Corrosion and Electrochemical Behavior of Sn-2Ag-0.5Cu Lead-Free Solders Solidified with Magnet Stirring
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JMEPEG https://doi.org/10.1007/s11665-019-04227-4
Corrosion and Electrochemical Behavior of Sn-2Ag-0.5Cu Lead-Free Solders Solidified with Magnet Stirring A.A. El-Daly, K.M. Zohdy, and M. Ragab (Submitted November 19, 2018; in revised form June 3, 2019) In this study, permanent magnet stirring (PMS) was applied to modify the crystallographic texture and corrosion resistance of Sn-2Ag-0.5Cu (SAC205) solders. The results indicate that PMS transforms the lamellar structure into equiaxed grains. Following the initiation corrosion process, the crystallographic texture shows a new class of grains that consists of single-crystal morphology with {111}Sn facets before applying PMS. This could result in severe corrosion of b-Sn crystals with non-uniform corrosion rate caused by the orientation effect of (111). The corrosion rate for SAC205 solder, measured by immersion tests in 3.5% NaCl solution at 25 °C, was significantly decreased from 31 to 8 mpy after applying PMS. This change in corrosion resistance of SAC205 solder can be attributed to the improved anodic and cathodic reaction activities caused by the modification of microstructure and crystallographic texture. The mechanism of pitting and sequential corrosion events is fully explained and discussed. Keywords
corrosion resistance, lead-free solders, microstructure, permanent magnet stirring
1. Introduction Because of the inherent toxicity and human health concerns of using lead, lead-free solders have been the materials of choice in electronic packaging industry (Ref 1, 2). Recently, several lead-free solder alloys have been recommended and developed for microelectronic industry. The interest in using Sn-Ag-Cu solders is mainly associated with their excellent mechanical property, relatively low coarsening rate, and high fatigue resistance (Ref 3, 4). This brilliant combination of such properties can be ascribed to the presence of fine structures attained by addition of little amount of alloying elements (Ref 5, 6). But the new alternative solder materials achieved by alloying elements could result in microstructural inhomogeneity and micro-galvanic influence of impurities and intermetallic compound (IMC) phases. Creating such deterioration reliability in solder joints of electronic devices would be a disadvantage (Ref 7, 8). In case of Sn-Ag-Cu alloys, the main challenge with eutectic Sn-Ag-Cu solder alloys is the presence of large amounts of Ag, and Cu, which leads to undesirable formation of discrete cathodic IMCs as Ag3Sn and Cu6Sn5 phases. Such IMCs are inert in chloride solution and display more passive behavior compared to the b-Sn matrix, thus allowing localized micro-galvanic coupling corrosion (Ref 9). Some researchers
A.A. El-Daly, Physics Department, Faculty of Science, Zagazig University, Zagazig, Egypt; K.M. Zohdy and M. Ragab, Higher Technological Institute, 10th of Ramadan City, Egypt. Contact e-mails: [email protected] and [email protected].
Journal of Materials Engineering and Performance
studied the effect of IMCs morphology during aging on the
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