A new selective technique for characterization of polycyclic aromatic hydrocarbons in complex samples: UV resonance Raman spectrometry of coal liquids.
Evidence is given showing that UV resonance Raman spectroscopy (UVRRS) is a powerful technique for the identification and quantitation of polycyclic aromatic hydrocarbons (PAH's) in a coal liquid. Results suggest that this technique will be of general utility for the study of systems containing chromaphoric species absorbing in the UV spectral region. In this paper it is demonstrated that (1) the resonance Raman spectra of PAH derivatives can be selectively observed in solutions of coal liquids and (2) different UV excitation wavelengths result in the selective Raman enhancement of different polycyclic aromatic hydrocarbon ring systems.
UV Raman spectroscopy of hydrocarbons
In this paper, the UV Raman spectra of a large number of saturated and alkyl–substituted monocyclic, bicyclic and polycyclic aromatic hydrocarbons are obtained at 220 and 233 nm excitation wavelengths. Also included are nitrogen– and sulphur–containing hydrocarbons. The spectra obtained are fluorescence free, even for such highly fluorescent compounds as perylene, consistent with earlier reports of UV Raman spectra of hydrocarbons. The hydrocarbon UV Raman spectra exhibit greatly improved signal–to–noise ratio when in the neat liquid or solution state compared with the neat solid state, suggesting that some surface degradation occurs under the conditions used here. Assignments are given for most of the bands and clear marker bands for the different classes of hydrocarbons are readily observable, although their relative intensities vary greatly. These results are discussed in the context of structure and symmetry to develop a consistent, molecular–based model of vibrational group frequencies.
“ PARTIALLY-ACTIVATED ” FLUX RESIDUE IMPACTS ON ELECTRONIC ASSEMBLY RELIABILITIES
The reliabilities of the flux residue of electronic assemblies and semiconductor packages are attracting more and more attention with the adoption of no-clean fluxes by majority of the industry. In recent years, the concern of “partiallyactivated” flux residue and their influence on reliability have been significantly raised due to the miniaturization along with high density design trend, selective soldering process adoption, and the expanded use of pallets in wave soldering process. When flux residue becomes trapped under low stand-off devices, pallets or unsoldered areas (e.g. selective process), it may contain unevaporated solvent, “live” activators and metal complex intermediates with different chemical composition and concentration levels depending on the thermal profiles. These partially-activated residues can directly impact the corrosion, surface insulation and electrochemical migration of the final assembly. In this study, a few application tests were developed internally to understand this issue. Two traditional liquid flux and two newly developed fluxes were selected to build up the basic models. The preliminary results also provide a scientific approach to design highly reliable products with the goal to minimize the reliability risk for the complex PCB designs and assembly processes.