The interfacial reaction between eutectic Sn-3.5Ag solder and ENIG substrate during reflow was investigated. During the reflow, the topmost Au layer dissolved into the molten solder, and the reaction layers of Ni3Sn4/Ni-Sn-P/Ni3P formed between Sn-Ag solder and Ni-P plating layer. After the reflow at 255 degrees C for 3 min, most of Ni3Sn4 intermetallic compound (IMC) spalled off the substrate. The formation of thick Ni3P and Ni-Sn-P layers was related to the direct reaction between solder and Ni-Sn-P layer by the IMC spalling.