Practically all microelectronic assemblies in use today utilize Sn-Pb eutectic solder for interconnection. Emerging environmental regulations worldwide have targeted the elimination of Pb usage in electronic assemblies, due to the inherent toxicity of Pb. This has made the search for suitable Pb-free solders an important issue for microelectronics assembly. This book describes some properties of Sn-9Zn and Sn-8Zn-3Bi lead-free solders interacting with copper substrate. Some information are given on the wetting behavior, the interfacial reaction, mechanical properties and growth kinetics of the solders on Cu substrate. The wetting reaction between Sn-Zn solder and Cu substrate is due the interaction between Zn and Cu atoms. This interaction leads to the formation of a flat Cu5Zn8 and scallop CuZn intermediate phases. The book also explains the degradation of the solder joint strength due to Zn-depletion zone and Kirkendall voids. Finally, The evolution of the Cu5Zn8 intermetallic and the activation energy for the growth of the intermetallic under solid state aging is explained in depth.