Diffusion soldering is the new joining technology which ensure both the high thermal stability of the obtained interconnections and the relatively low joining temperature. It is possible because of the presence of one or more intermetallic phases (IPs) in the joint area. They are formed during isothermal solidification reaction and subsequent solid state diffusion between the joined substrates and the interlayer of solder material. The last one material is the metal or alloy with the low melting temperature which also is environmental friendly (lead-free solder).
The investigation was focused on the microchemical and microstructural description of the Cu/In-Bi 22at.%/Cu interconnections obtained at the temperatures ranging from 85 °C to 150 °C. As a solder material the indium-bismuth alloy was applied because of its very low melting temperature (only 72 °C). The use of the scanning electron microscope equipped with an energy-dispersive X-ray spectrometer allowed to characterize the IPs appearing in the interconnection. Moreover, the mechanical properties of the phases were inspected on the Nano Identer XP instrument.
The first phase, BiIn was formed due to liquid - solid reaction between the In-Bi solder and the copper substrates. After 15 minutes of annealing at the 125 °C the second phase, Cu11In9 in the solid-solid reaction appeared. It is characterized by the melting temperature of 307 °C. The morphology of this phase observed at the higher temperatures (125, 150 °C) took the shape of scallops while in lower temperatures (85, 100 °C) it formed rather homogenous layer.
The load-displacement data provided the information about the hardness and the elastic-plastic deformation behaviour of the phases during nanoidentation tests. The indenter displacement in the BiIn was accommodated plastically and the phase was found to be very soft (with the hardness value of 0.17 GPa). On the contrary, the second’s phase Cu11In9 response on the loading - unloading treatment had an elastic-plastic character. Also the hardness was significantly larger (5.92 GPa). That means the Cu11In9 phase has the potential for brittle behaviour. Also the elastic modulus was determined for the both phases BiIn and Cu11In9 and was equal to 34.1 and 103.5 GPa, respectively.