Wetting kinetics and interfacial reaction of lead-based and lead-free solder alloys on copper and nickel metallizations in electronic packaging

In this research, which was motivated by technological and environmental concerns, we have conducted a systematic study of the wetting behavior and the interfacial reaction of lead (Pb)-based solders and Pb-free solders on metal surfaces: Cu sheet, Ni sheet, and Cu thin-films.; In the wetting experiment, it is shown that the wetting tip and wetting interface are not in static equilibrium due to the reaction. The wetting angle remains constant after the initial spreading, but eutectic SnPb solder forms a side band at the wetting tip. The side band formation at the wetting tip is characteristic of the eutectic SnPb solder, not necessary in other solders. In the reactive wetting, such as solder reaction, the free energy change by compound formation must be considered besides the surface energy balance.; In the cases of eutectic SnPb, eutectic SnBi, and eutectic SnAg on Cu, their interaction leads to Cu Sn intermetallic compound formation, but not compounds of Cu-Pb, Cu-Bi, and Cu-Ag. The Cu-Sn compounds do not form layered structures, rather the {dollar}rm Cusb6Snsb5{dollar} phase grow as scalloplike grains. The scalloplike grains grow larger but fewer with time, indicating that a ripening reaction has occurred among them. The ripening is not a constant volume process since it is accompanied by the interfacial reaction. A kinetic model of the compound growth at the interface of solder/Cu is presented, and good agreement is obtained between the values from the model and the experiment. The consumption rate of base metal (Cu) is measured by determining the change of the total volume of Cu-Sn compound as a function of time and temperature of reflow. On the other hand, in the system of solder/Ni, we observe that the {dollar}rm Nisb3Snsb4{dollar} phase forms at the solder/Ni interface and it grows very slowly compared to that in the solder/Cu system.; In the solder reaction on thin-film metallization, the spalling of the Cu-Sn compounds at the solder interface is an important issue of reliability. We have found that the spalling of compound grains on the thin film starts after the solder reaction has consumed all the Cu, and is assisted by the ripening among the grains. This spalling causes the dewetting of the solder on the exposed Cr or Ti underlayer.; In phasing Cr and Cu, the crystallites of Cr and Cu are intermixed. Since the Sn reacts only with Cu in the Cr-Cu phased structure, the intermetallics of Cu-Sn become locked into the Cr so that they will not separate easily. Although the phased Cr-Cu film structure has good resistance to spalling in the reflow with 95Pb-5Sn solder, we have observed spalling in the reaction of high-Sn solder on the phased structure due to its high Sn content. In addition, we have observed an asymmetric spalling phenomenon using a sandwich structure, in which two wafers were soldered face-to-face. The spalling tends to occurs predominantly at the bottom interface of the solder joint under the influence of gravity.