Microstructural Evolution of Lead-Free Solder Joints in Ultrasonic-Assisted Soldering

Solder joint reliability greatly depends on the microstructure of the solder matrix and the morphology of intermetallic compounds (IMCs) in the joints. Addition of strengthening phases such as carbon nanotubes and ceramic particles to solder joints to improve their properties has been widely studied. In this work, ultrasonic vibration (USV) of casting ingots was applied to considerably improve their microstructure and properties, and the resulting influence on fluxless soldering of Cu/Sn-3.0Ag-0.5Cu/Cu joints and their microstructural evolution was investigated. It was demonstrated that USV application during reflow of Sn-based solder had favorable effects on β-Sn grain size refinement as well as the growth and distribution of various IMC phases within the joints. The β-Sn grain size was significantly refined as the ultrasound power was increased, with a reduction of almost 90% from more than 100 μm to below 10 μm. Long and large Cu6Sn5 tubes in the solder matrix of the joints were broken into tiny ones. Needle-shaped Ag3Sn was transformed into flake-shaped. These IMCs were mainly precipitated along β-Sn phase boundaries. High-temperature storage tests indicated that the growth rate of interfacial IMCs in joints formed with USV was slower than in conventional reflow joints. The mechanisms of grain refinement and IMC fragmentation are discussed and related to the ultrasonic effects.