In electronic packaging, it is very important to understand and control the interfacial microstructure formed due to the reaction between the metallization scheme and solders. Often, the interfacial reaction is exploited/controlled to obtain an optimum properties of the joints. The strength and interfacial properties of the solder joints are determined by the interfacial microstructure. The evolution of interfacial microstructure in solder joints is governed by the diffusion path during processing and in service. Even though the semiconductor industry uses a wide variety of complex metallization schemes, recently the Pd-Ni metallization scheme on Cu has gained a lot of interest. Furthermore, the design of new metallization scheme(s) to control the interfacial microstructure and to improve the quality of solder joints is the cornerstone of advanced electronic packaging. We have carried out a systematic study of diffusion and interfacial reaction, both in the liquid and solid states, between Pb-Sn eutectic solder and the Pd-Ni metallization scheme on Cu.