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RESEARCH PAPERS

# Comparative Study of the Dissolution Kinetics of Electrolytic Ni and Electroless NiP Layers by Molten Sn3.5Ag Solder Alloy

[+] Author and Article Information
M. N. Islam, M. O. Alam, A. Sharif

Department of Electronic Engineering,  City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong

Y. C. Chan1

Department of Electronic Engineering,  City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong KongEEYCCHAN@cityu.edu.hk

1

Corresponding author. Telephone +852-2788-7130; fax: +852-2788-7579.

J. Electron. Packag 127(4), 365-369 (Dec 22, 2004) (5 pages) doi:10.1115/1.2056567 History: Received August 05, 2003; Revised December 22, 2004

## Abstract

Sn-based, Pb-free solders with high a Sn content and high melting temperature often cause excessive interfacial reactions at interfaces. Sn-3.5Ag solder alloy has been used to identify its interfacial reactions with two-metal layer flexile substrates. In this paper the dissolution kinetics of Sn3.5Ag solder on the electrolytic Ni and electroless NiP layer are investigated. It is found that during 1 min reflow the electrolytic Ni layer dissolves much less than the electroless NiP layer due to the formation of $Ni3Sn$ and $Ni3Sn2$ intermetallic compounds (IMCs) on the electrolytic Ni layer. The faster nucleation of $Ni3Sn4$ IMC on the NiP layer is proposed as the main reason for the higher initial dissolution rate of the electroless NiP layer. A P-rich Ni layer is formed underneath the $Ni3Sn4$ IMC due to the solder-assisted reactions. This P-rich Ni layer acts as a good diffusion barrier layer, which decreases the dissolution rate of the NiP layer as compared to that of the Ni layer, but weakens the interface of solder joints and reduces the ball shear load and reliability. Below a certain thickness, the P-rich Ni layer breaks and an increase in the diffusion of Sn atoms through the fractured P-rich Ni layer occurs that increases the growth rate of IMCs again, and thus the dissolution rate of the NiP layer becomes higher again than for the Ni layer. It is found that a $3μm$ thick NiP layer cannot protect the Cu layer for more than 120 min reflow at 250°C. An electrolytic $Ni∕solder$ system has a relatively higher shear load, a lower dissolution rate of the Ni layer, and is more protective for the Cu layer during extended times of reflow.

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Copyright © 2005 by American Society of Mechanical Engineers
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## Figures

Figure 1

Equilibrium phase diagram of Sn–Ag solder (Ref. 8)

Figure 2

Solder ball attachment on two-metal layer flexible substrate

Figure 3

Shear load of electrolytic Ni∕solder joints and electroless NiP∕solder joints

Figure 4

Fracture surface of (a) electrolytic Ni∕solder joint, and (b) electroless NiP∕solder joint after 180 min molten reactions

Figure 5

Interfacial structure of (a) electrolytic Ni∕solder joint, and (b) electroless NiP∕solder joint after 1 min reflow at 250°C

Figure 6

Equilibrium phase diagram of Ni–Sn binary system (Ref. 8)

Figure 7

IMCs layer thicknesses as a function of reflow time

Figure 8

(a) Electrolytic Ni and electroless NiP layer thicknesses as a function of time in the molten state, and (b) the dissolution rate of electrolytic Ni and electroless NiP layers

Figure 9

Interfacial structure of (a) electrolytic Ni∕solder joint and (b) electroless NiP∕solder joint after 30 min reflow at 250°C

Figure 10

Interfacial structure of (a) electrolytic Ni∕solder joint and (b) electroless NiP∕solder joint after 120 min reflow at 250°C

Figure 11

Interfacial structure of (a) electrolytic Ni∕solder joint and (b) electroless NiP∕solder joint after 180 min reaction in the molten condition at 250°C

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