This study examines the growth mechanism of micro void called “Kirkendall voids” within NiSnP nano-crystalline layer between
(Cu,Ni)6Sn5 intermetallic compound (IMC) and Ni3P formed during two double reflow processes. The micro voids in NiSnP layer formed at the first reflow grow faster under the
elevated reflow temperature than under the standard lead-free reflow, during the second reflow process. Despite the diffusion
barrier Ni(P), the inward diffusion flux of Sn from (Cu,Ni)6Sn5 into NiSnP layer is much slower than the outward flux of Sn from NiSnP layer into Ni3P, consequently leaving voids as NiSnP thickness increases. Results show that the thermal activation energy through the elevated
reflow temperature has a higher influence in micro void growth than the number of reflows for the inward and outward diffusion
flux difference of Sn within NiSnP layer in electroless Ni(P)/immersion Au and SnAgCu reaction system.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: In this work, the impact of microvoids on the microstructural evolution of η-Cu6Sn5 and ɛ-Cu3Sn in the Cu-Sn system is evaluated numerically. Through the use of the multiphase-field method, the systems of interest are allowed to evolve using a solid-state aging temperature of 453 K in conjunction with material parameters and reaction conditions adopted from previous research. The simulation results are then analyzed and compared with previous experiments in terms of the morphological evolution of the intermetallic compounds (IMCs), the IMC layer thicknesses, and the corresponding interfacial roughness. It is shown that the presence of microvoids at the ɛ/Cu interface interferes with the flow of mass throughout the phases, impeding phase transformations and grain coarsening. This ultimately affects the IMC coarsening rate and overall IMC layer thicknesses. Additionally, it was observed that the presence of microvoids at the ɛ/Cu interface affects the formation of both IMC layers and their corresponding interfaces, and the changes in roughness for the interfaces are quantitatively provided. Overall, the simulations are found to be within the range of accepted experimental values for the morphology of the IMC grains, the evolution of IMC layer thicknesses, and the evolution of interface roughness.
[Show abstract][Hide abstract] ABSTRACT: The effect of bath life of Ni(P) on the brittle-fracture behavior of Sn-3.0 wt.% Ag-0.5 wt.% Cu (SAC)/electroless nickel immersion gold (ENIG) was evaluated in this study. The bath lives of Ni(P) for the ENIG surface finish in this study were varied from 0 to 3 metal turnover (MTO), which were indirectly indicative of Ni(P) bath life, with "0 MTO'' denoting the as-make-up state and "3 MTO'' denoting almost waste plating solution. The SAC/ENIG sample when Ni(P) was plated in the 3 MTO bath (3 MTO sample) had thicker (Cu, Ni)(6)Sn-5 and P-rich layers than when Ni(P) was plated in the 0 MTO bath (0 MTO sample). The brittle-fracture behavior of the 0 and 3 MTO samples was evaluated by use of a high-speed shear (HSS) test with a strain rate of 0.1-2.0 m/s. The shear strength of the 0 MTO sample was higher than that of the 3 MTO sample. The incidence of brittle fracture increased as the bath life of Ni(P) of ENIG (= MTO of Ni(P)) increased. Observation by transmission electron microscopy (TEM) revealed nano-sized voids (or particles) in the Ni-Sn-P layer. As the MTO of the Ni(P) increased, the number of nano-sized voids in the Ni-Sn-P layer of the SAC/ENIG interface increased. The poor brittle-fracture behavior of the 3 MTO sample originated from the weak interface at the thick P-rich layer and from the large nano-sized voids.
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