Wetting Behavior of Solders

Abstract Lead bearing solders have been used extensively in the assembly of modern electronic circuits. However, increasing environmental and health concerns about the toxicity of lead has led to the development of lead-free solders. Wetting of solders on surfaces is a complex and important phenomenon that affects the interfacial microstructure and hence the reliability of a solder joint. The solder material reacts with a small amount of the base metal and wets the metal by intermetallic compound (IMC) formation. The degree and rate of wetting are the two important parameters that characterize the wetting phenomenon. Contact angle is a measure of the degree of wetting or wettability of a surface by a liquid. Spreading kinetics in a given system is strongly affected by the experimental conditions. In reactive systems like soldering, wetting and chemical interfacial reactions are interrelated, and hence for successful modeling, it is essential to assess the effect of interfacial reactions on kinetics of wetting. Solder wetting necessarily involves the metallurgical reactions between the filler metal and the base metal. This interaction at the solder/base metal interface results in the formation of IMCs. During soldering an additional driving force besides the imbalance in interfacial energies originates from the interfacial reactions. The formation of IMC has significant influence on contact angle. The presence of IMCs (thin, continuous, and uniform layer) between solders and substrate metals is an essential requirement for good bonding. Optimum thickness of an IMC layer offers better wettability and an excellent solder joint reliability. However, due to their inherent brittle nature and tendency to generate structural defects, a too thick IMC layer at the interface may degrade the joint. In this paper, the factors affecting the wetting behavior of solders and evolution of interfacial microstructure are reviewed and discussed.