Impurity and alloying effects on interfacial reaction layers in Pb-free soldering

摘要

The objective of this review is to study the effect of minor alloying and impurity elements, typicallypresent in electronics manufacturing environment, on the interfacial reactions between Sn and Cu,which is the base system for Pb-free soldering. Especially, the reasons leading to the observed interfacialreaction layers and their microstructural evolution are analysed. The following conclusions have beenreached. Alloying and impurity elements can have three major effects on the reactions between the Snbasedsolder and the conductor metal: Firstly, they can increase or decrease the reaction/growth rate.Secondly, additives can change the physical properties of the phases formed (in the case of Cu and Sn, eand h). Thirdly they can form additional reaction layers at the interface or they can displace the binaryphases that would normally appear and form other reaction products instead. Further, the alloying andimpurity elements can be divided roughly into two major categories: (i) elements (Ni, Au, Sb, In, Co, Pt,Pd, and Zn) that show marked solubility in the intermetallic compound (IMC) layer (generally take partin the interfacial reaction in question) and (ii) elements (Bi, Ag, Fe, Al, P, rare-earth elements, Ti and S)that are not extensively soluble in IMC layer (only change the activities of species taking part in theinterfacial reaction and do not usually participate themselves). The elements belonging to category (i)usually have the most pronounced effect on IMC formation. It is also shown that by adding appropriateamounts of certain alloying elements to Sn-based solder, it is possible to tailor the properties of theinterfacial compounds to exhibit, for example, better drop test reliability. Further, it is demonstrated thatif excess amount of the same alloying elements are used, drastic decrease in reliability can occur. Theanalysis for this behaviour is based on the so-called thermodynamic–kinetic method.