Improvement of electrical and mechanical properties of In-48Sn solder bumps for flexible LED signage using Cu-Ag nanoparticles

Abstract

This study aimed to enhance the mechanical strength of In-48Sn alloy solder bumps without the loss of electrical conductance for application of flexible electronic devices. As a simple method for controlling the mechanical strength, Cu nanoparticles were added to the In-48Sn solder paste during paste formulation. The addition of Ag-encapsulated Cu nanoparticles effectively improved the shear strength without loss of electrical conductance while the addition of Cu nanoparticles deteriorated both mechanical and electrical properties. The development of a dense or coarse microstructure during the soldering process was sensitively dependent on the surface condition and the content of nanoparticles. In particular, the uniform distribution of nanoparticles and alloy phases in β-In3Sn phase was critical for the control of mechanical strength of excessively ductile In-48Sn solder. As a result, the addition of 6 wt% Ag-encapsulated Cu nanoparticles improved the shear strength from 18.7 to 49.5 MPa and their electrical resistance from 0.23 to 0.1 mΩ. Flexible LED signage interconnected using these solder bumps performed very well after 100 000 cycles of bending test.