In conclusion, EM failure mechanisms in Pb-free solder joints and stress migration failure mechanisms in SnPb composite solder joints were investigated in this study. Both finite element analysis and theoretical analysis explained the experimental results well. When SnAg solder bumps with 2-μm Ni UBMs were stressed at 2.16 × 104 A/cm2 and 150 °C, open failure occurred in the bump that had electron flow direction from the chip side to the substrate side. However, when Sn-3.0Ag-0.5Cu solder bumps with Cu column UBMs were stressed at 2.16 × 104 A/cm2 and 150 °C, cracks formed along the interface of Cu6Sn5 IMCs and the solder on the substrate side. The 3-D simulation on current density distribution supports the contention that the current crowding effect was responsible for the failure on both the chip and the substrate side for the two kinds of solder bumps. Furthermore, being confirmed by IR microscopy, the alleviation of current crowding by the Cu column UBMs also helped decrease the Joule heating effect in the solder bump during current stressing. Therefore, the solder joints with Cu column UBMs have a higher EM resistance than the traditional flip-chip solder joints.
In addition, the EM failure mechanisms in SnAg solder joints with 5-μm-Cu/3-μm-Ni UBM were changed according to the flux divergence at the
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IMC/solder interface at various stressing conditions. It was found that both the void formation and the UBM consumption failure mechanisms occurred at the stressing conditions without a TM driving force confirmed by IR microscopy. When the Sn EM flux is larger, void formation at the interface was responsible for the failure. However, UBM dissolution and IMC formation dominated the failure mechanism when the Sn chemical potential flux toward the cathode side exceeded. This model successfully explains the experimental results.
On the other hand, stress migration brought about failures in solder joints. When the SnPb composite solder joints were subjected to a TCT between -55 and 125 °C after 500cycles, the Sn grains coarsened and formed anisotropic Sn stripes near the necking site in the solder joint due to the stress migration. Then the cracks, arising from the thermal stress, propagated along the Sn/Pb interfaces. After the prolonged 14410 cycles of TCT, the Sn stripes almost penetrated the entire solder joint, and the cracks expanded accordingly. The 3D simulation of the strain distribution supports the contention that the crack formation started at the Sn/Pb interfaces. Furthermore, instead of Sn stripes, only coarsened circular Sn grains were observed in the solder joints after a pure isothermal annealing at 150 °C. Therefore, stress migration of the Sn-rich phase triggered by the temperature cycling tests is a critical reliability issue in SnPb composite solder joints.
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Publication list
International journals
Liang, Y. C., Tsao, W. A., Yao, D. J., Lai, Y. S., Chen, C., Huang, A. T., Feb.
2012, Influence of Cu column under-bump-metallizations on current crowding and Joule heating effects of electromigration in flip-chip solder joints, J. Appl.
Phys. 111, 043705.
Liang, Y. C., Chen, C., Tu, K. N., Aug. 2012, Side wall wetting induced void
formation due to small solder volume in microbumps of Ni/SnAg/Ni upon reflow,
ECS Solid State Letters 1(4), 60.
Liang, Y. C., Lin, H. W., Chen, C., Chen, H. P., Tu, K. N., Lai, Y. S., Mar. 2013,
Anisotropic grain growth and crack propagation in eutectic microstructure under cyclic temperature annealing in flip-chip SnPb composite solder joints, Scripta Mater. 69, 25.
International conferences
Liang, Y. C., Chen, C., Oct. 2009, Direct measurement of hot-spot temperature
in flip-chip solder joints with Cu columns under current stressing using infrared microscopy, International Microsystems, Packaging, Assembly and Circuits
103
Technology Conference, pp. 158-161.
Liang, Y. C., Chen, C., Yao, D. J., Apr. 2010, Direct measurement of hot-spot
temperature in flip-chip solder joints with Cu columns under current stressing
using infrared microscopy, MRS Spring Meeting.
Liang, Y. C., Chen, C., Yao, D. J., Chang, T. C., Zhan, C. J., Juang, J. Y., Oct.
2010, Direct measurement of temperature distribution in flip-chip micro-bumps under current stressing by using infrared microscopy, International Microsystems,
Packaging, Assembly and Circuits Technology Conference.
Liang, Y. C., Chen, C., Yao, D. J., Chang, T. C., Zhan, C. J., Juang, J. Y., Sep.
2011, Joule heating effect in microbumps for 3D IC packaging, International
Union of Materials Research Societies - International Conference in Asia.
Liang, Y. C., Chen, C., Oct. 2011, Microstructure evolution in a sandwich
structure of Ni/SnAg/Ni microbump during reflow, International Microsystems,
Packaging, Assembly and Circuits Technology Conference.
Liang, Y. C., Tsao, W. A., Yao, D. J., Lai, Y. S., Chen, C., Dec. 2011, Influence
of Cu columns on current crowding effect in electromigration in flip-chip solder
joints, Electronics Packaging Technology Conference.
Liang, Y. C., Chen, C., Tu, K. N., Jan. 2013, Side wall wetting induced void
formation due to small solder volume in microbumps of Ni/SnAg/Ni upon reflow,
104
Pan Pacific Microelectronics Symposium.
Domestic journals
Liang, Y. C., Tsao, W. A., Chen, C., Nov. 2010, Influence of Cu columns on
current crowding effect in electromigration in flip-chip solder joints, ASE Tech.
Journal, Vol. 3, No. 2.