In situ transmission electron microscopy (TEM) study on the structural evolution behavior of nano Sn sheets under a thermal field

Abstract

Sn-based solders have been used widely in chip attachment and surface mount processes in the electronic packaging industry, where Sn plays an indispensable role as the main component of Sn-based solders. However, the application of pure Sn as a solder is limited. Therefore, we employed in situ transmission electron microscope technology to investigate the thermodynamic dynamic process of nanostructured Sn sheets. The heating process of pure Sn nano sheets has been observed to result in the formation of nanospheres, and the crystal structure of these nanospheres undergoes changes during continuous heat preservation. The presence of nanospheres and their unstable crystal structure contribute to a reduction in the contact surface area and instability at the contact point. These findings suggest that pure Sn may pose challenges as a solder material. The conclusion was further validated through the implementation of an in situ Cs-TEM thermal field experiment to conduct welding on Sn–Cu interface samples. It was observed that Sn nanospheres were generated at the Sn–Cu interface, and the contact angle between the resulting Sn nanospheres and the base material Cu exceeded 90°, thereby providing additional evidence to support the aforementioned conclusion.