HALOGENS AND THEIR MODE OF ACTION
The extensive use of various forms of chlorine and iodine as antiseptics has resulted in the accumulation of much information about efficiency of these halogens under various circumstances. There has been considerable study given to the factors affecting their germicidal activities and to the mechanisms of these effects.' Much less is known about bromine?. a* The qualitative similarities and quantitative differences in many of the properties of chlorine, bromine, and iodine afford an excellent opportunity for observing the effects of certain isolated factors on germicidal activity. Accordingly, this paper will be concerned, for the most part, with the extent to which a few basic chemical properties of the halogens, such as hydrolysis, ionization, and formation of complexes can be used to explain quantitative dfierences in antiseptic power. The mechanism of the reactions leading to the death of the organism will not be discussed here. The interested reader is referred to another paper in this series and to a published report6 for information on this subject. One would expect that the rate of bactericidal action would be characteristic of the particular halogen compound present. Under usual conditions of use, however, one may have present, in addition to elemental halogen, hypohalous acid formed by hydrolysis, hypohalite ion from ionization of the acid, complex ions involving elemental halogen with halide ion, and N-halogen compounds. Furthermore, factors such as concentration, pH, and temperature should have quantitatively different effects on the germicidal activities of the three halogens because the equilibrium constants of the reactions involved are different. Most of the work leading to quantitative conclusions has been carried out with bacterial spores, beginning with Levine and co-workers,6 Meyers,' Tilley and Chapin: and others. The advantages of using B. metiens spores, for example, have been pointed out by Charlton and Levine9 and need not be enumerated here. Accordingly, in FIGURE 1, we consider the comparative action on B. meliens spores over a suitable concentration range of chlorine,'O bromine, and iodine" at pH 7 and room temperatures. A solution of each halogen free of added halide was added to a buffered distilled water suspension of spores, extraneous organic matter being excluded as far as possible. The log of the time for 99 per cent kill is plotted against the log of the normality of the solution. Calculation shows that under these conditions chlorine exists chiefly as hypochlorous acid and bromine entirely as hypobromous acid, but iodine is present as molecular iodine. It is then not surprising that the usual order of halogen reactivities is not found and that iodine fails to show less germicidal power than bromine. It might be of some interest to note that on a weight concentration basis, iodine is considerably less active than chlorine, although still more active than bromine. Against
Germicidal Action of Bromine
Conclusions Bromine is shown to be an effective germicidal agent against non-spore-forming microörganisms. Spore-forming bacteria are much more resistant. The element has marked specificity for Escherichia coli and Eberthella typhosa. Mold-spores were more resistant than yeasts or non-spore-forming bacteria.
“ 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.