Solder Wetting and Spreading

Abstract

During the Second World War, a situation occurred—in retrospect rife with irony given the current subject of this book—when wartime demands for tin (Sn) led to an effort to develop Snfree solder alloys. The results of this effort were not entirely successful and produced lead (Pb)rich alloys that exhibited poor wetting properties in comparison to their Sn–Pb predecessors that were Sn-rich. A deep appreciation developed for the excellent wetting characteristics with which Sn endows many solder alloys. Now, some 60 years later, a markedly different imperative has served to initiate an effort to identify a set of candidate Pb-free solder alloys capable of replacing the existing Pb–Sn alloy set. This effort is spurred by health concerns over Pb in the waste stream and by economic concerns that thosewith Pb-freemicroelectronics products would attain an edge in the market place based on the environmentally benign character of their products. The Pb–Sn system is one of the most extensively characterized low-melting-point metal systems because of its longstanding success in soldering of microelectronic components. This alloy system has a long history based on its adaptability to increasingly sophisticated soldering processes and demanding soldering applications. Current industry processes use the Pb–Sn eutectic alloy (63Sn–37Pb) and higher Pb content alloys (e.g., 90Pb–10Sn, 97Pb–3Sn, etc.) extensively. The popularity of the Pb–Sn system grew out of its many desirable material properties [e.g., low melting point, limited dissolution of copper (Cu) substrates, etc.]. Recently, a variety of Pb-free alternative alloys have been investigated (e.g., Sn–Ag, Sn–Ag–Cu, Sn–Cu, Sn–In, Sn–Sb, Sn–Bi, and Sn–Zn) [1]. There is a general consensus that a single-alloy system may not have sufficient versatility to serve as a basis for drop-in replacements for the present Sn–Pb alloys, particularly eutectic Sn–Pb. However, one characteristic of the new Pb-free solder formulations that has emerged with some measure of certainty is that of high-Sn compositions.