Phase formation and reaction kinetics in M–Sn systems (M = Zr, Hf, Nb, Ta, Mo)

A systematic search for metal – tin systems which show a potentially high remelting temperature in diffusion-soldered bonds was done by investigation of phase formation and reaction kinetics in M–Sn systems (M = Zr, Hf, Nb, Ta, Mo). Phase equilibration experiments were done with powder samples prepared from pure elements, heated in evacuated silica capsules between 300 and 1000 °C for 1 h to 200 days, and studied by X-ray diffraction analysis. Diffusion couples M/Sn (M = Zr, Nb, Ta, Mo) were prepared from the foils of pure elements and heated in a bonding furnace or in evacuated silica tubes at 300 – 700 °C for 6 – 95 h. The couples were cross-sectioned and studied by scanning electron microscopy with energy-dispersive X-ray analysis. The rates of M–Sn reactions can be semiquantitatively ordered in the sequence Zr > Nb > Hf > Ta≫Mo (– Sn). The activation energy of the better defined reactions Zr – Sn, Nb–Sn, Hf–Sn are in the range of 67 – 93 (± 15) kJ/mol. A Ta–Sn phase diagram was constructed with similarities to the V–Sn system. Only Zr –Sn is a potential candidate for diffusion soldering of high-temperature stable bonds, Mo is suggested as an effective diffusion barrier against the attack of liquid tin.