Electromigration in flip-chip solder joints: Effect of temperature on failure mechanism and analysis of bump resistance curves
| Autor: | Lin, Chung-Kuang, 林宗寬 |
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| Rok vydání: | 2014 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 102 Temperature-dependent electromigration failure was investigated in solder joints with Cu metallization at 126°C, 136°C, 158°C, 172°C, and 185°C. At 126°C and 136°C, voids formed at the interface of Cu6Sn5 intermetallic compounds and the solder layer. However, at temperatures equal 158°C or greater than, extensive Cu dissolution and thickening of Cu6Sn5 occurred, and few voids were observed. We proposed a model considering the flux divergence at the interface. At temperatures below 131°C, the electromigration flux leaving the interface is larger than the in-coming flux. Therefore, voids formed at the interface. Yet, the in-coming Cu electromigration flux surpasses the out-going flux at temperatures above 131°C. This model successfully explains the experimental results. This study also examines the formation of Sn-rich phases in the matrix of Cu-Sn-Ni intermetallic compounds (IMCs) after current stressing of 1.2 × 104 A/cm2 at 160°C. The Sn-rich phases were formed at the cathode end of the solder joints with Cu metallization, and this formation was attributed to the decomposition of Cu6Sn5 IMCs. When the Cu6Sn5 IMCs were transformed into Cu3Sn during current stressing, Sn atoms were released. Due to the insufficient supply of Cu atoms, Sn atoms accumulated to form Sn-rich phases among the Cu-Sn-Ni IMCs. Resistance curves play a crucial role in detecting damage of solder joints during electromigration. In general, resistance increases slowly in the beginning, and then rises abruptly in the very late stage; i.e., the resistance curve behaves concave-up. However, several recent studies have reported concave-down resistance curves in solder joints with no satisfactory explanation for the discrepancy. In this study, electromigration failure mode in Sn2.5Ag solder joints was experimentally investigated. The bump resistance curve exhibited concave-down behavior due to formation of IMCs. In contrast, the curve was concave-up when void formation dominated the failure mechanism. Finite element simulation was carried out to simulate resistance curves due to formation of IMCs and voids, respectively. The simulation results indicated that the main reason causing the concave-down curve is rapid formation of resistive Cu6Sn5 IMCs in the current-crowding region, where resistivity is nine times larger than that of Cu. Therefore, when Cu reacted with Sn to form Cu6Sn5 IMCs, the resistance increased abruptly, resulting in the concave-down resistance curve. Cu3Sn was constantly found in the solder joint after current stressing. In this study, two different types of Cu3Sn formed according to the stressing temperature of solder joints. The solder joint was under 1.30 × 104 A/cm2 current stressing test at 170°C, the solder joint could transform to layer Cu3Sn joints. However, when the stressing temperature increased to 222°C and the current density was 2.27 × 104 A/cm2, an interesting porous Cu3Sn formed at the solder joint. The formation mechanism of porous Cu3Sn, Could be explained by the phase transformation and side wall wetting phenomenon. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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