Study on the floating kinetics of graphene in molten Sn-based alloy based on in-situ observation of X-ray radiography

It has often been observed that the agglomeration and floating of graphene during soldering graphene nanosheets-reinforced Sn–Ag–Cu (GNSs/SAC) alloys, which considerably deteriorates properties of the solder joint. Moreover, the mechanisms of the above behavior have not been clarified, which severely hinders the application of the composite solder. In this study, two-dimensional in-situ X-ray radiography is used to observe graphene agglomeration and floating in molten GNSs/SAC. It is found that graphene agglomerates do not start floating as soon as the alloy melts but lags behind the melting. Based on colloidal theory, molten GNSs/SAC alloy can be regarded as colloidal suspension. The net interaction potentials between graphene nanoparticles are always negative and the approaching nanoparticles can agglomerate spontaneously. There exists meta-stable equilibrium between floating and Brownian motion in the molten GNSs/SAC system. Furthermore, a kinetic model of particles motion is derived and the delayed floating time is calculated to be in the range of 5.5–16.9 s for 0.05 wt% GNSs/SAC alloy, which is quantitatively validated using X-ray radiography. According to the model, the agglomeration and floating behavior of graphene are time dependent. Therefore, we propose a transient current bonding (TCB) technique to shorten the bonding time by 3 orders of magnitude and effectively inhibit the agglomeration and floating of graphene. Graphene uniformly distributed in the SAC matrix also improves the shear strength of the solder joints.