Reliability evaluation on a submicron Ni(P) thin film for lead-free soldering

The solderability between a Sn–Ag–Cu alloy and a submicron Ni(P) film (0.2 μm) was examined using a focused ion beam and field-emission transmission electron microscope. The Ni(P) was electrolessly deposited between the Au and Cu layers, which possessed a low P content (less than 5 wt.%) and a nanocrystalline structure. After one typical reflow, the Ni(P) film was mostly eliminated from the interface, where Cu6Sn5 with a significant Ni content [(Cu0.6Ni0.4)6Sn5] nucleated. The subsequent diffusion of Sn to the underlying Cu through molten solder channels among the (Cu0.6Ni0.4)6Sn5 grains yielded a second Cu6Sn5 layer at the (Cu0.6Ni0.4)6Sn5/Cu interface. The removal of Ni from the Ni(P) during soldering reaction allowed P to nucleate as nanocrystalline Ni3(Sn,P) between the two Cu6Sn5 layers, which subsequently translated into a chain of amorphous P–Sn–O pores. The propagation of the porous P–Sn–O destroyed the stability of (Cu0.6Ni0.4)6Sn5 and drove the compound layer to separate from the second Cu6Sn5 after the third reflow. These observations suggest that the exhaustion of the Ni(P) induced spallation of the compound layer, thereby degrading the reliability of the joining interface.

► Solderability of a submicron Ni(P) film was evaluated.
► The exhaustion of the Ni(P) induced intermetallic spalling phenomenon.
► Ni3Sn phase might nucleate at the temperature of ~ 260 °C.
► The Ni(P) thickness is a key factor for the reliability of solder joints.