Effect of Morphological Change of Ni 3 Sn 4 Intermetallic Compounds on the Growth Kinetics in Electroless Ni-P/Sn-3.5Ag

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FOR stable intermetallic compound (IMC) formation and the joining of lead-free solders to electrical interconnections, the most popular metallizations are copper (Cu) and nickel (Ni). The formation of stable IMCs at solder joints is important in providing electrical and thermal pathways and mechanical stability. Therefore, the growth kinetics of the IMCs that are formed on Cu and Ni has been extensively investigated.[1–17] Some of the studies have suggested diffusion-controlled reactions (n = 1/2), while other studies have proposed slower growth kinetics (n £ 1/3). Several studies[5,6] have reported a 1/3-power-law dependence for the growth of Cu6Sn5. The scallop-like morphology of Cu6Sn5 grains allows atomic transport along liquid channels or grain boundaries and in between Cu6Sn5 grains. The grains were assumed to grow in a self-similar manner with a constant aspect ratio between the perpendicular length and lateral width. In their fluxdriven ripening (FDR) theory, the growth of the

YOONCHUL SOHN is with the Department of Welding & Joining Science Engineering, College of Engineering, Chosun University, 309 Pilmun-daero, Dong-gu, Gwangju 61452, Korea. Contact e-mail: [email protected] Manuscript submitted November 26, 2019.

METALLURGICAL AND MATERIALS TRANSACTIONS A

scallop-like grains is supplied by two fluxes: the interfacial reaction flux and ripening flux. Meanwhile, Schaefer et al.[8] suggested a kinetic model based on fast diffusion along Cu6Sn5 GBs as the rate-controlling process. In the analysis, the effective heights of the GBs were smaller than the average heights of Cu6Sn5 grains because of GB grooving. Neglecting volume diffusion and taking self-similar growth, i.e., constant aspect ratio of Cu6Sn5 grains, the analysis also predicted a 1/3-power-law dependence on time in agreement with FDR theory. Reaction studies have been further developed in terms of the rate of IMC formation[9,10] and detailed microstructure analysis of the solder joints.[11,12] For example, the fast formation of Cu-Sn IMCs was demonstrated for the application of high-temperature-stable interconnects, which could be fabricated with ultrasonic-assisted soldering[9] and micro-resistance spot welding processes.[10] In the spot welding experiment, accompanying the speedy electromigration of Cu atoms in the molten Sn solder, the columnar dendritic Cu6Sn5 compounds were formed due to constitutional supercooling and were subsequently transformed into Cu3Sn compounds, showing that the joule heat-induced temperature coupled with the passage of electric current significantly enhanced the interfacial reaction at the liquid Sn/solid Cu metallization interface. Recently, some researchers have published studies on Ni3Sn4 growth kinetics for Ni metallization in liquid

Sn-based solders compared to FDR theory and GB diffusion kinetics.[18–22] Shen et al.[20] reported that the actual growth of Ni3Sn4 involves the net effect of several interrelated phenomena, such as diffusion through the layer via volume diffusion and GB diffusion, GB groovi

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