Evolution in Lead-Free Solders

Microstructural Evolution and Interfacial Interactions in Lead-Free Solder Interconnects 

This chapter reviews Uteratme on the microstructure of solder joints and the interactions of solders with substrates and related solder joint reliability issues. The substrates here are limited to Cu, Ni-coated Cu, electroless nickel/ immersion gold (ENIG), and hot air solder leveled (HASL) Sn-Pb. The solders are mainly Sn, eutectic or near eutectic Sn-Ag, and Sn-Ag-Cu alloys. Specific reliability issues discussed here include black pads of ENIG, gold embrittlement, compatibility of Pb-free solders with Pb-containing surface finish, and Kirkendall voids. 

The phase diagrams described previously tell us only the phases formed during the equilibrium solidification. The microstructure, defined here as the size, shape, and distribution of the solid phases, is determined by the kinetics, i.e., the nucleation and growth mechanisms during solidification. 

Growth. The growth mechanisms of eutectic Sn-Pb and eutectic Sn-Ag-Cu are also different. In the eutectic solidification process (Ref 5), the eutectic alloys are divided into two types normal and anomalous 63Sn-37Pb is a normal eutectic, while the Pb-free eutectic alloys (eutectic Sn-Ag, Sn-Cu, and Sn-Ag-Cu) are anomalous. 

Normal alloys are associated with regular microstructures of a lamellar or rod form. Dining their solidification process, the constituent phases of a normal eutectic alloy grow into the liquid phase in a coupled or coordinated mechanism, which usually requires that the growth rates of the constituent phases are the same, and their growth mechanisms are non-faceting. For a normal binary eutectic alloy, two metals usually have similar melting points, and the two constituent phases have the same volume fraction. 

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