Cruciform pattern formation in Sn/Co couples
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CoSn3 phase is formed in Sn/Co couples reacted at 200 °C. The reaction phase shows a unique cruciform pattern that is observed for the first time in the solid/solid reaction. The reaction phase layers are thick and uniform along the edges of the Co substrate, and there are no reaction phases at the corners. A continuous reaction layer is formed in Sn/Co couples reacted at 180 °C. A metastable CoSn4 phase is formed at the corner, and the reaction phase along the edge of the Co substrate is the CoSn3 phase. The reaction CoSn3 phase region shows a cruciform pattern if the CoSn4 phase region is ignored. It is concluded that the Sn flux is much faster than the Co flux, and the cruciform pattern of the reaction CoSn3 phase layer is formed either by cracking or transformation to the CoSn4 phase at the corners where stresses are most intensified.
I. INTRODUCTION
The emergence of Pb-free solders is one of the most important issues in the electronic industry. Co and Co alloys have been examined for their applicabilities as Pb-free solders and as under-bump metallization materials for Pb-free solders.1–6 Our recent study of the interfacial reactions of solders with Co alloys indicates a possibility of cruciform pattern formation in Sn/Co couples. Cruciform pattern formation7–12 is an interesting phenomenon referring to the formation of interfacial reaction phases with the unique shape of a crucifix. The unique cruciform patterns were observed in gas/solid and liquid/ solid reaction systems7–12; however, no cruciform pattern resulting from solid/solid reaction has ever been found previously. This work studies the Sn/Co solid/solid reactions. In addition to the observations of the cruciform pattern formation, various kinds of Sn/Co couples are examined and the Sn/Co interfacial reaction mechanisms are investigated. II. EXPERIMENTAL
The Sn/Co couples were prepared with pure Sn shots (99.99 wt% purity, Kojundo Chemical, Saitama, Japan) and Co foil (99.95 wt% purity, Alfa Aesar, Ward Hill, MA). The proper amount of Sn was weighed, placed in a graphite mold, and melted in a furnace at 280 °C. The Co foil with a dimension of 0.45 mm × 0.6 mm × 15 mm was immersed in the molten Sn, and the sample was rapidly
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0422 3404
http://journals.cambridge.org
J. Mater. Res., Vol. 22, No. 12, Dec 2007 Downloaded: 13 Mar 2015
quenched with cold water. The Sn/Co reaction couple of 1 mm thickness was cut from the quench ingot, and it was a couple with one rectangular Co in the center encircled with Sn on the outside. For better observation of the stress influence on interfacial reactions, a “thin-strip” type reaction couple was prepared. A Co stripe of 0.15 mm × 0.5 mm × 10 mm was attached on a glass, immersed into molten Sn in a graphite mold, and then quenched immediately with water. The glass was then removed from the specimen, and the flat surface of the original glass side was taken as the bottom of the specimen. The upper part of the speci
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