| 其他摘要 | Interfacial reactions are critical for the reliability of microelectronic interconnects, especially for flip chip solder joints. Usually, the solder interconnects are subjected to both thermal and electrical loading when an electronic device is in operation. Thus, the study about interfacial reactions of lead-free solders under electrical and thermal loading gets very important. In the present work, the electromigration behavior of Sn-9Zn solder bump interconnect was investigated, and a new method to restrain the electromigration in lead-free solder interconnects was developed. Meanwhile, interfacial reactions of FeNi/SnZn and Ni/Sn-xZn under thermal loading were also investigated.
Polarity effect on the interfacial reactions from high-density electric currents was investigated in a eutectic Sn-9Zn/Cu interconnect with a large disparity in the effective charge between the solder constituents. A reverse polarity effect was found where the intermetallic compound (IMC) layer at the cathode grew significantly thicker than that at the anode under electric loading. Such an abnormal polarity effect was shown to result from electromigrations of Sn and Zn along opposite directions as dictated by the disparity in their effective charges. As Sn migrated to the anode under electron wind force, the resulting back stress drove Zn atoms to drift to the cathode. A kinetic analysis of the Zn mass transport explained the differential growth of the IMCs at the two electrodes.
The relationship between IMCs at the interfaces and current direction for Ni/Sn-9Zn/Cu combination was investigated. When the electrons traveled from the Ni side to the Cu side, uniform layers of Ni5Zn21 and Cu5Zn8 were formed at the Ni/SnZn and Cu/SnZn interfaces. However, upon reversing the current direction, where electron flow went from the Cu side to the Ni side, thicker Cu6Sn5 phase replaced Ni5Zn21 phase at the Ni/SnZn interface, whereas at the Cu/SnZn interface, thicker β-CuZn phase replaced the γ-Cu5Zn8 phase. A kinetic model, based on the Zn and Cu mass transport in the sample, was presented to explain the growth of the IMC at the anode and cathode.
A new method has been developed to restrain the electromigration in SnBi solder interconnects. By introducing prestrains into the solder joints, Bi interfacial segregation under current stressing was effectively retarded. After prestraining of the SnAgCu solder joints, no polarity effect was found in that the IMC layer at the anode has the same growth rate as the cathode. Therefore prestrain treatment provides an effective approach to restrain the electromigration in the solder interconnects. Such an inhibiting effect indicated that dislocations acted as internal “traps” for electromigration damage rather than fast migration paths as expected from a vacancy mechanism.
Polarity effect on the interfacial reactions under high-density electric currents was investigated in the Cu/FeNi/Sn/FeNi/Cu solder interconnects. A reverse polarity effect was found where the FeSn2 IMC layer at the cathode grew significantly thicker than that at the anode under electric loading. Such an abnormal polarity effect was shown to result from electromigrations of Fe along opposite directions of electron flow. In the same time, a Cu-rich layer arose next to the FeSn2 layer at the anode side. A kinetic analysis of the Fe and Cu mass transport explained the differential growth of the IMCs at the two electrodes.
Interfacial reactions and growth kinetics of IMC layers formed between Sn-9Zn solder and electroplated Fe-42Ni metallization were investigated at 120, 150 and 170 oC for up to 360 h. Experimental results show that the IMC formed at the interface was mainly δ-FeZn8.87 phase, not the FeSn2 phase. The reason is that the attraction between Zn and Fe is stronger than that of Sn and Fe. The growth exponent n for δ-FeZn8.87 phase was found to be about 0.5, which indicates that it grows by a diffusion-controlled process even at a very high temperature. The activation energy for the growth of δ-FeZn8.87 phase was determined to be 42 KJ/mol, in good agreement with the reported data.
Nickel is widely used as a solderable diffusion barrier in several types of surface finishes for components and printed circuit boards. Upon reflow and in aging, Sn-Ni intermetallic and P-rich layers form at the interface and degrade the reliability of the solder joints if not controlled. So it would be desirable to limit the growth of Ni-Sn IMC and the P-rich layer. In our study, we found that the typical Ni-Sn reaction product, Ni3Sn4 phase, was substantially changed by adding small amounts of Zn in Sn. With Zn addition, the ternary Ni4(Sn1-x,Znx) phase, rather than the common Ni3Sn4 compound, formed at the interface during reflow and aging. In the Ni4(Sn1-x,Znx) phase, the lattice parameters contracted with increasing Zn content, in agreement with the Vegard’s law. Since diffusion of the reactive species through the denser ternary IMC was more difficulty than through the binary Ni3Sn4, the Zn-containing solder showed a much slower electroless Ni-P consumption rate than Sn. |
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