Occurrence of brittle interfacial fracture at an electroless Ni(P)/immersion gold–solder joint has long been a serious problem not yet fully understood. In our previous report on the electroless Ni(P) [J. Mater. Res.19, 2428 (2004)], it was shown that crystallization of the Ni(P) film and growth of the Ni3SnP layer were accelerated after the intermetallic compound (IMC) spalling, and accurate failure locus of the brittle fracture due to so-called “IMC spalling induced microstructure degradation of the Ni(P) film” is presented for the first time in this communication. For Sn–3.0Ag–0.5Cu solder joints, (Ni,Cu)3Sn4 and/or (Cu,Ni)6Sn5 ternary IMCs formed at the interface, and neither spalling nor interfacial fracture was observed. For Sn–3.5Ag joints, Ni3Sn4 compound formed, and the brittle fracture occurred through the Ni3SnP layer in the solder pads where Ni3Sn4 had spalled. Since the Ni3SnP layer is getting thicker during or after Ni3Sn4 spalling, control of IMC spalling is crucial to ensure the reliability of Ni(P)/solder system.