The increasing power density of modern electronic systems intensifies challenges related to heat dissipation and long-term reliability. Insulated metal substrates (IMS), particularly three-dimensional (3D), are increasingly used as integrated thermal–mechanical solutions in high-power electronics. However, their complex geometry and material interfaces introduce new reliability concerns, especially at solder joints. This study investigates the evolution of intermetallic compounds (IMCs) in solder joints formed on 3D aluminum IMSs with ENIG metallization, focusing on SAC305 and Sn42Bi57Ag1 solder alloys. Solder joints were subjected to environmental aging under high-temperature, high-humidity, and thermal-shock conditions to simulate realistic service environments. Microstructural and compositional analyses of the interfacial IMC layers were performed, together with measurements of IMC thickness evolution. The results show that aging significantly modifies the chemical composition and morphology of IMC layers in both solder systems. In SAC305 joints, progressive development of (Cu,Ni)₆Sn₅ phases with increasing Cu participation was observed. In Sn42Bi57Ag1 joints, Bi affected reaction kinetics but did not alter the diffusion-controlled nature of IMC growth. Thickness measurements indicate higher sensitivity of SAC305 joints to environment-assisted interfacial degradation, while Sn42Bi57Ag1 joints exhibit greater susceptibility to stress-assisted IMC growth during severe thermal cycling. These findings highlight the distinct reliability behaviors of tested solders on 3D IMSs and provide insight into their suitability for high-power electronic applications.