Reliable solder joint performance is critical for advanced electronic packaging, wherein interfacial microstructure significantly impacts mechanical reliability. This study compared conventional reflow, laser, and intense pulsed light (IPL) soldering in terms of microstructural evolution, mechanical properties, and aging reliability. Conventional reflow soldering required preheating and a long process time, resulting in a thick interfacial (Cu, Ni)6Sn5 intermetallic compound (IMC) layer with coarse grains. By contrast, IPL soldering produced a thinner IMC layer due to its shorter process time (∼12 s), whereas laser soldering resulted in the thinnest IMC layer due to rapid heating/cooling and minimal time above liquidus. joint behavior was assessed via nano-indentation, shear testing, and finite element analysis (FEA). Laser-soldered joints exhibited the highest hardness due to their refined microstructure, followed by IPL and reflow soldering. However, IPL soldering achieved higher shear strength, attributed to the thin but relatively rough IMC layer morphology and the absence of interfacial voids, which enhanced mechanical interlocking and reduced stress concentration and crack propagation. Aging at 150 °C for up to 1500 h caused IMC growth and shear strength degradation in all joints and altered fracture paths in laser-soldered joints, exposing pre-existing interfacial voids on the fracture surface. FEA confirmed that the fracture behavior was primarily governed by aging-induced changes in IMC thickness and adhesion. Overall, IPL soldering demonstrated the most favorable performance, offering an optimal balance of IMC morphology, mechanical reliability, and voids suppression. These findings demonstrate the potential of IPL soldering as a reliable interconnection technology for next-generation electronic packaging.