Ultrafine-Grain and Isotropic Cu/SAC305/Cu Solder Interconnects Fabricated by High-Intensity Ultrasound-Assisted Solidification

With the miniaturization of portable electronic devices, the size of solder joint interconnects is decreasing to micrometer levels. These joints possess only several or even one or two grains, resulting in anisotropy and failure issues. Direct ultrasound-assisted solidification of Cu/SAC305/Cu interconnects for grain refinement and fabrication of isotropic solder joints is presented herein. These joints consist of many beta-Sn grains. The average cross-sectional area of the Sn-rich phase is significantly reduced by up to 99% when compared with conventional as-reflowed samples. The ultrasonic power density exhibits a threshold value for affecting the microstructures. Below 200 W cm(-2), the beta-Sn grains were refined and had circular shape. The Ag3Sn phase grew in a manner similar to branched coral to sizes reaching 30 mu m, or as rods aggregated together with Cu6Sn5 tube fragments. Above 200 W cm(-2), the microstructures were coarsened and Ag3Sn had plate-like shape. The thickness of Cu6Sn5 intermetallic layers at the Cu/solder interfaces was reduced by more than 26%. The relationships among the ultrasonic power, nucleation rate, local temperature drop, and pressure were identified. At the highest power density of 267 W cm(-2), the nucleation rate was about 4.05 x 10(14) m(-3) s(-1), the local temperature drop was 248 K, and the local pressure was on the order of several GPa.