In recent years, recovery of metals from electronic waste within the European Union has become increasingly important due to potential supply risk of strategic raw material and environmental concerns. Electronic waste, especially a mixture of end-of-life electronic products from a variety of sources, is of inherently high complexity in composition, phase, and physiochemical properties. In this research, a closed-loop hydrometallurgical process was developed to recover valuable metals, i.e., copper and precious metals, from an industrially processed information and communication technology waste. A two-stage leaching design of this process was adopted in order to selectively extract copper and enrich precious metals. It was found that the recovery efficiency and extraction selectivity of copper both reached more than 95% by using ammonia-based leaching solutions. A new electrodeposition process has been proven feasible with 90% current efficiency during copper recovery, and the copper purity can reach 99.8 wt %. The residue from the first-stage leaching was screened into coarse and fine fractions. The coarse fraction was returned to be releached for further copper recovery. The fine fraction was treated in the second-stage leaching using sulfuric acid to further concentrate precious metals, which could achieve a 100% increase in their concentrations in the residue with negligible loss into the leaching solution. By a combination of different leaching steps and proper physical separation of light materials, this process can achieve closed-loop recycling of the waste with significant efficiency.

A Cleaner Process for Selective Recovery of Valuable Metals from Electronic Waste of Complex Mixtures of End-of-Life Electronic Products.

Optimally engineered ultrathin gold (Au) layer as an effective surface passivation for low-temperature, low-pressure fine-pitch Cu–Cu bonding is demonstrated in this paper. The Au passivation layer not only performs the role of protecting the underlying Cu surface from unwanted oxidation but it also helps in reducing the effective surface roughness, which are two major requirements for thermocompression bonding. In addition, Au, being a noble metal, ensures efficient surface passivation of Cu even at elevated temperatures. Furthermore, Au-passivated Cu surfaces show significantly high {111}-oriented surface planes with random grain structures at the bonding interface, which account for enhanced diffusion ability. Herein, an optimized Au passivation layer of 3 nm has resulted in high-quality fine-pitch Cu–Cu thermocompression bonding at 140 °C and 0.3-MPa pressure. The bonded samples have further been subjected to various reliability studies in order to confirm the efficacy of the proposed bonding scheme, along with mathematical modeling to cross check using Fick’s second law of approximation. The bonded samples have attributed to a high bond strength (>200 MPa) and a very low and stable specific contact resistance ( $\sim 1.43 \times 10^{\mathrm {-8}}\Omega $ cm $^{2}$ ) under robust conditions, which is significantly better than the values previously reported in the literature.

Interface and Reliability Analysis of Au-Passivated Cu–Cu Fine-Pitch Thermocompression Bonding for 3-D IC Applications

We report on the surprisingly strong, broadband emission of coherent terahertz pulses from ultrathin layers of semiconductors such as amorphous silicon, germanium and polycrystalline cuprous oxide deposited on gold, upon illumination with femtosecond laser pulses. The strength of the emission is surprising because the materials are considered to be bad (amorphous silicon and polycrystalline cuprous oxide) or fair (amorphous germanium) terahertz emitters at best. We show that the strength of the emission is partly explained by cavity-enhanced optical absorption. This forces most of the light to be absorbed in the depletion region of the semiconductor/metal interface where terahertz generation occurs. For an excitation wavelength of 800 nm, the strongest terahertz emission is found for a 25 nm thick layer of amorphous germanium, a 40 nm thick layer of amorphous silicon and a 420 nm thick layer of cuprous oxide, all on gold. The emission from cuprous oxide is similar in strength to that obtained with optical rectification from a 300 μm thick gallium phosphide crystal. As an application of our findings we demonstrate how such thin films can be used to turn standard optical components, such as paraboloidal mirrors, into self-focusing terahertz emitters.

/pdf/enhanced-terahertz-emission-by-coherent-optical-absorption-2bs78ucaky.pdf

Enhanced terahertz emission by coherent optical absorption in ultrathin semiconductor films on metals

Interdiffusion study is conducted in the Au–Cu system, which has complete solid solution in the higher temperature range and ordered phases in the lower temperature range. First experiments are conducted at higher temperatures, where atoms can diffuse randomly. Higher values of interdiffusion coefficients are found in the range of 40–50 at.% Cu. This trend is explained with the help of thermodynamic factor and possible concentration of vacancies. Following an experiment is conducted at 623 K (350 °C), where the ordered phases are grown. The interdiffusion coefficients at this temperature are compared after extrapolating the data calculated at higher temperatures.

Diffusion mechanism in the gold-copper system

Atmospheric corrosion resistance of electroplated Ni/Ni–P/Au electronic contacts

The value of the room-temperature copper-gold interdiffusion coefficient derived by extrapolating from high-temperature measurements is an underestimate by several orders of magnitude. Plating copper with a gold film has several disadvantages. Two specimens were analyzed by using Auger electron spectroscopy. Once the full thickness of the gold film is penetrated, copper accumulates on the surface, and a layer of high concentration of copper exists immediately below the gold-air interface. Electrical resistivity of an alloy is much higher than the resistivity of either component. This high-resistivity layer may be localized within the skin depth of propagating electromagnetic waves; in cases where copper has reached the surface, it is permanently within the skin depth at any frequency. A nickel diffusion barrier, commonly applied between copper and gold, is unsuitable in many microwave and millimeter-wave applications because of ferromagnetism of nickel at room temperature. The compound that forms on the surface of untreated copper at room temperature in a reasonably clean atmosphere is cuprous oxide. Its properties make it a better alternative to gold in microwave and millimeter-wave engineering. >

Diffusion of copper into gold plating

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Diffusion of copper into gold plating