Kejun Zeng

Tin-lead (SnPb) solder reaction in flip chip technology

Solder reactions between SnPb and one of the four metals, Cu, Ni, Au, and Pd have been reviewed on the basis of the available data of morphology, thermodynamics, and kinetics. The reactions on both bulk and thin film forms of these metals have been considered and compared. Also the two kinds of reactions, above and below the melting point of the solder, have been considered and compared. The rate of intermetallic compound formation in wetting reactions between the molten solder and the metals is three to four orders of magnitude faster than those between the solid state solder and the metals. The rate is controlled by the morphology of intermetallic compound formation. In the wetting reaction between molten SnPb and Cu or Ni, the intermetallic compound formation has a scallop-type morphology, but in solid state aging, it has a layer-type morphology. There are channels between the scallops, which allow rapid diffusion and rapid rate of compound formation. In the layer-type morphology, the compound layer itself becomes a diffusion barrier to slow down the reaction. Similar morphological changes occur between SnPb and Au or Pd. The stability of scallop-type morphology in wetting reaction and layer-type morphology in solid state aging have been explained by minimization of surface and interfacial energies. The unusually high rate of scallop-type intermetallic compound formation has been explained by the gain of rate of free energy change rather than free energy change. Also included in the review is the use of a stack of thin films as under-bump-metallization, such as Cr/Cu/Au, Al/Ni(V)/Cu, and Cu/Ni alloyed thin films.

Morphology, kinetics, and thermodynamics of solid-state aging of eutectic SnPb and Pb-free solders (Sn–3.5Ag, Sn–3.8Ag–0.7Cu and Sn–0.7Cu) on Cu

Intermetallic compound (IMC) growth during solid-state aging at 125, 150, and 170 °C up to 1500 h for four solder alloys (eutectic SnPb, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and Sn-0.7Cu) on Cu under bump metallization was investigated. The samples were reflowed before aging. During the reflow, the solders were in the molten state and the formation of the IMC Cu 6 Sn 5 in the cases of eutectic SnPb and Sn-3.5Ag had a round scallop-type morphology, but in Sn-0.7Cu and Sn-3.8Ag-0.7Cu the scallops of Cu 6 Sn 5 were faceted. In solid-state aging, all these scallops changed to a layered-type morphology. In addition to the layered Cu 6 Sn 5 , the IMC Cu 3 Sn also grew as a layer and was as thick as the Cu 6 Sn 5 . The activation energy of intermetallic growth in solid-state aging is 0.94 eV for eutectic SnPb and about 1.05 eV for the Pb-free solders. The rate of intermetallic growth in solid-state aging is about 4 orders of magnitude slower than that during reflow. Ternary phase diagrams of Sn-Pb-Cu and Sn-Ag-Cu are used to discuss the reactions. These diagrams predict the first phase of IMC formation in the wetting reaction and the other phases formed in solid-state aging. Yet, the morphological change and the large difference in growth rates between the wetting reaction and solid-state aging cannot be predicted.

Interfacial reactions between lead-free SnAgCu solder and Ni(P) surface finish on printed circuit boards

Transmission electron microscopy and scanning electron microscopy were employed to analyze the interfacial microstructure between Sn-Ag-Cu solder alloys and Ni(P)/Au metallizations. The intermetallic compound Cu/sub 6/Sn/sub 5/, containing a small amount of dissolved Ni, was found to form preferentially on the Ni coating. This compound layer served as a barrier for direct reaction of Sn with the Ni(P) coating. On the Ni(P) side, a nickel phosphide was identified. Thermodynamic evaluation of the Cu-Ni-Sn system was carried out to rationalize the enrichment of Cu at the solder/finish interface. Effects of the interfacial reactions on joint reliability are discussed.

Wetting reaction versus solid state aging of eutectic SnPb on Cu

The reaction kinetics of eutectic SnPb solder on Cu were studied and compared in the liquid state at 200 to 240 °C and in the solid state aged at 125–170 °C. The ternary phase diagrams of SnPbCu, the morphology of intermetallic compound (IMC), and the kinetics of growth of the intermetallics were used in the comparison. The temperature difference between these two reactions is only 30 °C, but the kinetics of reaction, as well as the morphology of IMC formation, are very different. The kinetics in the wetting reaction is four orders of magnitude faster than that in solid state aging. The Cu6Sn5 intermetallic morphology in solid state aging is a layer type, but it has a scallop-type morphology in the wetting reaction. The morphology strongly affects the kinetics. While the kinetic difference can be attributed to the difference in atomic diffusivity between the liquid state and the solid state, it is the morphology that determines the kinetic path in these reactions. We conclude that a fast rate of reaction,...

