MICROSTRUCTURE AND CHEMICAL COMPOSITION OF (Cu+5at Ni)/Sn-Ag-Sn/(Cu+5at Ni) INTERCONNECTIONS

摘要

Comprehensive scanning (SEM) and transmission (TEM) electron microscopy studies of the (Cu+5at% Ni)/Sn-Ag-Sn/(Cu+5at% Ni) diffusion-soldered interconnecttions were performed. Sn and Ag are frequently used as substitutes of Pb-Sn eutectic alloys in electronic devices. The addition of the Ni to the substrate leads to the acceleration of intermetallic phase growth accompanied with the shape change of the IC. On the other hand, the thin (5 μm) Ag interlayer is applied in order to avoid formation of Kirkendall's voids in the centre of the Cu(Ni)/Sn/Cu(Ni) joints. The microstructure and chemical composition characterisation of the structural components after soldering were carried out using the scanning electron microscope (Fei XL30 E-SEM) equipped with an energy-dispersive X-ray spectrometer (Edax Gemini 4000). An intensive study of phenomena occurring mainly at the interfaces between the different phases, were accomplished on the transmission electron microscope Tecnai G2 FEG super Twin (200 kV) equipped with the high angle angular dark field (HAADF) detector and integrated with an energy-dispersive X-ray spectroscopy (EDS) system manufactured by Edax. The thin foils were cut out from the carefully selected regions of the interconnection using a Quanta 3D focused ion beam. The electron microscopy techniques with particular emphasis on the EDS and selected area electron diffraction allowed to reveal the presence of two intermetallic compounds ((Cu_(1-x)Ni_x)_6Sn_5 and Ag_3Sn) in the whole reaction zone for the various soldering times (2, 5, 10, 15 and 20 minutes) and temperatures (240 - 260 ℃). The (Cu_(1-x)Ni_x)_6Sn_5 phase has grown in the whole reaction zone. Additionally, the Ag_3Sn was detected in the centre of interconnection in the form of the relatively large, rounded grains. Furthermore, the EDS analysis (point to point and mapping) made across the substrate/IC interface showed the presence of layer significantly enriched in nickel.