Layered intermetallic compounds and stress evolution in Sn and Ni(P) films
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In situ measurements of stresses due to the phase transformation in Sn and Ni(P) films were analyzed relating to the formation of layered intermetallic compounds such as Ni3Sn4, Ni3Sn2, Ni3P, and the crystallization of Ni(P) films. When Sn/Ni(11.7P) films were heated up to 480 °C, the first tensile stress developed due to formation of Ni3Sn4 and Ni3P around 220 °C, and the second one appeared at 335 °C due to formation of Ni3Sn2 as well as the self-crystallization of Ni(11.7P). For Sn/Ni(3P), a tensile stress developed mildly with the temperature between 300 and 410 °C due to formation of Ni3Sn2 and precipitation of Ni3P. The onset temperatures of self-crystallization of Ni(P) and Ni3P precipitation decreased due to the Ni–Sn reaction.
I. INTRODUCTION
Electroless Ni films as under bump metallurgy (UBM) have attracted much interest as an interconnect material with Sn-based solders. Electroless plated Ni films inherently contain phosphor because of their typical deposition process, and the P content decreases with the pH of the plating solution.1,2 The Ni(P) films in the as-prepared state have nanocrystalline structure at low P content (9 wt%).2–5 Amorphous Ni(P) films are usually used in microelectronics packaging. With the presence of Sn, the reaction between Sn and Ni(P) results in the formation of Ni3Sn4, which subsequently enhances the crystallization of Ni(P) by making the Ni(P) layer a P-rich layer and then forming a Ni3P layer. It is known as reactionassisted crystallization.6 Interconnect materials between chips and substrates always undergo several thermal cycles due to packaging processes such as solder reflows. Many intermetallic compounds (IMCs) are produced during these packaging processes, which affect the solder joint reliability.7 This is why most of the previous work on the Ni(P) or Ni UBM has been focused on the kinetics of IMC formation such as Ni3Sn4, Ni3Sn2, Ni3Sn, and Ni3P and their effects on joint reliability.8–10 The intermetallic compounds are brittle and are frequently associated with sites of brittle failure. As such, the residual stress related to the formation of IMCs is an important factor in solder joint and
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0258 J. Mater. Res., Vol. 22, No. 7, Jul 2007
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package reliability. Both compressive and tensile intrinsic stresses can result when phase transformation alters material densities and thus generates intrinsic stresses in the films on substrates.3 Previously, it was reported that the intrinsic tensile stress evolved due to the crystallization of Ni(P) films3 and the tensile or compressive stress developed due to the phase transformation of forming Ni3Sn4 and Ni3P phases between Ni(P) and Sn layers11 where the stress evolutions could be used to study their phase transformation kinetics.12 In the present work, the roles of diffusioncontrolled phase transformations were experimentally investigated for formation of the multilayered IM
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