Creep deformation characteristics of tin and tin-based electronic solder alloys
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I. INTRODUCTION
LEAD-BEARING alloys, particularly lead-tin eutectic, have been the popular solder materials for interconnection and packaging of commercial microelectronics assemblies due to their favorable processing, good mechanical properties, and lower cost. For the past several years, concerns on the toxicity of lead, coupled with governmental legislative pressures, have stimulated research in developing lead-free solder materials for electronic packaging applications.[1,2] Despite extensive research on several alloy systems, no real substitute has so far been found as a drop-in replacement. Sn-3.5Ag eutectic and Sn-5Sb alloy have been identified as two of the potential alternate materials for replacing Pbbearing alloys.[3] Sn-3.5Ag alloy has better fatigue resistance among more than 30 common soft solders,[4] while Sn-Sb alloy has superior creep resistance.[5,6] The contact angle for tin-antimony alloys is higher (43 deg 4) than that of leadtin eutectic (17 deg 4), but is considered adequate from wettability considerations. During normal service conditions, the low melting solder alloys are subjected to creep and fatigue deformation. Ambient operating temperature and local heating during operation correspond to high homologous temperatures (as high as 0.7) for these alloys.[7] The maximum interconnect temperatures can reach as high as 350 K; automotive underhood and mainframe and supercomputer applications impose much higher operating temperatures. Fatigue deformation occurs due to induced stresses from differential thermal expansion between the substrate and the solder joints, and so thermal fatigue is also a major cause of concern. In this article, we discuss our recent results on the creep deformation characteristics of tin, Sn-3.5Ag, and Sn-5Sb alloys. In addition to the creep studies on bulk specimens, single lap shear tests were performed on solder joints, with a configuration of a 33 33 solder bump array
on a dummy silicon chip. Creep results obtained at room temperature under shear loading conditions on the bump array are compared with the bulk tensile creep data. II. MATERIALS AND EXPERIMENTAL METHODS Pure tin metal was obtained in the form of 10-mm-thick bars from Taracorp Imaco Inc., Sn-5Sb alloy in the form of 1-mm-thick sheet from Alpha Metals Inc., and Sn-3.5Ag alloy in the form of 1-mm-thick specimens from Indium Corp., Utica, NY. Specimens with a gage length of 25 mm, gage width of 4 mm, and gage thickness of 1 mm were used for the creep tests. Tin samples were first machined from the received block to the required dimensions, were polished, and were then annealed at 453 K for 30 minutes before creep testing (melting point of pure tin is 505 K). Sn-5Sb and Sn-3.5Ag alloys were received in the annealed condition and so these were tested in the as-received condition. Constant load creep tests were performed in air at 296, 327, 373, 423, 448, and 473 K, and at stresses varying from 1 to 30 MPa. A single specimen was used at each test temperature. Strain/time dependence of the creep rate w
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