Preferential growth and orientation relationship of Ag 3 Sn grains formed between molten Sn and (001) Ag single crystal
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The current study shows that there is a preferred orientation relationship between Ag3Sn and Ag in reaction between molten Sn and Ag. Due to the preferred orientation relationship, the morphology of Ag3Sn grains formed on (001) Ag single crystal is different from those formed on (011), (358) single crystal Ag, and polycrystalline Ag Facet scallop-type Ag3Sn grains formed irregularly on (011), (358) single crystal Ag, and polycrystalline Ag; whereas the regular Ag3Sn grains with parallel edges grew on (001) Ag single crystal, and they were elongated along two perpendicular directions. The orientation relationship between Ag3Sn grains and (001) Ag single crystal was determined using electron backscattered diffraction. The preferential growth of regular Ag3Sn grains with parallel edges formed on (001) Ag single crystal can be attributed to their minimum misfit.
I. INTRODUCTION
Sn-Pb solders would be replaced by the lead-free solders in electronic packaging field because of the toxic nature of Pb.1,2 At present, Sn-Ag, Sn-Cu, Sn-Zn, and SnBi alloys are the most promising lead-solders, which are potential candidates of lead-free solders in electronic packaging field in the future.2 These soldering joints provide the electrical connection and the mechanical bonding. But intermetallic compounds (IMCs) always form at the interface between the solders and substrates during the reflow procedure.2–5 Therefore, the solders as well as the morphologies of the formed IMCs should play important roles in the electrical connection and the mechanical properties. However, nearly all of the investigations only focus on the morphology of IMCs formed on polycrystalline substrates, mainly on Cu substrate so far.6 More recently, new findings show that the Cu6Sn5 grains with strong texture and special morphology formed on the (001) and (111) Cu single-crystal substrates.7–9 The morphologies, orientation relationships, and evolution of Cu6Sn5 grains formed between molten Sn and Cu single crystals have been carefully investigated in these research studies.7–9 It is indicated that the nucleation, growth, and ripening of Cu6Sn5 grains on Cu single crystal are different from those on polycrystalline Cu. However, in electronic packaging field, in addition to the Cu and Ni substrates, Ag is also a good substrate due to its good wettability, antioxidation, and electronic and thermal conductivity.10–13 This process raises an a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0237 J. Mater. Res., Vol. 24, No. 6, Jun 2009
http://journals.cambridge.org
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interesting question: whether the special morphology and the preferential orientation of Ag3Sn IMC can appear on Ag single-crystal substrate with low index. In this article, to avoid the effect of other elements, the pure Sn/(001) Ag single-crystal couple was used as an example to decipher the question above. II. EXPERIMENTAL PROCEDURE
In this study, Ag single crystal was used as a substrate and Sn foil was used as a solder. First,
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