Characterising the polymorphic phase transformation at a localised point on a Cu₆Sn₅ grain

Cu₆Sn₅ is an important intermetallic for electric interconnects and anode material of Li-ion batteries, with a polymorphic transformation from hexagonal eta-Cu₆Sn₅ at temperatures higher than 186 °C to monoclinic eta′-Cu₆Sn₅ at lower temperatures. The polymorphic transformation of Cu₆Sn₅ has the potential to generate internal stresses in soldered joints during both the soldering process and subsequent device operation involving thermal cycling. Here we developed a systematic method to characterise the polymorphic transformations at localised points in a single grain of Cu₆Sn₅ that is in intimate contact with adjacent Sn rich and Cu₃Sn phases to establish a time–temperature transformation (TTT) diagram of the eta to eta′-Cu₆Sn₅ transformation. The method involves in situ temperature-controlled high-voltage transmission electron microscopy (HVTEM) of a single-targeted Cu₆Sn₅ grain with observation of diffraction patterns during multiple heating and cooling experiments on a 0.5 μm thick sample made by focused ion beam milling (FIB). The results are compared to previously reported powder X-ray diffraction data. It is concluded that there is good agreement between both techniques and the transformations observed in the powder are representative of those in individual grains present in polycrystalline structures. Based on the newly developed TTT, it was established that the high-temperature eta-Cu₆Sn₅ could be stablised down to room temperature if the cooling rate exceeded 20 °C min⁻¹.