Thermal expansion and elastic properties of high gold-Tin Alloys

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I.

INTRODUCTION

ALLOYS of gold and tin have been used in the semiconductor industry for approximately 30 years. [~,2'31Because of its relatively low eutectic temperature (278 ~ the alloy at 29 at. pct Sn has been most useful for package sealing and die attachment. Until 1974, this alloy was thought to comprise two intermetallic compounds, at the approximate stoichiometry AusSn and ~ at AuSn. Although there were few apparent problems in using this alloy, Bernsteintl'2] reported an unusual dip in the thermal expansion coefficient vs temperature curve. Bemstein was concerned that this dip was caused by phase transformation and that the phenomenon could cause cracking problems. The details of his thermal expansion measurements were not given, but his results are shown in Figure 1, where the dip occurs at approximately 200 ~ He went on to show that a 2 pct substitution of indium for tin resulted in total suppression of this thermal expansion dip. Giessent4] found considerable metastable phase activity near the ~ composition in splat-cooled alloys. He also observed a variant of the ~ phase which he referred to as ~,. Ishihara e t al. tSl used rapid quenching techniques to identify several metastable phases in the Au-Sn system, but it is unlikely that any of those metastable phases could cause problems in the semiconductor industry. In 1974, Osada et al. t6j discovered that the hexagonal ~ phase undergoes an order-disorder phase transformation at approximately 190 ~ The ordered hexagonal phase, ~', can be seen in the most recent phase diagram for the Au-Sn system, tT] It seems clear that the thermal expansion anomaly that concerned Bernstein was due to the ~ ~' transformation. Since the alloy is often used for sealing electronic packages, this phase transformation may jeopardize hermeticity. This concern indicates that Bernstein's measurements should be confirmed even though the current use of the alloy seems trouble-free. Reported here are measurements of linear thermal expansion coefficients, densities, and elastic moduli. The measurements were made at temperatures ranging from F.G. YOST, M.M. KARNOWSKY, W.D. DROTNING, and J.H. GIESKE, Technical Staff Members, are with Sandia National Laboratories, Albuquerque, NM 87185. Manuscript submitted October 13, 1989. METALLURGICAL TRANSACTIONS A

23 ~ to 250 ~ on alloys prepared at the ~-hase, eutectic, and &hase compositions. These findings are interpreted in terms of the current version of the Au-Sn phase diagram.

II.

EXPERIMENT

Three Au-Sn alloys were prepared for the measurement of density, coefficient of linear thermal expansion, elastic moduli (both longitudinal and transverse), and Poisson's ratio. The alloys were prepared with the intention of obtaining the ~ phase at AuSn, the eutectic composition at 29 at. pct Sn, and the 8' phase at AusSn. High-purity (99.9+) Sn and Au constituents were sealed in 10-mm-diameter quartz fiat-bottomed tubes under a vacuum of 1.3.10 -3 Pa. The samples weighing approximately 20 grams each were melted three times, with tube inversion

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