<p>The reliability of lead-free solder joints in microelectronic packaging is strongly influenced by the formation and growth of interfacial intermetallic compounds (IMCs) during reflow soldering. In this study, the effects of graphene introduced through two different approaches dispersed graphene nanoplatelets obtained by liquid-phase exfoliation (LPE) and a monolayer graphene interlayer grown by chemical vapor deposition (CVD) on the interfacial microstructure and mechanical properties of Sn–Ag–Cu (SAC305) solder joints on copper substrates were investigated. Four composite solders were investigated: conventional SAC305/Cu, CVD graphene interlayer on Cu (CVD-GF–0.0 wt. % GNS), and SAC305 composite solders containing 0.05 wt. % and 0.50 wt. % LPE-derived graphene nanosheets. The solders were fabricated and reflowed under controlled conditions. In parallel, CVD graphene was synthesized directly on copper substrates under a CH₄/H₂ atmosphere at 1000 °C, acting as a diffusion-modifying interlayer. Interfacial microstructures were characterized by optical microscopy, focusing on the morphology and thickness of Cu₆Sn₅ and Ag₃Sn intermetallic compounds. Mechanical behavior was assessed by nanoindentation, yielding Vickers microhardness, elastic modulus, and plastic deformation parameters. The results show that both graphene additions lead to a noticeable reduction in IMC layer thickness and a more uniform interfacial morphology compared to conventional SAC305/Cu joints. Improvements in mechanical properties were also observed, which are attributed to the combined effects of restricted Cu diffusion across the interface and heterogeneous nucleation during solder solidification. These findings demonstrate that graphene, whether incorporated into the solder matrix or applied as a CVD-grown interfacial layer, effectively tailors interfacial reactions and enhances the mechanical response of SAC305 solder joints.</p>