Corrosion of Ga-doped Sn-0.7Cu Solder in Simulated Marine Atmosphere
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Sn-Pb solder alloys had been widely used for a long time in the electronic packaging industry due to their excellent physical and mechanical properties as well as low cost.[1] Because of the high toxicity of Pb, in terms of harm to human health and the environment,[2,3] Sn-Pb solders have been gradually banned by many industrial countries since 2004. Many lead-free solders have replaced the traditional Sn-Pb solder.[4] Among them, Sn-0.7Cu alloy is a promising alternative because of the low cost and good mechanical properties.[5] It has recently come into significant commercial use, especially for wave soldering.[6] Since the emergence of lead-free solders, many factors which may affect the properties and reliability of solder joints, such as mechanical properties,[7] interface reaction,[8,9] whisker growth,[10–16] electromigration,[17] oxidation,[18–20] etc., have been extensively studied. However, few studies were reported with respect to the corrosion properties of the lead-free solders,[21–24] although corrosion is of great importance to the reliability of microelectronic devices used in a corrosive environment. Generally, Sn-based leadfree solders exhibit an outstanding corrosion resistance in normal environments due to the protective oxide film; but, when the alloys are exposed directly to corrosive media such as salty marine environment, corrosion may take place because of the moisture and halogen ions.[25] In addition, corrosion may lead to the tin whisker growth and to the decrease of the mechanical properties.[26,27] ZHONG YAN, Engineer, is with the Shenyang National Laboratory for Material Science, Institute of Metal Research, Chinese Academy of Science, 72 Wenhua Road, Shenyang, 110016, P.R. China, and also with the Shenzheng Huawei Technol. Inc., Shenzheng, P.R. China. AI-PING XIAN, Professor, is with the Shenyang National Laboratory for Material Science, Institute of Metal Research, Chinese Academy of Science. Contact e-mail: apxian@ imr.ac.cn Manuscript submitted January 9, 2012. Article published online November 10, 2012 1462—VOLUME 44A, MARCH 2013
Some researchers[28,29] have studied the corrosion behavior of Sn-Ag-Cu solders, but the results are controversial when they are compared with those obtained for Sn-Pb solder. A preliminary study of the corrosion properties of Sn-0.7Cu solder in a marine atmosphere had been also carried out previously by the present authors,[30] and the results showed that the corrosion resistance of the alloy was poor and needed to be improved, perhaps by doping with a trace element such as Ga. In the research described herein, salt spray testing was used to study the corrosion behavior of Sn-0.7Cu solder and the effect of trace amounts of Ga and P in the alloy on the corrosion behavior. The results may be taken as a reference to choose lead-free solders used in electronic devices equipped in marine facilities. II.
EXPERIMENTAL PROCEDURES
A. Sample Preparation Commercially pure tin (better than 99.90 pct purity) and pure copper (99.99 pct purity) were melted together
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