Tin whiskers of bulk solders generated under resonance
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Truan-Sheng Lui, Yea-Luen Chang, and Li-Hui Chen Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, Republic of China (Received 24 November 2004; accepted 18 March 2005)
In this study, we investigated the vibration properties of Sn–Zn and Sn–Zn–Al alloys. It is notable that, with a small amount of Al addition, Sn whiskers were generated during resonant vibration. This may be the first time that Sn whiskers were found on bulk materials. Microstructural refining and hardening caused by alloying of Al stunted the fiberous type Sn deformation within Sn–Zn eutectics. Vibration-induced extrusion of Zn-free regions in the vicinity of cell boundaries may account for the appearance of the whiskers.
The development of lead-free solders is currently of great interest to the electronic packaging industry due to the ban on the usage of lead. Sn–Zn eutectic alloy has recently been considered as a candidate for a lead-free solder material because of its low melting point (198 °C), excellent mechanical properties, and low cost.1,2 However, since Zn-containing alloys have disadvantages related to oxidation and wetting,3,4 new Sn–Zn based alloys are now under development. It has been reported that the addition of Al to the Sn–Zn system reduces hightemperature oxidation and improves the wettability.5,6 However, deterioration in tensile properties of a Sn–Zn– Al alloy with Al content of 0.5 wt% occurred with prolonged natural aging.7 Given that failure may occur due to vibration when solder joints are assembled, e.g., in vehicles and aircraft,8–10 especially when the vibration frequency meets the resonance, the vibration fracture resistance should be taken into consideration in the alloy design of lead-free solders. The objective of this study is to explore the effect of Al additions on the resonant vibration behavior of Sn–Zn–Al alloys. Near-eutectic Sn–Zn alloys with Al content of 0, 0.2, and 0.5 wt% were prepared (designated as Sn–Zn, Sn–Zn–0.2Al and Sn–Zn–0.5Al, respectively) and cast into a Y-shaped graphite mold. All specimens were naturally aged at room temperature for 30 days to stabilize the microstructure before testing. The microstructural characteristics were analyzed quantitatively with an image analyzer and Vickers hardness tester. Each data point was the average of 15 tests. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0195 J. Mater. Res., Vol. 20, No. 6, Jun 2005
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A simple cantilever beam vibration system, shown schematically in Fig. 1(a), was used for the vibration experiments. The test specimen, rectangular with dimensions 20 × 100 × 4 mm was mounted and fixed on end to the vibration shaker [Fig. 1(b)]. Two V-notches near the clamp were made to induce stress concentration. The vibration force was monitored using an acceleration sensor, and the deflection amplitude of the specimen at the end opposite the vibration shaker was recorded by a deflection sensor
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