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e Sn-3 wt pct Ag-0.5 wt pct Cu (SAC305) solder is one of the popular Pb-free solders for consumer electronics packaging. To further reduce its melting point of about 490 K (217 C), a small amount of bismuth (Bi) is doped into the SAC305 solder, that is, Sn-3 wt pct Ag-0.5 wt pct Cu-3 wt pct Bi.[1] The addition of Bi is also found to be able to lower the growth rate of intermetallic compounds formed at the joint interface.[2] As miniaturization of solder joints continues, electromigration becomes an inevitable reliability issue.[3] Electromigration refers to the phenomenon of directional atomic movement induced by current stressing,[4] which can induce significant microstructural changes (void, hillock formation, and phase segregation) and affect the growth of intermetallic compounds in the solder joints.[5] Electromigration of Bi-added SAC305 solder at current densities from 1 9 105 to 5 9 105 A/cm2 has been investigated.[6] It is found that electromigration induces Cu dissolution from the cathode-side Cu electrode into the solder, resulting in the formation of highdensity Cu6Sn5 phase across the entire solder after 72 hours of current stressing. This article reports an interesting and unique phenomenon of the Cu6Sn5

CHIH-FAN LIN, Ph.D. Student, SHANG-HUA LEE, Masters Student, and CHIH-MING CHEN, Professor, are with the Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan, R.O.C. Contact e-mail: [email protected] Manuscript submitted February 20, 2012. Article published online June 14, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

formation within the solder at the early stage of electromigration (approximately 24 hours). The Cu6Sn5 phase was found selectively within a specific Sn grain, exhibiting strong dependence on the crystallographic orientations of the Sn grains. A bilayer of Ti/Cu was deposited on a SiO2/Si substrate using sputtering. The thicknesses of Ti and Cu are 200 and 4000 A˚, respectively. The Cu layer was then grown to 15 lm thick using electroplating. Using photolithography and lift-off techniques, the Ti/Cu bilayer was patterned to a stripe with two square pads on both ends. The dimension of the stripe is 10 mm in length and 300 lm in width. Using photolithography and lift-off techniques for the second time, a trench with a length of 500 lm was patterned in the middle of the stripe. A tiny solder piece (Sn-3 wt pct Ag-0.5 wt pct Cu-3 wt pct Bi) coated with flux was placed in the trench. The sample was reflowed at 523 K (250 C) for 3 to 5 seconds and then cooled to room temperature. After cooling, the solder piece transformed into a bump in the trench and joined with the Cu stripe on its two ends. By careful polishing using ultrafine sandpapers and 0.3 lm Al2O3 suspension, the solder bump was polished down until the solder thickness was the same as the Cu stripe. The sample configuration and the experimental setup are schematically shown in Figure 1. The sample was placed on a hot plate. Then, the solder stripe was current stressed at a current density of 5 9 104 A