Undercooling and microhardness of Pb-free solders on various under bump metallurgies
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Sung K. Kang IBM T.J. Watson Research Center, Yorktown Heights, New York 10598
Hyuck Mo Leea) Department of Materials Science and Engineering, KAIST, Gusung-Dong 373-1, Yusung-Gu, Daejeon, Republic of Korea 305-701 (Received 18 October 2007; accepted 14 January 2008)
The undercooling behavior of pure Sn, Sn–0.7Cu, Sn–3.5Ag, and Sn–3.8Ag–0.7Cu solder alloys was observed in terms of various under bump metallurgies (UBMs). Four different UBMs (electroplated Cu, electroplated Ni, electroless Ni(P), and electroless Ni(P)/immersion Au) were used. The amount of the undercooling of Pb-free solder alloys was reduced when reacted with electroplated Cu UBM and Ni-based UBMs. The Ni-based UBMs were more effective than Cu UBM in reducing the undercooling of Pb-free solders. When Ni3Sn4 was formed during the interfacial reactions with Ni-based UBMs, the reduction of undercooling was significant, especially for pure Sn and Sn–3.5Ag. The effects of UBMs on the undercooing of Pb-free solder alloys are discussed by comparing intermetallic compounds formed during interfacial reactions with UBMs. In addition, the microstructural changes as well as the microhardness of four solders with or without UBMs are discussed, which could be related to their undercooling behaviors.
I. INTRODUCTION
Sn-based, near eutectic binary or ternary solder alloys are promising Pb-free candidates to replace Pbcontaining solders in electronic packaging. These include Sn–0.7Cu, Sn–3.5Ag, and Sn–3.8Ag–0.7Cu (in wt% unless specified otherwise). The extensive searches for Pbfree solder alloys in the last several years were conducted to produce reliable Pb-free solder joints.1–5 Because most Sn-based Pb-free solders consist of more than 90 wt% Sn and minor amounts of alloying additions such as Cu and Ag, their physical, chemical, and mechanical properties are heavily influenced by the properties of pure Sn.5 The undercooling behavior of Sn-based Pb-free solders is also influenced in this way. Undercooling implies that solidification does not occur even though the liquid phase passes its equilibrium phase-transformation temperature. In other words, the solidification does not occur at the equilibrium melting point but at a relatively lower temperature. This is attributed to the difficulty in nucleating a solid phase from a liquid phase. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0133 J. Mater. Res., Vol. 23, No. 4, Apr 2008
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In the case of Sn without any impurities, the maximum undercooling observed was 107–120 K.6,7 The amount of undercooling in Sn-based Pb-free solders reported by recent works is about 30 °C, which is much larger than the undercooling observed in Pb-rich solders.8–10 The large amount of undercooling in Ag-containing Pb-free solders such as Sn–3.5Ag and Sn–3.8Ag–0.7Cu can cause and accelerate the formation of large Ag3Sn plates because it provides a longer growth time for the proeutectic Ag3Sn plates in a supersaturated liquid during co
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