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FIGURE 47.9 Schematic plan view and detailed side view of an adjustable oven characterization fixture (oven diagnostic tool) composed of pairs of thermocouples, shielded from one another by a rigid, heat-resistant insulator. The thermocouple positions are adjustable in width and length for investigating temperature distribution along the tunnel width.
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to determining oven performance. The diagnostic board, whether a product board or a separate test board or other device, should be thermally symmetrical. It should be made from a low thermal conductivity material, or better yet, a thermal insulator. It should also be of low thermal mass. Attached to this are thermocouples mounted top-side and bottom in an evenly spaced array across the width of the material. They should be mounted close enough to the conveyor rails to gauge their influence on the thermal profile. Thermocouple beads should be identical in size and spacing from the insulating material. Furthermore, they should also be mounted slightly off the insulating material such that the heated oven environment can freely swirl around them. Measuring with such a device provides a true picture of the oven s thermal environment. Figure 47.9 shows a schematic of one such device developed for oven diagnostics.
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47.3.10.2 Printed Circuit Board Thermometry. Knowledge of good thermometric practices and thermocouple use is critical to acquiring accurate and useful board time-temperature profiles and oven thermal performance information. 47.3.10.2.1 Good Thermocouple Practices. A thermocouple is composed of two atomically disparate metals or alloys in contact with one another. The wires are joined by twisting them tightly together or preferably by welding (Fig.47.10a,b).When both legs of the thermocouple are held at the same temperature and joined at a discrete point, an electronic difference between the two metals results in a measurable voltage. This electromagnetic field (EMF) is temperature-dependent and once calibrated can be used for accurate temperature measurement.
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FIGURE 47.10 Thermocouples: (a) twisted couple; (b) welded couple; (c) welded couple with an inadvertent couple (see arrow).
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Thermocouples are simple to make. The two different wires need only touch one another to form the couple. This simplicity in construction can also be problematic. If the two wires inadvertently contact (see Fig. 47.10c), thermocouple output will be related to the temperature at that accidental juncture. In the case of a thermocouple sandwiched between a component lead and PWB pad (see Fig. 47.11) and an accidental junction formation nearby, the thermocouple will register the oven air temperature rather than the conducted temperature of the component lead, the solder, and the PWB. Since the oven air temperature is generally much hotter than the PWB traveling through it, the resultant soldering time-temperature profile (based on the false thermocouple reading) may be cooler than desired and cold (under-reflowed) solder joints may result. 47.3.10.2.2 Welded versus Twisted. A thermocouple wire pair should be welded to help preclude noise-induced measuring errors from intermittent wire pair contact. Small, inexpensive bench-top welders are available for this purpose. The two halves of the thermocouple pair should touch at only one point that closest to the area desired for temperature measurement. The welded pair has a significant advantage in that the resulting weld bead can be made small and uniform in shape.As a rule of thumb, the weld bead diameter can be made as small as about 1.5 times the diameter of the individual conductors that compose the couple. The temperature will be sensed only at the weld bead. Regardless of whether the couple is made or bought, always check with a magnifier or microscope to ensure the integrity of the bead and that couple wires are only touching at the bead and not at any point below it. During manufacture of a thermocouple assembly, the usual practice is to twist the thermocouple wires together prior to welding. Upon welding, the twisted material will fuse and melt back. After welding, it may be necessary to untwist the pair just behind the bead. Of course, this is risky because many of the thermocouple materials are brittle before welding and are even more so afterward. 47.3.10.2.3 Most Common Couple. The most common thermocouple used for PWB soldering application is the K type, Ni-Cr/Ni-Al (chromel-alumel) couple, as it possesses a
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