In-situ isothermal aging TEM analysis of a micro Cu/ENIG/Sn solder joint for flexible interconnects

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 & DEG;C. However, Cu 6 Sn 5 which is critical for a microelectronic interconnection, will go through a phase transition at temperatures between 186 and 189 & DEG;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 ri & PRIME;-Cu 6 Sn 5 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 & DEG;C, reacting with the ENIG coating and Cu to form (Ni x Cu 1- x ) 3 Sn 4 , AuSn 4 , and Cu 6 Sn 5 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 Cu 3 Sn. The scallop-type Cu 6 Sn 5 and the seahorse-type Cu 3 Sn constituted a typical two-layered structure in the solder joint. Most importantly, the transition between ri and ri' was captured near the phase transition temperature for Cu 6 Sn 5 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. & COPY; 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.