Oxidation and reduction behavior of pure indium
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Fundamental knowledge on the oxidation behavior of pure indium, commonly used as a low-temperature, fluxless soldering material in micro-electro-mechanical system (MEMS) devices, is of importance as it influences the solder joint reliability. A thermodynamic model of the oxidation and reduction behavior of indium is developed by constructing an Ellingham diagram, and by using H2(g) reactions. Partial pressure (p) of H2O was shown to be the critical parameter in creating a reducing environment in the applicable solder reflow temperature range. Verification of the thermodynamic models was then carried out through heating and melting of indium in controlled glove box environments by adjusting p(H2)/p(H2O). The nanometer scale thickness of the oxide layer grown on indium was measured by a spectroscopic ellipsometer. The growth mechanism for oxidation in air below 220 C follows Uhlig’s logarithmic law where electron transport is the rate-controlling mechanism, implying that there is an incubation period for the onset of initial oxidation. Its activation energy was found to be 0.65 eV. I. INTRODUCTION
Pb-based solders have been used since the inception of the electronics industry because of their excellent mechanical properties and low solidification temperature. However, recent political initiatives such as those passed by the European Union to eliminate Pb-based solder have led the industry to look for less-toxic alternatives.1,2 One such alternative is pure indium and indium alloy solders. Pure indium has a specific niche in compression bonding where long elongation is required.3 It also has been used in applications sensitive to large thermal expansion mismatches due to its ability to accommodate large plastic strains.4 Since pure indium is known to oxidize in air, flux is used to prevent oxidation, which adversely affects both wetting and the overall joint strength.5–7 In applications where flux cannot be used because of contamination and reliability issues, a carefully controlled environment must be used to control the oxidation behavior according to thermodynamic equilibrium. In the study by Kim et al., the effect of oxidation on solder joint reliability was examined by performing wetting angle and lap shear experiments.8 The influence of oxidation on wetting angle was quantified by reflowing indium samples with predetermined oxide thickness. Indium’s wetting angle on a Ti/Au pad was found to increase with thicker oxide layers, an indication of poor bonding. For a more direct measurement of joint a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0040
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 2, Feb 2009 Downloaded: 28 Mar 2015
strength, indium with various initial oxide thicknesses was reflowed between two silicon substrates with Ti/Au metallization and tested by lap shear specimens. Similar to the wetting angle experiment, joint strength was found to be adversely effected by oxide thickness, which ultimately limits joint reliability. Infor
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