Microstructure evolution and mechanical properties of Cu-Sn intermetallic joints subjected to high-temperature aging

In this work, full Cu3Sn joints were aged at 500°C, 550°C, and 600°C for various durations. Microstructure evolution and grain morphology of the joints were systematically investigated by OM, SEM, EBSD, and TEM technology. In addition, the mechanical properties of hardness and shear strength for the CuSn intermetallic compounds were studied using a Micro Vickers hardness tester and uniaxial tensile equipment. Upon heating to 500°C, the phase transformation pathway was Cu3Sn–Cu41Sn11–α(Cu); aging at 550°C demonstrated a pathway of Cu3Sn–Cu41Sn11–Cu41Sn11&α(Cu)–α(Cu); and finally, heating to 600°C resulted in a phase transformation pathway of Cu3Sn–Cu20Sn6–Cu20Sn6&α(Cu)–α(Cu). It was found that longer aging times and higher temperatures had effects on grain size and morphology. For the 500°C group, the average grain size of the fully formed Cu41Sn11 phase increased from 1.93 μm at an aging time of 20 min to 4.20 μm at 30 min. The grain sizes of the fully formed Cu41Sn11 phase in joints subjected to 550°C for 11 min was lower than 2.36 μm, which were in turn smaller than joints treated at 500°C for 30 min (4.2 μm). Treatment with higher temperatures resulted in more cores and a higher growth rate of Cu41Sn11 grains formed on the interface of Cu/Cu3Sn, which eventually led to the small grain size of the Cu41Sn11 phase. The grain morphologies of the formed Cu41Sn11 phase and Cu20Sn6 phase were mainly column grain. After an aging time of 11 min for the 550°C group, the grain morphology of α(Cu) included in a two-phase microstructure (Cu41Sn11&α(Cu)) mainly exhibited dendritic properties at the grain boundary of Cu41Sn11, its crack tendency broke the hardenability of single grains for the Cu41Sn11 phase, which resulted in a decline in strength from 107 MPa to 42 MPa for Cu41Sn11&α(Cu) compared to the Cu41Sn11 phase. Nevertheless, for the joints treated at 600°C after an aging time of 7 min, the dispersion of α(Cu) particles mainly followed granular shapes within the one large Cu20Sn6 grain in the two-phase microstructure (Cu20Sn6&α(Cu)). This blocked the extension of dislocation, which finally led to an enhancement in strength for the Cu20Sn6 & α(Cu) phase (51 MPa) compared to the Cu20Sn6 phase (73 MPa).