Growth below solidus temperature

The formation of intermetallic compounds in solder joints during both reflow and subsequent annealing below the solidus temperature is also discussed. The classic Pb-Sn/metallization systems serve as a reference point [4—13]. In this context, the case of thermal cycling of solder joints of Cu/Ni/Au/Pb-Sn and the formation of Au-Ni-Sn intermetallic compounds at metallization interfaces is considered in detail [5,41-62]. The fast diffusion of Au controls the formation of Au-Ni-Sn alloys in this system [54—56]. In fact, some fundamental observations of the diffusion characteristics in Sn provide useful generalizations for all Sn-based solder systems, including Pb-free solders. Many Pb-free solders have near-eutectic compositions of Sn and various elements, often in ternary compositions. Those elements are often in relatively low concentrations, so the reactions between pure Sn and various metallizations are discussed as a basis for comparison. The formation of intermetalhc compounds at Pb-free solder/metaUization interfaces is discussed, and an attempt is made to understand the basic mechanisms of the growth processes and their relation to the reliability of interconnections. 

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