Failure mechanism of Ta diffusion barrier between Cu and Si

The reaction mechanisms in the Si/Ta/Cu metallization system and their relation to the microstructure of thin films are discussed on the basis of experimental results and the assessment of the ternary Si–Ta–Cu phase diagram at 700 °C. With the help of sheet resistance measurements, Rutherford backscattering spectroscopy, x-ray diffraction, a scanning electron microscope, and a transmission electron microscope, the Ta barrier layer was observed to fail at temperatures above 650 °C due to the formation of TaSi2, the diffusion of Cu through the silicide layer, and the resulting formation of Cu3Si precipitates. However, in order for the TaSi2 phase to form first, the Ta diffusion barrier layer must be thick enough (e.g., 50–100 nm) to prevent Cu diffusion into the Si substrate up to the temperature of TaSi2 formation (∼650 °C). Independent of the Ta layer thickness, Cu3Si was present as large nodules, whereas the TaSi2 existed as a uniform layer. The resulting reaction structure was found to be in local equil...

TaC as a diffusion barrier between Si and Cu

The reaction mechanisms and related microstructures in the Si/TaC/Cu metallization system have been studied experimentally and theoretically by utilizing ternary Si–Ta–C and Ta–C–Cu phase diagrams as well as activity diagrams calculated at 800 °C. With the help of sheet resistance measurements, Rutherford backscattering spectrometry, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy, the metallization structure with the 70 nm thick TaC barrier layer was observed to fail completely at temperatures above 725 °C because of the formation of large Cu3Si protrusions. However, the formation of amorphous Ta layer containing significant amounts of carbon and oxygen was already observed at the TaC/Cu interface at 600 °C. This layer also constituted an additional barrier layer for Cu diffusion, which occurred only after the crystallization of the amorphous layer. The formation of Ta2O5 was observed at 725 °C with x-ray diffraction, indicating that the oxygen rich amorphous layer h...

Intermetallic reactions between lead-free SnAgCu solder and Ni(P)/Au surface finish on PWBs

Due to its toxicity, Pb is likely to be eliminated eventually from electronic products and, therefore, it is important to understand and control the compatibility of the Sn-Ag-Cu solder alloys with Ni(P)/Au metallizations. Transmission electron microscopy and scanning electron microscopy were employed to analyze the interfacial microstructure. The intermetallic compound Cu/sub 6/Sn/sub 5/, containing a small amount of dissolved Ni, was found to form preferentially on the Ni coating. This compound layer served as a barrier for the reaction of Sn with the Ni coating. On the Ni(P) side, a nickel phosphide was identified. Thermodynamic evaluation of the Cu-Ni-Sn system was carried out to rationalize the enrichment of Cu at the solder/finish interface. Effects of the interfacial reactions on joint reliability are discussed.

Amorphous layer formation at the TaC/Cu interface in the Si/TaC/Cu metallization system

An amorphous Ta(O,C)x layer was found to form at the TaC/Cu interface in the Si/TaC/Cu metallization system. The formation of the layer was induced by oxygen trapped in the as-deposited films, since on the basis of thermodynamic evaluation of the ternary Ta–C–O system, the dissociation of the TaC layer and the formation of the Ta2O5 and graphite can be expected to occur during subsequent annealings in this case. However, as observed experimentally, the formation of the amorphous Ta(O,C)x preceded the formation of the stable tantalum oxide.

Effect of oxygen on the reactions is Si/Ta/Cu and Si/TaC/Cu systems

The effect of oxygen on the reactions in the Si/Ta/Cu and Si/TaC/Cu metallization systems was investigated by utilizing the assessed Ta-O binary and the evaluated ternary Ta-C-O phase diagrams together with detailed transmission electron microscopy (TEM) and secondary ion mass spectrometry analyses (SIMS). The presence of some form of amorphous tantalum oxide at the Ta/Cu and TaC/Cu interfaces was experimentally verified. The formation of the interfacial layers was explained with the help of the assessed phase diagrams as well as with the available kinetic data.