New under bump metallization (UBM) is required for future microelectronics to minimize reliability concerns with solder interconnects. In the present work, FeNiP thin films as potential UBM were prepared by electroless deposition and their solderability and interfacial reactions with lead-free solders were investigated. New complexing system was designed in order to reduce the difference in the electrode potential between Ni2+ and Fe2+. The iron concentrations of deposited films were improved by complexing with EDTA-2Na and DTPA. FeNiP films were obtained on the surfaces of copper and silicon wafer. The iron content of the electroless deposits was controllable from 0 to 97 at.% by systematically controlling bath temperature, concentration of EDTA-2Na and mole ratio of Fe2+/Ni2+ in the plating bath. The solderability of FeNiP films with Sn and eutectic SnAgCu solders was investigated by the static wetting tests and the wetting balance measurements. Both the final contact angles and the maximum wetting force of as-plated FeNiP films were better than the widely used NiP UBM, although the wetting time was longer in some instances. The as-plated FeNiP platings showed better solderability as the iron content of the film increased. Furthermore, electroless FeNiP had better wetting property than electrodeposite FeNi at the same Fe concentration, especially after annealing at high temperatures. The improved wetting property of electroless FeNiP is believed to be related to a porous oxide layer which was readily removed by the flux. The interfacial microstructures of FeNiP/Sn couples were investigated after reflowing for different times. It was found that the interfacial reactions depended strongly on the Fe percentage in the electroless FeNiP alloys: FeSn2 single layer was observed at the interface in a Fe-rich alloys at 55 and 75 at.%Fe, while both FeSn2 and Ni3Sn4 layers were found at the interface at a low Fe concentration of 25 at.%. The preferential status of FeSn2 phase at the interface was believed to result from the lower Gibbs free energy for FeSn2 formation than that of Ni3Sn4 in the Sn/FeNiP system. Moreover, the growth rate of interfacial intermetallic compounds and the amount of intermetallic whiskers grown on the FeNiP coatings decreased with the increase of Fe percentage. It was much lower than that of Ni3Sn4 phase in NiP/Sn couple. The solid-state interfacial reaction of Sn/xFeNiP and SnAgCu/xFeNiP couples were investigated systemically. During solid-state aging with Sn solder, the interfacial microstructures changed according to Ni3Sn4 single phase--Ni3Sn4 and FeSn2 coexisting--FeSn2 single phase with the increase of Fe content in FeNiP alloys in the direction 0--25--55-85at.%. The activation energy for the growth of FeSn2 phase in Sn/55FeNiP and Sn/85FeNiP couples were 46.6kJ/mol and 93.8kJ/mol, respectively. These are much larger than 27.2kJ/mol of Ni3Sn4 phase in Sn/NiP couple. Higher activation energy means slower growth of IMC. Compared with electrodeposited 55FeNi, IMC growth was obviously slower in 55FeNiP/Sn couple. For xFeNiP/SnAgCu systems, double reaction phases formed at the interface: FeSn2 phase which faced the FeNiP substrate and Cu6Sn5 phase which adhered to SnAgCu solder. The morphology of Cu6Sn5 phase changed from layer to particle and reduced gradually with the Fe contents of FeNiP films increasing from 25 to 85at.%. Both in xFeNiP/Sn and xFeNiP/SnAgCu couples, no P-rich layer was observed at the interfaces after aging process for all the three systems. Application of electrodeposited Fe film and electroless FeP film for UBM on solder joint was discussed. Experimental results showed that wettability of as-plated Fe and FeP with Sn and SnAgCu solders were better than that of electroless NiP. Compared with Fe, FeP showed better wetting property before and after aging at 180℃ because of its looser oxide layer. An extremely thin layer of FeSn2 formed at the interfaces for Sn/Fe, Sn/FeP, SnAgCu/Fe and SnAgCu/FeP couples after reflowing. After aging for Sn/Fe and Sn/FeP couples, the IMC formed at the interfaces was still FeSn2 single layer. A few Cu6Sn5 and Ag3Sn were distributed on the surface of FeSn2 layer when SnAgCu/Fe and SnAgCu/FeP couples were aged. Moreover, IMC growth for FeP UBM was slower than on Fe UBM. The activation energy for the growth of FeSn2 phase in Sn/Fe and Sn/FeP couples were determined to be 88.1kJ/mol and 106.2kJ/mol, respectively, which are much larger than that of Ni3Sn4 in Sn/NiP.
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