Origin and evolution of voids in electroless Ni during soldering reaction

The interfacial voids in Ni2SnP were studied by high-resolution transmission electron microscopy. These voids were found to be responsible for many types of failure in electronic solder joints, but so far, detailed investigation of these voids has been pursued in a only few studies because of the need for advanced analytical techniques. The interaction of Sn3Ag0.5Cu with electroless Ni/immersion Au was investigated in this study. The microstructures during reflow were quenched to understand the evolution of these voids. Different peak reflow temperatures were adopted to analyze the change in the void density. Then, the effect of the number of reflow cycles on the void density was investigated. Two types of interfacial voids were found: one in the Ni3P region and the other in the Ni2SnP region. The voids in Ni2SnP were connected to each other to form a void line. The void density increased with the number of reflow cycles, but not with the peak reflow temperature. These results confirmed that the voids in Ni2SnP were more fragile and responsible for most of the interfacial cracks. Void nucleation and coalescence were modeled on the basis of the Johnson–Mehl–Avrami–Kolmogorov theory. The experimental data agreed well with the model data. The mechanism of the void nucleation and coalescence was then discussed.

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► Nano voids (2–4 nm) found at electroless Ni substrate.
► These voids grow to about 18 nm during reflow.
► Higher soldering cycles lead to higher density of voids that are responsible for intermetallic compound spalling and fracture of solder joints.
► This phenomenon is will matched to Johnson–Mehl–Avrami–Kolmogorov theory.