Neutron diffraction study of structure, stability and phase transitions in Cu-In-Sn alloys as alternative Pb-free solders

CUELLO, G. J.; AURELIO, G.; SOMMADOSSI, S.

Grenoble

Conferencia; European Powder Diffraction Conference EPDIC 2012; 2012

The study of the Cu-Sn-In ternary system has become of great importance in recent years, due to new environmental regulations forcing to eliminate the use of Pb from bonding technologies for electronic devices. In this work we present a neutron diffraction study of the constitution and crystallography of a series of alloys around the 60 at.% Cu composition, and with In contents ranging from 0 to 25 at.%, quenched from 300ºC. The alloys were characterized by scanning electron microscopy, probe microanalysis and high-resolution neutron diffraction. The Rietveld refinement of neutron diffraction data allowed to improve the currently available model for site occupancies in the hexagonal η-phase in the binary Cu-Sn as well as in ternary alloys. For the first time, structural data is reported in the ternary Cu-Sn-In η-phase as a function of composition, information that is of fundamental technological importance as well as valuable input data for the ongoing modeling of the ternary phase diagram. In addition, we have followed the real-time sequence of phase transformations in successive heating and cooling ramps between 100ºC and 550ºC, using neutron powder thermodiffractometry complemented with calorimetric studies of the phase transitions. Our results give experimental support to the current assessment of the ternary phase diagram in this composition and temperature range, confirming the sequence of transitions η → (η + L) → (ε + L) with transformation temperatures of 210º C and 445º C, respectively. The use of neutrons allowed to overcome common difficulties in phase identification with powder XRD due to absorption and preferred orientation issues. Even the transitions to liquid phases could be successfully identified and monitored in situ, turning the neutron thermodiffraction technique into a valuable tool for phase diagram studies of emerging lead-free solder candidates.