Low-temperature-solderable intermetallic nanoparticles for 3D printable flexible electronics

Functional materials for flexible and wearable smart devices have attracted much attention in recent years. This paper describes structure and properties of uniquely prepared, functional interconnectable nanoparticles (NPs) of Cu₆Sn₅ intermetallic compound that can allow 3D flexible packaging and nano-circuits. In situ TEM analysis confirms that size-controllable Cu₆Sn₅ NPs as small as ∼6.40 nm can be made sinterable at the start temperature as low as ∼130 °C, which is much lower than its bulk melting point (MP) of 415 °C. After sintering, its high MP provides mechanical and thermal stability. Based on the in situ TEM observation and calculation, particle size and distribution affects the sintering process. More interestingly, the relative orientations of adjacent particles also play an important role. A new orientation related sintering mechanism noted as orientation unification (OU) is revealed as two adjacent particles exhibit orientation change to slowly match their orientation with each other during the heating process. The interesting interaction between nano-Cu₆Sn₅ and micro-Cu substrate during in situ TEM heating gives first hand atomic level proof of the formation of Cu₃Sn. The nano-Cu₆Sn₅ joints possess high enough bonding strength and great high temperature working capability. This intermetallic nano-soldering approach can pioneer a novel strategy of circuit connection, by providing high working temperature interconnection materials for 3D flexible packaging and ultra-high-density micro/nano interconnections.