Two-Phase η′ + η Region in Cu₆Sn₅ Intermetallic: Insight into the Order–Disorder Transition from Diffusion Couples

Abstract

The ongoing electrification and miniaturization increase the quality demands on solder joints. A bottleneck for solder joint reliability can be the intermetallic Cu₆Sn₅ phase, which undergoes a phase transition, implying a volume change in a relevant temperature range. There are contradicting reports on the sign and magnitude of this volume change, which possibly implements stresses and cracks in solder joints. To clarify the characteristics of the phase transition, different samples were manufactured by applying industrial-like standards and isothermal heat treatments around the predicted phase transition temperature. Using x-ray diffraction, a coexistence of ordered η′ and disordered η was detected in samples treated at 438–445 K. The lattice parameters show that the volume of the disordered η phase is approximately 0.64–0.65% smaller than the one of the ordered η′ phase. A comparison with order–disorder transitions in structurally related phases shows that the volume change based on order–disorder transitions is normally of opposite sign and around 0.1–0.2%. Therefore, an effect of different compositions is considered responsible for the volume change. Adopting the exact composition Cu₆Sn₅ (Cu_1.20Sn) for the η′ phase, it was estimated, based on density functional theory calculations from the literature, that the coexisting η phase assumes lower Cu content of Cu_1.171Sn at 438 K and Cu_1.174Sn at 445 K. In contrast, the lattice parameters of η′, generated at different temperatures, imply a largely temperature-independent composition of Cu_1.20Sn. This leads to adjustments of the Cu-Sn phase diagram.