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FIGURE 47.11 Arrows indicate impinging forced-convection flows from oven blowers. The inadvertent thermocouple junction (crossed wires) will record oven local air temperature rather than the intended measurement target the solder-joint area.
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thermal measurement range best suited for soldering of circuit boards regardless of alloy type. There are two main devices in use for measuring the output of the thermocouple: the reflow tracker and digital thermometer. 47.3.8.2.4 Measurement Instrumentation. One of the two most primitive measurement devices is the electronic (digital) thermometer with output hand-recorded or tied to a chart recorder. The other (though much more sophisticated) device is the reflow profiler or tracker, an electronic package that can not only output the time-temperature plot of multiple thermocouples but can also make certain assessments of the data, such as heating ramp rate, preheat time, time above liquidus, cooling ramp rate, and other attributes. Tracker systems are computer-based and battery-powered and can generate detailed reports and graphs of a reflow cycle. Most are sold with a thermal protective cover that enables them to travel through the oven, thus eliminating the resistive losses and thermal heat-sinking effects of excessively long thermocouple wires. Some even output temperature by wireless transmitter so that measurement data can be observed in real time. Once a selected above-ambient threshold temperature has been reached, such as 30 C, the tracker begins recording the experienced thermal environment per unit time. It samples and
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records the circuit board s thermocouples at short, programmable time intervals (several times per second) for the entire trip through the oven s tunnel. When the tracker emerges from the exit end of the oven s tunnel, it is removed from its protective thermal barrier and docked to a computer.The data stored in the electronic tracker are uploaded to the computer, conditioned, displayed, reported, and printed for analysis. The user defines the reporting format, including the type of data sought, thermocouple plots, and other relevant, run-related data. Some systems are capable of making predictive corrections to the profile based on current measured conditions and recorded oven settings. This can further enhance the ease of adjusting a complex multizoned reflow oven to meet the requirements of a particular job. Whether a simple or complex model is selected, the reflow tracker is invaluable for setting up the oven and checking process repeatability. When an electronic tracker is used, its presence in the oven may have an impact on the resulting profile data by affecting the local aerodynamics within the oven. The trackers also have thermal mass, but all trackers are sold with an insulating cover to protect their electronics. It is best to check the influence of the tracker s presence as it traverses through the oven tunnel. To do this, use long thermocouple wires and, once the oven has warmed to a steady-state operating condition, run the profile tracker with the tracker a fixed distance behind the profile board. Note that the tracker is usually placed to follow the board in reflow. Were the tracker to precede the thermocoupled profile board, it might cause the heaters to ramp up as the oven attempts to compensate for the thermal mass of the tracker or compensate from the disrupted or deflected air flow. Although that is what happens during a reflow cycle as a cooler board enters a heated zone of the oven, the oven would attempt to adjust for the board s thermal mass. The added thermal mass or disruption of the air flows from the profile tracker would not be a factor in the normal reflow process. Rather than assessing the reflow tracker effect on the profile, if you maintain the tracker one board length away from the instrumented board, you can eliminate the likelihood of the tracker having any influence on the measured profile. 47.3.10.3 The Profile Board. The profile board is the keystone for any soldering process. It is a board loaded with its complement of appropriate components. Thermocouples are attached to critical locations to ensure that proper soldering temperatures are reached during the bonding cycle. Typically thermocouples are embedded under BGAs (innermost joint and outermost joint) and on fine-pitch and even some passive components. The idea is to ensure that the most difficult, thermally challenging parts are receiving enough heat and that the lightest components are not overheating at the same time. Sometimes it is impossible to achieve the perfect balance, and in such cases the thermal edge is given to the area array components to ensure good soldering beneath while slightly overheating the lighter components. It is good practice to attach thermocouples to components on the leading edge of the board, the trailing edge of the board, the sides, and the middle. It may take more than one trip through the oven to collect data for all these locations. There is no substitute for a profile board. Some have tried weighing the board for an estimate of thermal mass. Generic profiles with overly hot peak temperatures have been applied and other supposed short-cuts have been attempted. None of these approaches can be 100 percent successful. There is no replacement for preparation and proper thermal assessment with a profile board. Typically for oven reflow soldering, one profile board is prepared for the secondary-side reflow profile (in which only secondary-side components are mounted), then another for primary-side reflow profile (in which both secondary- and primary-side components are mounted). Another board is prepared for wave soldering (all SMDs loaded plus PTH parts). These boards also double as rework profile boards. It is wise to attach thermocouples to power and ground pins if that information is readily available. These connection points are the most thermally challenging points for a particular device. This is especially true on thick motherboards where there may be multiply interconnected, thick ground, or power planes embedded within the board, which are slow to heat and slow to cool.
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