In-situ observation on microfracture behavior ahead of the crack tip in 63Sn37Pb solder alloy
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I. INTRODUCTION
SOLDER joint reliability during service is important for prolonged longevity of a printed circuit board. Since the solder is softer than other components, most of the strains and failures take place in the solder. Therefore, the mechanical behavior of the solder alloys is necessary to be clearly understood for developing the reliability assessment of electronic packaging. Eutectic Sn-Pb solder has been widely used for electrical contacts because of their low melting temperature, good wettability on copper, good plasticity, and reasonable electrical conductivity.[1] Excessive studies have been done on the failure mode and fracture mechanism of the Sn-Pb eutectic alloy.[1–5] Grain boundary sliding was found to be the dominant mechanism at lower strain rates (103 s1) and caused the initiation of intergranular cracks on the free surface of 63Sn37Pb tensile test specimens. However, almost all the studies were carried out using scanning electron microscopy (SEM) that has a low spatial resolution. As a consequence, unfortunately, the fracture behavior in nanometer scale in the solder was rarely mentioned in these documents, even though it is crucial to clarify the microcharacteristics of the failure. Transmission electron microscopy (TEM) provides a spatial resolution high enough for the dislocation behavior in the material to be observed directly. In fact, since the early 1980s, the in-situ tensile tests under the TEM had been performed to study the microcrack nucleation and propagation.[6,7,8] The detailed experimental research work was focused on the brittle and traditional ductile materials.[6–12] Results indicated that cleavage cracks would continuously propagate, and the discontinuous voids resulting from the blunting of the microcracks in the dislocation-free zone (DFZ) would link with each other and with the main crack, leading to ducYING DING, Ph.D. Candidate, CHUNQING WANG, Professor, YANHONG TIAN, Lecturer, and BINBIN ZHANG, Master’s Candidate, are with the Microjoining Laboratory, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, People’s Republic of China, 150001. Contact e-mail: [email protected] or [email protected] Manuscript submitted July 15, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
tile fracture. However, for the visco-plastic material, such as the 63Sn37Pb solder alloy, which has obvious creep properties due to its high homologous temperature (T/Tm 0.5) at room temperature, there was a lack of understanding of the physical process ahead of the crack tip. It seems there is a black box encumbering the further comprehension on the fracture mechanism in solders. On the other hand, during the service process, both the components and solders would be aged under frequent thermal operations. The high-temperature storage experiments were always performed in the research to accelerate the aging process. The goal of the present work was to check the dislocation behavior ahead of the crack tip in the 63Sn37Pb solder alloys under the as-cast and aged co
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