparent activation energies in the range of 48 to 50 kJ/mol for the base metals: Cu, Au, Ag, Pd, Pt, and Ni. We obtained satisfactory correlations between the dissolution rates of Cu, Au, Ag, Pd, Pt, and Ni and their respective melting temperatures and heats of fusion.[5] On the other hand, a poor correlation was observed between dissolution rate and boiling temperature. Therefore, the dissolution mechanism was likely determined by atomic movements characteristic of a melting process rather than vaporization. This is an important distinction when trying to develop computational materials models for dissolution phenomena.[6] PAUL T. VIANCO, Principal Member of the Technical Staff, ROBERT D. WRIGHT, Technologist, and PAUL F. HLAVA, Member of the Technical Staff, are with Sandia National Laboratories, Albuquerque, NM 87123. Contact e-mail: [email protected] JOSEPH J. MARTIN, Contractor, is with Orion Inc., Albuquerque, NM. Manuscript submitted September 12, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS
Cheng et al. examined the dissolution of the Ag thick film layer in molten 51In-49Sn solder.[7] The Ag thick film has a very porous structure. We performed a multivariable, linear regression analysis on the data from that report. A linear time dependence was calculated (tn; n 5 1.1 6 0.3) that was similar to that assumed by Klein-Wassink. However, the apparent activation energy of 24 6 6 kJ/mol was lower than the 48 to 50 kJ/mol range cited in the KleinWassink analysis. The lower apparent activation energy was possibly caused by either the porous structure of the Ag thick film or the different solder composition. A rigorous study was needed to address the dissolution rate kinetics and interface microstructure between molten, Sn-based, Pb-free solders
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