Laser solder ball jetting of SAC305/ENIG joints: Microstructure, kinetics, and reliability under multiple reflow cycles

In this work, laser solder ball jetting (LSBJ) was used to bond electroless nickel immersion gold (ENIG) coated Cu pads on FR4 substrates. Lead-free SAC305 (Sn–3Ag–0.5Cu) solder balls (Ф 100 μm) were used for the LSBJ process. After LSBJ, the solder joints were reflowed for various reflow cycles (0, 1, 3, 5, and 7), each cycle consisting of 60 s. The microstructure and mechanical properties of the LSBJ solder joints so formed were studied after multiple reflow cycles. It was found that SAC/ENIG-Cu solder joints had higher shearing strength during multiple reflows. The shearing force of the solder joints remained almost stable with the increase in the number of reflow cycles. The fracture of SAC/ENIG-Cu solder joints in LSBJ (before reflow) mainly occurred in the bulk solder. In contrast, after reflow, it showed various combinations of bulk solder fracture, ball lift, and Cu-pad lift interface failure. The microstructure evolution of the solder joints during multiple reflows showed a continuous change in the chemical composition of the IMC layer. Large Ag–Sn and Cu₆Sn₅ phases near the IMC layer were found within the SAC solder joints on the Cu pads. The LSBJ process enables precise solder deposition and uniform bonding quality during initial joining; however, multiple reflow cycles can lead to progressive IMC growth, affecting long-term reliability. These results demonstrate that while LSBJ offers distinct advantages in controlled soldering, its performance stability under multiple reflows requires further optimization.