Internal reflow

The kinetic parameters of Tc-HMPAO retention in the human brain have been studied (Lassen et al. 1988), and measurements of CBF have been compared to Xe (Andersen et al. 1988). CBF was quantified using dynamic single-photon emission computer tomography (SPECT) (Murase et al. 1992). The role of SPECT with Tc-HMPAO in ischemic stroke has been evaluated (Heiss 1983 Podreka et al. 1987). Reflow hyperemia in subacute stroke has been described as luxury perfusion (Lassen 1966) and was later explained by hyperfixation of " Tc-HMPAO (Lassen and Sperling 1993). Increased regional blood flow in subacute stroke has been measured with " Tc-HMPAO (Moretti et al. 1990). The effect of acetazolamide on CBF was reported in patients with severe internal carotid artery stenosis/occlusion (Asenbaum et al. 1995), and has been studied in primates (Dormehl et al. 1997). " Tc-HMPAO SPECT was evaluated in patients with cerebrovascular disease by comparison with F-fluoromethane positron emission tomography (PET) (Heiss et al. 1990). A pattern of focal abnormalities in CBF was derived in patients with degenerative dementia (Holman et al. 1992). 

Connectors comprise approximately 75 to 80 percent of odd-form components. See Fig. 40.6. For example, there are universal serial bus (USB) connectors, dual inline memory module (DIMM) connectors, phone jacks, and zero-insertion-force (ZIF) connectors, as well as a variety of surface-mount connectors. Other odd-form components include sockets, electrolytic capacitors, transformers or large inductors, LEDs, relays, switches, as well as speakers and vibrators for handheld communications devices. In surface-mount technology, odd-form components are most often through-hole packages, including both passive and active devices as well as connectors. When addressing such odd-form components, the first step is to check the availability of an equivalent surface-mount version since there has been a shift toward surface-mount versions of many of these devices. Of course, the materials used to construct the surface-mount replacement must survive the reflow furnace environment, and especially the Pb-free reflow temperatures, particularly connectors because of their complex internal construction. In some cases, it has been possible to replace the through-hole connector with one of a variety of solderless products. 

FIGURE 40.31 A hot-air shroud that is used to simultaneously reflow all of the solder joints on a EGA package. (Courtesy of OK InternationaL)... 

Lea, C, Inert IR Reflow The Significance of Oxygen Concentration of the Atmosphere, Proceedings of Surface Mount International, San Jose,August 1991, pp. 27-29. 

Temperature Compatibility. If pin-in-paste soldering is to be used, check that components are temperature-compatible with the oven reflow process. The high temperatures and long exposures associated with oven reflow soldering may cause unsuited molded component bodies to melt or warp. Connector contact normal force may be affected if the molded connector body softens or distorts. Solder joints or wire bonds internal to some devices may become disbanded, and some, such as electrolytic capacitors, may leak or even explode as a result of an oven reflow cycle. Check the component manufacturer s specification for thermal hmits and compatibihty with oven reflow soldering. 

When massive, densely populated boards are introduced to the vapor reflow oven, vapor collapse can occur—a condensation rate that outpaces that of vaporization. The result is that the internal atmosphere of the oven thins dramatically to the point that it cannot sustain adequate reflow. Vapor-phase machines that rely on immersion heaters are prone to this phenomenon. More recent machines include massive heating element housings that provide sufficient thermal inertia to preclnde this problem. 

INTERNAL LAYER A conductive pattern contained entirely within a mnltUayer board. IPC Institute for Interconnecting and Packaging Electronic Circuits. A leading printed wiring industry association that develops and distribntes standards as well as other information of value to printed wiring designers, users, suppliers, and fabricators. IR Infrared heating for solder-reflow operation. 

Reflow solder Wave solder Component finish or I/O Board finish Component internal... 

Hymes, L. Pitsch, T. Successful implementation of a no clean strategy for reflow, wave, and hand soldering. Proceedings of the Surface Mount International Conference, Edina, MN, SMTA Edina,... 

FIG. 87 Flux collection system for air reflow including dual internal and external exhausts. (From Ref 48.)... 

Kokko, P. Microstructure and Suitability of Sn-Ag-Cu Solder to Reflow Soldering in Normal Conditions. Internal Report. Helsinki University of Technology, 1997. in Finnish. 

A fused-Sn plating is one that has been reflowed (melted and resolidified) subsequent to electroplating. Typically, an electroplated part is dipped into a hot-oil bath and held at a temperature above the melting point of Sn. The objective of the fusing operation is to allow the Sn to slowly sohdify and, thereby, remove any internal stresses that may have resulted from the electrodeposition process. Fused Sn is reputed to not grow whiskers unless the surface of the tin is scratched, compressively stressed, or plastically deformed [2]. 

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