<p>The comparative interfacial reactions and mechanical reliability of ENEPIG (electroless Ni/electroless Pd/immersion gold) and Ni-free IGEPIG (immersion gold/electroless Pd/immersion gold) surface finishes on high-density interconnect (HDI) substrates were systematically investigated using SAC305 (Sn-3.0Ag-0.5Cu) and Ni-containing LF35 (Sn-1.2Ag-0.5Cu-0.05Ni) lead-free solders. IGEPIG was evaluated as a Ni-free alternative to conventional ENEPIG for low-insertion-loss packaging applications. Interfacial microstructures were analyzed by scanning electron microscopy (SEM), focused ion beam (FIB), and electron probe microanalysis (EPMA) after multiple reflow cycles and long-term isothermal aging. Mechanical reliability was evaluated using both low-speed and high-speed ball shear testing. The results show that the Ni diffusion barrier in ENEPIG effectively suppresses Cu diffusion and stabilizes the formation of (Cu,Ni)₆Sn₅ intermetallic compounds (IMCs), resulting in slower IMC growth during thermal reflow and aging. In contrast, the Ni-free IGEPIG system promotes accelerated formation of Cu₆Sn₅ and Cu₃Sn due to direct Cu diffusion from the substrate. When LF35 solder was used, trace Ni addition led to the formation of (Ni,Cu)₆Sn₅, partially moderating IMC growth in the IGEPIG system. Despite faster IMC thickening, IGEPIG joints maintained stable mechanical reliability under thermal stress. These findings clarify the role of diffusion-barrier design in ENEPIG and provide insight into interfacial reactions in Ni-free IGEPIG surface finishes, highlighting the trade-off between electrical performance and metallurgical stability for next-generation low-insertion-loss HDI substrate applications.</p>