Intermetallic Compounds in Solder Alloys: Common Misconceptions

Abstract

Intermetallic compounds (IMC) or intermediate phases are formed between two or more metallic elements in many metal alloy systems. During soldering, an IMC is formed at the soldered interface as the molten solder reacts with an element in the substrate. IMCs also can form within the bulk solder as the joint solidifies. IMCs have critical roles in the solder joint quality and reliability. Unlike most metal alloys, an intermetallic compound typically has a fixed stoichiometry and is in variance with the conventional phases or constituents in the metal system (e.g., alpha and beta). IMCs have a different crystal structure than any of its constituents and some but never all the characteristics and properties of its constituents. Ductility is an important solder joint property, and the low intrinsic ductility of IMCs is associated with brittle behavior and reliability risk in service. However, a review of published solder field failures shows little evidence that IMC properties or IMC evolution under service conditions reduce solder joint reliability. Most IMC-induced solder joint failures are found to result from incorrect material specification or uncontrolled soldering processes. This paper describes the IMCs that occur typically in eutectic, Sn63Pb37 solder and near-eutectic SAC305 or other tin-based Pb-free solder alloys, including how they impact solder joint reliability. The paper also describes the potential impact of IMCs on the solder joint reliability for the newest generation of Pb-free high-performance solder alloys.