This manuscript reports the isothermal annealing effect on the mechanical and microstructure characteristics of Sn-0.7Cu-1.5Bi solder joints. A detailed microstructure observation was carried out, including measuring the activation energy of the intermetallic compound (IMC) layer of the solder joints. Additionally, the synchrotron µX-ray fluorescence (XRF) method was adopted to precisely explore the elemental distribution in the joints. Results indicated that the Cu6Sn5 and Cu3Sn intermetallic layers thickness at the solder/Cu interface rises with annealing time at a rate of 0.042 µm/h for Sn-0.7Cu and 0.037 µm/h for Sn-0.7Cu-1.5Bi. The IMC growth's activation energy during annealing is 48.96 kJ mol-1 for Sn-0.7Cu, while adding Bi into Sn-0.7Cu solder increased the activation energy to 55.76 kJ mol-1. The µ-XRF shows a lower Cu concentration level in Sn-0.7Cu-1.5Bi, where the Bi element was well dispersed in the β-Sn area as a result of the solid solution mechanism. The shape of the IMC layer also reconstructs from a scallop shape to a planar shape after the annealing process. The Sn-0.7Cu hardness and shear strength increased significantly with 1.5 wt.% Bi addition in reflowed and after isothermal annealing conditions.
The growth and formation of primary intermetallics formed in Sn-3.5Ag soldered on copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish after multiple reflows were systematically investigated. Real-time synchrotron imaging was used to investigate the microstructure, focusing on the in situ growth behavior of primary intermetallics during the solid-liquid-solid interactions. The high-speed shear test was conducted to observe the correlation of microstructure formation to the solder joint strength. Subsequently, the experimental results were correlated with the numerical Finite Element (FE) modeling using ANSYS software to investigate the effects of primary intermetallics on the reliability of solder joints. In the Sn-3.5Ag/Cu-OSP solder joint, the well-known Cu6Sn5 interfacial intermetallic compounds (IMCs) layer was observed in each reflow, where the thickness of the IMC layer increases with an increasing number of reflows due to the Cu diffusion from the substrate. Meanwhile, for the Sn-3.5Ag/ENIG solder joints, the Ni3Sn4 interfacial IMC layer was formed first, followed by the (Cu, Ni)6Sn5 IMC layer, where the formation was detected after five cycles of reflow. The results obtained from real-time imaging prove that the Ni layer from the ENIG surface finish possessed an effective barrier to suppress and control the Cu dissolution from the substrates, as there is no sizeable primary phase observed up to four cycles of reflow. Thus, this resulted in a thinner IMC layer and smaller primary intermetallics, producing a stronger solder joint for Sn-3.5Ag/ENIG even after the repeated reflow process relative to the Sn-3.5Ag/Cu-OSP joints.