Effects of under bump metallization and nickel alloying element on the undercooling behavior of Sn-based, Pb-free solder
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Sung K. Kang IBM T.J. Watson Research Center, Yorktown Heights, New York 10598
Sun-Kyoung Seo Department of Materials Science and Engineering, KAIST Yuseong-gu, Daejeon, Republic of Korea 305-701
Da-Yuan Shih IBM T.J. Watson Research Center, Yorktown Heights, New York 10598
Hyuck Mo Leea) Department of Materials Science and Engineering, KAIST Yuseong-gu, Daejeon, Republic of Korea 305-701 (Received 13 June 2008; accepted 12 November 2008)
A significant reduction of the undercooling of Sn-based solder alloys was previously reported when they were reacted with various under bump metallurgies (UBMs). In the present study, new experiments have been designed and carried out to understand the undercooling behavior of various Cu- and Ni-doped solders on Ni UBM. Two competing mechanisms were further investigated that include the formation of intermetallic compounds (IMCs) at solder/UBM interface and the change of solder composition because of the dissolution of Ni UBM into solder. Two types of IMCs, including both Ni3Sn4 and Cu6Sn5 that were formed at the interface, were correlated with the undercooling of Sn–0.2Cu and Sn–3.8Ag–0.2Cu solders. In addition, the compositional changes of various Sn-based solders after reactions with Ni UBM were analyzed. On the basis of the experimental results, it was found that the significant reduction in undercooling is primarily caused by dissolved Ni atoms from Ni UBM and the concurrent formation of Ni3Sn4 IMC in the solder matrix. Finally, the beneficial effect of Ni dissolution is thermodynamically favorable as confirmed by the thermodynamic calculations and differential scanning calorimetry measurements with various Ni-doped solder alloys.
I. INTRODUCTION
Near-eutectic binary or ternary Sn-based solder alloys are the promising Pb-free candidates to replace Pbcontaining solders, such as 37Pb-63Sn or 97Pb-3Sn in electronic packaging applications. The popularly used Sn-based solders include Sn–0.7Cu, Sn–3.5Ag, and Sn–3.8Ag–0.7Cu (in wt% unless specified otherwise). Because most Sn-based solders consist of more than 90 wt% Sn and a small amount of alloying additions such as Cu and Ag, the physical, chemical, and mechanical properties are heavily affected by the unique bodycentered-tetragonal crystal structure of pure Sn.1 The significant undercooling behavior of Sn-based solders a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0046
534
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 2, Feb 2009 Downloaded: 29 Jan 2015
clearly indicates one of such propensities that molten Sn solder is difficult to solidify. When Sn-based solders are solidified, the solidification does not occur readily even though the liquid phase passes its equilibrium phasetransformation temperature. This phenomenon is called “undercooling” and is attributed to the difficulty in nucleating a solid phase from a liquid phase. In the case of pure Sn without any appreciable amount of doped impurities, the maximum undercooling observed was 107–120 K.2,3 The a
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