| 摘要 | Interfacial reaction studies between eutectic Sn-In and several substrate metalizations have been carried out in an effort to further understand this Pb-free solder. Copper, Nickel, NiP and NiFe, with various Fe concentrations, were used as reflow substrates with Sn-In eutectic solder. In addition, extensive research was conducted on the effects of current stressing on Sn-In Solder/Cu and Ni interconnects.
Structural changes resulting from high-density electric currents were examined in a eutectic Sn-In/Cu interconnect. Under electrical loading, Sn and In migrated in opposite directions, creating a partition of the Sn- and In-rich phases between the anode and the cathode. At the anode, a net gain of Sn atoms resulted in the formation of massive, columnar hillocks on the surface, but a net loss of In led to dissolution and disappearance of the In-rich intermetallic compound layer. At the cathode, the exodus of Sn left valleys adjacent to the In-rich regions on the surface, while the amount of the In-rich phase grew, due to the net influx of In at the expense of the anode In-rich intermetallic layer.
Similar electromigration research was conducted on Sn-In/Ni interconnects. The effects of current stressing were very similar in that the partitioning of the phases took place as a result of electromigration. However, the IMC thickness was not altered or enhanced by the partitioning effect of the current. This indicated that the changes in the Sn-In/Cu interconnects were brought upon strictly by electromigration and not thermomigration. Additional annealing and kinetic experiments carried out on the Sn-In/Ni interconnect revealed the effects of temperature on the IMC of the system. IMC compositions and structural morphology were obtained using SEM and EDS analysis. A Ni(In,Sn)2 intermetallic phase formed upon reflow and grew exponentially with temperature and time. The rate controlling mechanisms, based on n-values obtained from experimental data, are discussed along with the activation energies for the reactions.
The growth kinetics and interfacial studies carried out on Sn-In/Cu, and NiP gave different results than those done on electroplated Ni substrates. The reaction rate was much faster on the Cu substrates while the NiP showed very slow reaction rates as well as growth behavior when reflowed with Sn-In eutectic solders. In some cases, two mechanisms contributed to the IMC growth, under solid-state conditions, dependant on the annealing temperature used. The activation energies for Cu and NiP were obtained and are 44 and 86 kJ mol-1, respectively.
Interfacial reactions between eutectic Sn-In solder alloy and Ni-10%Fe were examined by cross-sectional scanning and transmission electron microscopy (SEM/TEM). It was found that Fe addition to an electroplated Nickel substrate greatly alters the interfacial reactions and intermetallic compositions after reflow with SnIn solder. Upon reflow, two phases can be detected in the IMC, a Ni3Sn4 and a FeSn2 phase. Under solid state annealing conditions, the IMC grows at a very rapid rate in two continuous layers. The composition of these layers was found to be Fe15Ni23In17Sn44 adjacent to the substrate and Ni30In19Sn50 adjacent to the solder. The solid-state activation energy was also calculated for a pure Nickel/Sn-In IMC growth to be 114.74 kJ mol-1, and 43.97kJ mol-1, for the Ni10%Fe/SnIn IMC.
In a related study, interfacial interactions in a Ni(x)Fe-SnIn eutectic solder (x=30, 55 at%), have been examined. Transmission and scanning electron microscopy (TEM/SEM) were utilized to investigate the structure, composition, and morphology of the intermetallic compounds (IMC’s). Upon reflow, Ni3Sn4 and FeSn2 phases appeared at the interface along with Cu6Sn5 in the solder. Annealing experiments revealed the formation of a bi-layer IMC, Fe-rich adjacent to the NiFe metalization and Ni-rich on the solder side. Kinetic studies established the apparent activation energies for both systems to be 51.8 kJ mol-1 and 85.1 kJ mol-1, for 30 and 55%Fe contents, respectively. In the Fe-rich system, globular Ni3Sn4 crystals were formed upon reflow, but were changed into a cubic/faceted structure after annealing. |
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