Synthesis of Sn3.5Ag0.5Cu nanoparticle solders and soldering mechanism

Solder has been long playing an important role in the assembly and interconnection of integrated circuit (IC) components on substrates, i.e., ceramic or organic printed circuit boards. The main function of solder is to provide electrical, thermal, and mechanical connections in electronic assemblies. Lead, a major component in Sn/Pb solder, has long been recognized as a health threat to human beings, which is the main reason for the requirement of environmental-friendly lead-free solder. A variety of lead-free solder alloys have been investigated as potential replacements for Sn/Pb solders, but there is still no perfect alternative. Three alloy families, Sn-Ag-Cu, Sn-Ag and Sn-Cu, seem to be of particular interest. However, concerns with this alloy family, including higher soldering temperature, poorer wettability due to their higher surface tension, and their compatibility with existing soldering technology and materials, have impeded their steps in completely replacing Sn/Pb solder. As the melting point can be dramatically decreased when the size of the particles is reduced to nanometer size, especially under 20 nm, and nanosolders have much better wettability at the same time. Furthermore, after heated and cooled, nanomaterials become bulk materials, which make them have the ability to endure a higher function temperature. Thus it is of great significance to conduct in-depth investigation on the synthesis of nanosolders and their soldering performance. In this work, Sn3.5Ag0.5Cu nanoparticles as a promising alternative of Sn/Pb solder was developed. The morphology, atomic structure, phase composition, and element composition of nanoparticles were characterized by SEM, TEM, XRD, and EDS, respectively. Size change of Sn3.5Ag0.5Cu nanoparticles under different sintering temperatures and sintering times was discussed. Microstructure of Cu/nanosolder/Cu sandwich structure under different soldering peak temperatures and soldering times was investigated. Shear strength and failure mode of the Cu/nanosolder/ Cu sandwich structure under different pressure were also discussed. The results showed that the average diameter of nanoparticles was less than 10 nm with an agglomeration growth tendency. When sintering temperature was relatively low, the neck size increased steadily as temperature and time increased. In contrast, when sintering temperature was relatively high, the agglomeration mainly happened in the initial process and neck size changed little as the time increased. Thickness of intermetallics of Cu/nanosolder/Cu sandwich structure increased with the soldering temperature increased while the size and quantity of voids decreased. Shear strength of bonded sample increased with the increasing pressure, and got the maximum 14.2 MPa when the pressure reached 10 N.