Sn-based, Pb-free solders with high a Sn content and high melting temperature often cause excessive interfacial reactions at interfaces. Sn-3.5Ag solder alloy has been used to identify its interfacial reactions with two-metal layer flexile substrates. In this paper the dissolution kinetics of Sn3.5Ag solder on the electrolytic Ni and electroless NiP layer are investigated. It is found that during 1 min reflow the electrolytic Ni layer dissolves much less than the electroless NiP layer due to the formation of and intermetallic compounds (IMCs) on the electrolytic Ni layer. The faster nucleation of IMC on the NiP layer is proposed as the main reason for the higher initial dissolution rate of the electroless NiP layer. A P-rich Ni layer is formed underneath the IMC due to the solder-assisted reactions. This P-rich Ni layer acts as a good diffusion barrier layer, which decreases the dissolution rate of the NiP layer as compared to that of the Ni layer, but weakens the interface of solder joints and reduces the ball shear load and reliability. Below a certain thickness, the P-rich Ni layer breaks and an increase in the diffusion of Sn atoms through the fractured P-rich Ni layer occurs that increases the growth rate of IMCs again, and thus the dissolution rate of the NiP layer becomes higher again than for the Ni layer. It is found that a thick NiP layer cannot protect the Cu layer for more than 120 min reflow at 250°C. An electrolytic system has a relatively higher shear load, a lower dissolution rate of the Ni layer, and is more protective for the Cu layer during extended times of reflow.