Metallurgical reactions of Sn-3.5Ag solder with various thicknesses of electroplated Ni/Cu under bump metallization
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C.Y. Liu Department of Chemical Engineering and Materials Engineering, National Central University, Chung-Li, Taiwan, Republic of China
S.S. Lin and K.H. Chen Megic Corporation, Ltd., Hsin-chu 300, Taiwan, Republic of China (Received 1 June 2005; accepted 6 July 2005)
Nickel has been widely used as an under-bump metallization (UBM) material in the microelectronics industry. The solid-state reaction between the eutectic SnAg solder bumps and three thicknesses of Ni/Cu UBM was investigated, with 5 m-Cu/3 m-Ni, 3 m-Cu/2 m-Ni, and 0 m-Cu/1 m-Ni. It was found that the shear strength of the solder bumps decreased after the solid-state aging at 150 °C for 200 h, and it did not change much after it was prolonged for 500 and 1000 h. Aging of the Ag3Sn intermetallic compound (IMC) and grain growth of the solder are responsible for the decrease in the shear strength. Furthermore, the shear test results indicated that the fracture mode switched from ductile to brittle for the solder bumps with 1 m Ni after aging longer than 200 h, causing the strength of the solder to decrease abruptly. This is attributed to the consumption of the peripheral Ni layer after the solid-state aging for 1000 h. The Ni consumption rate was measured to be 0.02 m/h1/2 at 150 °C.
I. INTRODUCTION
Flip-chip technology has been adopted in highperformance electronics devices because a large number of tiny solder bumps can be fabricated into an area array on a chip as input/output (I/O) interconnections.1 Pbcontaining solder has been used for over 30 years to form flip-chip joints, reacting with under bump metallurgy (UBM).2,3 However with increasing environmental concerns, the microelectronics industry is paying greater attention to lead-free solder alternatives.4,5 One of the most promising alternatives is SnAg3.5 solder for flip chip packages, and manufacturing. SnAg3.5 solder bumps on a wafer by electrical plating or printing technology is now commercially available. Therefore, it can be implemented in fine-pitch solder joints. The metallurgical reactions between the Pb-containing solders and different UBMs have been studied extensively over the past decade. Copper and nickel are the two common UBM materials used in the packaging industry. Copper reacts with the solder alloy very quickly
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0334 2772
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 10, Oct 2005 Downloaded: 21 Mar 2015
and provides excellent wetting.6 Yet, for thin Cu film, the fast consumption rate of the Cu UBM may cause spalling of the interfacial intermetallic compound (IMC) after longer reflowing time.7 Therefore, Ni UBM has been also used due to its lower reaction rate with the solder alloy.8 Due to its high stress, the Ni film cannot be deposited too thickly. Thus, the combination of the Cu and Ni films was adopted in the packaging industry, and several approaches were studied, including Al/Ni(V)/Cu,9 co-sputtering Cu-Ni alloy,10,11 and electroplated Cu/Ni films.
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