Paste release

Johnson R, J Pankow, D Bender, C Price, J Zogorski (2000) MTBE to what extent will past releases contaminate community water supply wells Environ Sci Technol 34 210A-217A. 

PBBs are no longer produced or used in the United States. Thus, the general population exposure to PBBs will only be from past releases. For people residing in the lower peninsula of Michigan, especially near PBB contaminated areas, exposure to PBBs may still be occurring today. However, environmental levels have decreased since the 1970s and current exposure, if any, will be at low levels. For other regions of the United States, the levels of exposure will either be very low or none. 

According to the Toxics Release Inventory, in 1996, the estimated releases of manganese of 8,896,662 pounds (4,043,937 kg) to air from 1,978 large processing facilities accounted for about 15% of total environmental releases (TRI93 1995). Table 5-1 lists amounts released from these facilities. The TRI data should be used with caution, however, since only certain types of facilities are required to report. This is not an exhaustive list. Also, because these data reflect past releases, they may not be representative of current releases at these facilities. 

There are many reasons for surface treatment of the fibers. In one process,introduction of functional groups is described. The equipment designed for this process includes a cleaning vessel, a vacuum drier, and a plasma treatment vessel. The fiber is first treated with a solvent, which is subsequently removed in the vacuum drier to remove all residual solvent. The surface is then modified by a plasma treatment. Cleaning removes dirt from the natural fibers and impurities from man-made fibers. Water, hydrocarbons, and halogenated hydrocarbons are used in an enclosed system. Surface etching and cleaning techniques which were used in the past released solvents and other materials to environment, especially because the fibers were not sufficiently dried. 

To qualify as a continuous release, the facility must establish a basis for asserting that the release is continuous as defined by the statute. A release may be reported for a period of time necessary to establish that the pattern of the release is continuous and stable, meaning multiple reports are not necessary. Past release data, engineering estimates, knowledge of a facility s operations, and/or best professional judgment may be used to establish the pattern of a release. A single telephone call to the NRC, SERC, and LEPC will alert authorities of a facihty s intent to report a release as a continuous release. 

Some valuable information about optimal materials and fabrication methods for a lead-free stencil also has been discovered. The message is that apertures with low-friction internal surfaces will perform better in terms of paste transfer and paste-release efficiency. 

What has become clear from such experiments is that established SnPb pastes support greater process robustness than their lead-free successors. Only when the aspect ratio is reduced below the practicality threshold of 0.6 did paste release for the SnPb control paste in the previous experiment fall below that of the lead-free formulations. However, the performance of lead-free pastes in this respect will almost certainly improve with fimther development because they are in the beginning of their evolutionary cycles. 

It is common sense that friction between the paste and the aperture walls has a strong influence on paste transfer. This has implications both for materials and fabrication technologies. For example, the electro-forming process results in inherendy smooth aperture surfaces. This is enhanced by the low coefficient of friction displayed by nickel, from which electro-formed stencils are generated. On the other hand, laser-cut apertures present a relatively high friction surface, even though they are demonstrably adequate for many applications. If market pressures demand, stencils cut by laser from nickel-rich stainless steel may provide a workable balance of outright paste-release efficiency, turnaround time and overall stencil cost. 

Figure 2 shows deposit height and area data, as well as the volume of paste released by individual apertures in a set of test stencils fabricated using electro-form and laser-cut processes. Because paste volume is closely related to the quality of the mechanical joint created between the device interconnect and the land during reflow, this investigation makes valuable data available in the quest to develop high-yield assembly processes based on lead-free materials. 

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