Oxidization-Induced Tin Whisker Growth on the Surface of Sn-3.8Ag-0.7Cu-1.0Er Alloy
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I.
INTRODUCTION
ELECTRONIC components industries are undergoing a transition from Pb-containing electronic products to environmentally friendly lead-free electronics. The drive to eliminate lead from electronics has resulted in renewed concern regarding the phenomenon of tin whiskering, because lead is very effective in prohibiting the nucleation and growth of tin whiskers.[1] Though the phenomenon of tin whisker growth on a tin alloy was first reported by Hunsicker and Kempf in 1947, questions such as how a tin whisker forms and grows and what the growth mechanism is are still being researched and discussed today.[2–6] A tin whisker is a single crystal of tin that grows spontaneously from the surface of plated cover or the surface of tin-based solder joints. They are typically only a few microns (lm) in diameter, but can grow to lengths from a few microns to tens of microns. The whiskers can break loose, causing mechanical damage in slip rings, optical components, or microelectro-mechanical systems (MEMS). Also, in low-pressure environments, it is possible for arcing to occur from the tin whisker to an adjacent conductor, causing serious failures in avionics. Furthermore, with the improvement of the packaging density of electronic products, problems caused by tin whiskers will become even more serious, and they push us to study and search a more effective way to inhibit the growth of tin whiskers. In most cases, the spontaneous growth rates of tin whiskers are about 0.01 to 0.1 A˚ s 1, as summarized in the literature.[7] Under such a low growth rate, some important phenomena exhibited by the tin whiskers during their growth are scarcely captured by researchers. In the present work, an amazingly rapid whisker growth was observed on the surface of the oxidized ErSn3 phase in the Sn3.8Ag0.7Cu1.0Er solder joints. It was evidenced that after storage at room temperature HU HAO, YAOWU SHI, ZHIDONG XIA, YONGPING LEI, and FU GUO, Professors, are with the School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, People’s Republic of China. Contact e-mail: [email protected]. edu.cn Manuscript submitted June 27, 2008. Article published online June 13, 2009 2016—VOLUME 40A, AUGUST 2009
for several hours, the tin whiskers began to form and grow at a high rate of 0.7 A˚ s 1. However, tin whiskers formed much earlier during storage at 150 C, and the maximal growth rate of the tin whiskers reached about 1000 A˚ s 1. Furthermore, some new whiskers with special morphology were also observed in the experiment, and they are very useful in further understanding the growth mechanism of the tin whisker.
II.
EXPERIMENTAL
The material investigated was Sn-3.8Ag-0.7Cu-1.0Er solder, and its preparation process was the same as that in a prior work.[8] The ingot was cold-rolled into a 0.3-mm-thick sheet and cut into a disk with a weight of 0.2 g. The solder disks were dipped in ZnCl2-NH4Cl (22 wt pct ZnCl2 + 2 wt pct NH4Cl + 76 wt pct deionized water) flux and then placed on the Cu substrates with dimensi
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