Hydrogen in Electrolessly Deposited Ni/Au and Ni/Pd/Au Films

Abstract

For the surface finishing of copper layer on printed circuit boards, autocatalytic electroless deposition of nickel-phosphorus alloy and immersion deposition of gold (ENIG, Ni-P/Au) or autocatalytic electroless deposition of Ni-P, palladium and immersion deposition of Au (ENEPIG, Ni-P/Pd/Au) are widely used. The Ni-P/Pd/Au surface finish is suitable not only for lead-free soldering but also for wire bonding [1]. For soldering, it was reported that hydrogen plasma improved bonding behavior and realized a flux-free process [2]. Electrochemically deposited metal films include hydrogen atoms. The hydrogen adsorbed in metal films causes various phenomena such as atomic diffusion, alloying, crystal growth, void formation. We have been investigating the atomistic state and behavior of hydrogen in electrodeposited metals by thermal desorption spectroscopy [3, 4]. In this study we measured changes in hydrogen amounts in the Ni-P/Au and Ni-P/Pd/Au surface finishes [5]. We used two different electroless Pd baths using formic acid (Ni-P/Pd/Au) and sodium phosphinate (Ni-P/Pd-P/Au) as reducing agents. The Ni-P films contained large amounts of diffusible hydrogen that desorbs at room temperature. During autocatalytic electroless deposition of Pd and Pd-P on Ni-P films, the greater part of the hydrogen in the Ni-P films desorbed. Although no hydrogen evolution accompanied immersion Au deposition, the hydrogen amounts in Ni-P/Au, Ni-P/Pd/Au, and Ni-P/Pd-P/Au films were much greater than in films without a Au layer. Moreover, at room temperature, they remained almost unchanged for one month after deposition, indicating that the thin Au layer (< 0.1 μm) prevents hydrogen desorption from the metal films. The high hydrogen concentration in Ni-P/Au films improved solder wettability.[1] Y. Oda, M. Kiso, S. Kurosaka, A. Okada, K. Kitajima, and S. Hashimoto, Proc. Internal. Microelectronic and Packaging Soc., IMAPS (2008).[2] T. Hagihara, T. Takeuchi, and Y. Ohno, Proc. 10th Electronics Packaging Technology Conf. (IEEE), 595 (2008).[3] N. Fukumuro, S. Kojima, M. Fujino, Y. Mizuta, T. Maruo, S. Yae, and Y. Fukai, J. Alloy. Comp., 645, S404 (2015).[4] N. Fukumuro, K. Tohda, and S. Yae, this symposium.[5] Y. Oda, Y. Sagara, N. Fukumuro, and S. Yae, J. Surf. Finish. Soc. Jpn., 70, 163 (2019).