Investigation of Joule heating effect in various stages of electromigration in flip-chip solder joints by infrared micro
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Investigation of Joule heating effect in various stages of electromigration in flip-chip solder joints by infrared microscopy Hsiang Yao Hsiao1, Chih Chen1*, D. J. Yao2 1 Department of Material Science and Engineering National Chiao Tung University, Hsinchu 300, Taiwan 2 Institute of Microelectromechanical System National Tsing Hua University, Hsinchu 300, Taiwan Tel: +886-3-5731814; Fax: +886-3-5724727 *Email: [email protected] Abstract The Joule heating effect at various stages under electromigration of flip-chip Sn3.5Ag solder joints was investigated under a current of 0.5 A at 100°C. During various stages of electromigration, voids may form and propagate and Joule heating effect may vary at different void sizes. To verify the void nucleation and propagation on Joule heating effect during electromigration process, the solder bump was stressed for different lengths of time and then examined by Kelvin bump probes and infrared microscopy. We found that voids started to form at approximately 1.2 times of the initial bump resistance. Then the voids propagated when the bump resistance increased. In addition, the temperature of the solder joints increased with the bump resistance and the increase of current stressing time. It increased very slowly in initial stages. In the last stage, the temperature of the solder bump increased rapidly due to the increase of the bump resistance and the local Joule heating effect. 1. Introduction With the rapid increase of current density and the higher requested performance for portable devices, flip-chip has been adopted due to its excellent electrical performance and better heat-dissipation ability [1]. Electromigration has becomes an important issue for flip-chip solder joints in reliability [2]. As the demand of high performance of electronic applications increases, the current density will carry 4 to 6 times in the future [4]. Under the high applied current, Joule heating effect in the solder bumps also becomes significant in the accelerated electromigration test. Because of the current crowding effect during electromigration, voids nucleate in the solder bump near the entrance of electronic flow. With the stressing time increased, the voids grew and propagated to the interface of the under bump metallization (UBM) and solder. However, there are only a few studies on the Joule heating effect in the solder joints during electromigration [3-6]. In this work, thermal infrared (IR) microscopy and SEM were used to measure the temperature distribution in the solder joint and the void appearance under electromigration. This study provides a clear understanding of the Joule heating effect on electromigration of solder joints during current stressing. 2. Experimental Fig. 1 shows the schematic diagram of the SnAg3.5 solder joint used in this study. The bump height is 70 ȝm and the diameter of the under-bump-metallization (UBM) opening is 120 ȝm on the chip side. The UBM consists of 0.1-Pm Ti/5.0-Pm Cu. The joints were polished laterally to approximately their centers, a
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