Interfaces in lead-free soldering

Abstract

Structural integrity of circuits is greatly dependent on interfacial microstructure. In this paper, the status of the current understanding of various interfaces appearing in lead-free soldering is reviewed, and recent data on interfaces in electronic interconnections, primarily analyzed by transmission electron microscopy (TEM), is presented. The compound Cu₆Sn₅ is formed, as localized precipitates attach to the interface of a Cu substrate with Sn plating, even in an as-received condition. After long-time exposure at room temperature, it grows into a Cu₆Sn₅ layer along the interface. When the temperature is raised slightly or Sn in a plating layer is consumed by the reaction, a Cu₃Sn layer can grow between a Cu₆Sn₅ layer and a Cu substrate. In soldering, most Sn alloys involving pure Sn, Sn-Ag, or their ternary alloys form two intermetallic compounds, e.g., Cu₆Sn₅ and Cu₃Sn, on a Cu substrate, with the former much thicker than the latter. The Ni plating forms Ni₃Sn₄/Ni₃Sn₂ double layers at the interface with Sn alloys in soldering with the latter layer very much thinner. In contrast, Fe-42Ni alloy forms (Fe,Ni)Sn₂ double layers by the reaction with Sn and Sn-Ag(-Cu). When Zn becomes one of the elements of the solder, Zn first reacts with a substrate. Thus, the Sn-Zn alloy forms different intermetallic compounds at an interface with Cu, i.e., the CuZn/Cu₅Zn₈ double layers. The Sn-Zn alloy also forms a thin AuZn layer when thin Au plating is on a substrate.