Abstract: Sn/ENIG has recently been used in flexible interconnects to form a more stable micron-sized metallurgical joint, due to high power capability which causes solder joints to heat up to 200 °C. However, Cu₆Sn₅ which is critical for a microelectronic interconnection, will go through a phase transition at temperatures between 186 and 189 °C. This research conducted an in-situ TEM study of a micro Cu/ENIG/Sn solder joint under isothermal aging test and proposed a model to illustrate the mechanism of the microstructural evolution. The results showed that part of the Sn solder reacted with Cu diffused from the electrode to form η´-Cu₆Sn₅ during the ultrasonic bonding process, while the rest of Sn was left and enriched in a region in the solder joint. But the enriched Sn quickly diffused to both sides when the temperature reached 100 °C, reacting with the ENIG coating and Cu to form (Ni_xCu_1-x)₃Sn₄, AuSn₄, and Cu₆Sn₅ IMCs. After entering the heat preservation process, the diffusion of Cu from the electrode to the joint became more intense, resulting in the formation of Cu₃Sn. The scallop-type Cu₆Sn₅ and the seahorse-type Cu₃Sn constituted a typical two-layered structure in the solder joint. Most importantly, the transition between η and η' was captured near the phase transition temperature for Cu₆Sn₅ during both the heating and cooling process, which was accompanied by a volume shifting, and the transition process was further studied. This research is expected to serve as a reference for the service of micro Cu/ENIG/Sn solder joints in the electronic industry.

Key words: In-situ TEM observation, Isothermal aging, Micro Cu/ENIG/Sn solder joint, Cu₆Sn₅ phase transition