Solid-state reactions and stress evolutions between SnAg and Ni(P) thin films
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Jin Yu Center for Electronic Packaging Materials, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Yuseong-gu, Daejeon 305-701, Korea (Received 17 June 2008; accepted 31 October 2008)
Phase transformations in SnAg–Ni80P20 films were studied ex situ in parallel with in situ measurements of the corresponding transformation-induced stresses. Layered formation of Ni3Sn4 and Ni3P phases at an early stage of a reaction between SnAg and Ni80P20 films resulted in a tensile stress similar to the stress evolution in Sn–Ni80P20 films, despite the additional formation of Ag3Sn phase. Ag3Sn phase did not significantly affect the degree of stress evolution because of its islandlike and sporadic formation on the top surface of the Ni3Sn4 layer. Isothermal annealing showed that compressive stress, which was induced by the dominant formation of Ni3Sn4, developed after an initial evolution of tensile stress.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
Lead-free solder bumps, such as Sn–Ag, Sn–Cu, and Sn–Ag–Cu, can be produced via electroplating methods and used in applications related to flip-chip packaging,1–6 wherein low-cost electroless Ni(P) films are used as under bump metallization (UBM).1,3,7–9 Many studies on the reaction between UBM [Cu, electroless Ni(P)] and solders, such as PbSn, Sn–Ag, Sn–Cu, and Sn–Ag–Cu, have been conducted in efforts to understand the kinetics of the formation of intermetallic compounds (IMC) as well as their microstructures, as these factors relate to the reliability of solder joints.2,3,6,10,11 It is well known that IMC, such as Ni3Sn4, Ni3Sn2, Ni3Sn, and Ni3P, can be produced in Sn–Ni(P) or Sn–Ni systems.8–10,12–14 Given that IMCs are brittle and often associated with fracture sites, residual stresses related to the formation of IMCs at solder joints are important.10,11 Previously, it was reported that intrinsic tensile stress develops during the crystallization process of amorphous Ni(P) films.15 Additionally, it was found that the formation of Ni3Sn4 and Ni3P in a Sn–Ni(P) system caused the evolution of both compressive and tensile stress, respectively.16,17 In this work, the effects of an alloy element on transformation-induced stress evolutions in SnAg–Ni (P) films are presented in association with microstructural changes in the IMCs of Ni3Sn4, Ni3P, and Ag3Sn.
Electroless Ni(P) films were deposited over Al films that were sputter-deposited onto [100] Si wafers (8 mm 25 mm). SnAg films were subsequently electroplated onto the Ni(P) films. The thicknesses of the Sn(or SnAg)/Ni(P)/Al/Si layers were approximately 0.5, 1.8, 0.7, and 400 mm, respectively, and the thickness of the SnAg films was approximately 1.5 mm for a thermal aging experiment. Ni(P) film containing nearly 20 at.% phosphorous15 is termed Ni80P20 in this work. Details of the deposition procedures of Ni80P20 and Sn films were sourced from available studies.15–17 The electroplating solution DIPSOL TS-3000 (DIPSOL Chemical Co., Tokyo, Japan) was used to deposit the SnAg films
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