Microstructure evolution during electromigration in eutectic SnPb solder bumps
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A technique has been developed to facilitate analysis of the microstructural evolution of solder bumps after current stressing. Eutectic SnPb solders were connected to under-bump metallization (UBM) of Ti/Cr-Cu/Cu and pad metallization of Cu/Ni/Au. It was found that the Cu6Sn5 compounds on the cathode/chip side dissolved after the current stressing by 5 × 103 A/cm2 at 150 °C for 218 h. However, on the anode/chip side, they were transformed into (Nix,Cu1−x)3Sn4 in the center region of the UBM, and they were converted into (Cuy,Ni1−y)6Sn5 on the periphery of the UBM. For both cathode/substrate and anode/substrate ends, (Cuy,Ni1−y)6Sn5 compounds were transformed into (Nix,Cu1−x)3Sn4. In addition, the bumps failed at cathode/chip end due to serious damage of the UBM and the Al pad. A failure mechanism induced by electromigration is proposed in this paper.
I. INTRODUCTION
Flip-chip technology has become one of the most important packaging methods in IC packaging. One of its advantages is that a large number of very small solder bumps can be fabricated into an area array on a chip as input/output (I/O) interconnections. To meet the performance requirements, the I/O number keeps increasing. Thus the size of the bumps progressively shrinks, and electromigration has become an important reliability issue for flip-chip packages.1,2 Eutectic SnPb solder has been adopted for the interconnection due to its low melting point and excellent mechanical properties. Nevertheless, because of its low melting point, the flip-chip SnPb bumps can fail at the stressing condition of 5 × 103 A/cm2 at 150 °C.3 Therefore, electromigration behavior of eutectic SnPb solder on various UBM has been studied.4–8 In addition, electromigration failure mechanism has been proposed.7,8 Due to smaller opening on the chip side of the flip-chip bump, the failure occurs predominantly in the chip side. Thus the under-bump metallization (UBM) on the chip side plays an important role in the electromigration failure. However, the microstructure evolution for the SnPb solder on the Cr/Cr-Cu/Cu UBM at the failure site during the stressing process is still unclear. Furthermore, all the previous studies used cross-sectional observation, which
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0305 2394
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 8, Aug 2004 Downloaded: 13 Mar 2015
can observe only part of the contact openings. In this study, the plan-view technique was used to examine the microstructure of the contact openings, which enables the panoramic observation of the contact openings of the chip side and substrate side. Thus it provides better understanding of the microstructure evolution during current stressing and of the electromigration failure mechanism. II. EXPERIMENTAL
Flip-chip packages of eutectic SnPb solder bumps were prepared as described next. The UBM consisted of 0.7-m Cu, 0.3-m Cr-Cu, and 0.1-m Ti. Eutectic SnPb solder paste was printed and deposited on the UBM of the wafer.
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