barcode scanner code in asp.net FIGURE 37.3 Continuity measurement circuits. in Software

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FIGURE 37.3 Continuity measurement circuits.
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Attempt to learn the resistance of each switch. Some systems require the operator to install a shorting plate over all test points, and the system then learns the sum of the switch resistances plus contact resistances to this plate. Note that this technique can actually add error if a high contact resistance value is learned for a particular test point, perhaps due to dirt on a probe. The excessive resistance value learned is subtracted from all subsequent measurements made with that probe, masking some high product resistances. The net result is a slightly increased possibility of passing a board that should be failed.
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37.3.1.3 Four-Wire Switch Matrix Construction. The left side of Fig. 37.3 represents a Kelvin or four-wire switch matrix construction. Notice that each test point will require four switches, so that any test point can be connected to the high-side current drive, high-side voltage sensor, low-side voltage sensor, or low-side current return via separate paths. Because the voltage-sensing portion of the measurement system has an extremely high input resistance, almost zero current flows through the extra pair of switches that connect the test points to this circuit. The result is that there is zero voltage drop across these sense switches, and the measurement unit sees exactly the voltage across the unknown load. Because the measurement system knows exactly the current flowing through the load, and exactly the voltage across it, it can calculate the load resistance accurately. The dominant error term in this situation is generally the contact resistances in the fixture and fixture-product interface. Also, the smaller transistors leak less current in the off state, thereby permitting isolation testing at higher thresholds with smaller errors. The continuity accuracy benefit of this technique is realized only if high-quality fixtures with low contact resistance are employed. 37.3.1.4 Continuity Threshold. The continuity resistance threshold parameter is usually specified in the range from a few ohms to 1,000. Several standards useful in suggesting continuity thresholds are summarized in Table 37.1. IPC-9252 has actually weakened the recommendations of IPC-ET-652, which it replaces. As discussed earlier in the chapter, these must be applied with judgment. Generally, a lower continuity threshold provides a more stringent test of the board. Networks with resistances of 5, 10, or 25, although rare when using copper traces of moderate length, can significantly impact the functionality of precision measurement instruments or high-speed computer products. At the same time, it should be noted that there are practical and economic considerations in determining how low the continuity test threshold should be set. Part of the limitation comes from the test system s measurement and switch matrix capabilities and, to a greater part, from the type of test fixture used. At the time of this publication, a 10-continuity resistance test threshold is a common lower limit for production testing with good-quality systems and fixtures.
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TABLE 37.1 Examples of Continuity Resistance Test Threshold Standards IPC-ET-652 (obsolete) < 50 Ohms < 20 Ohms < 20 Ohms MIL-55110D (obsolete) < 10 Ohms < 10 Ohms < 10 Ohms
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IPC Board Class Class 1: General Electronic Class 2: Dedicated Service Class 3: High Reliability
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IPC-9252 < 50 100 Ohms < 50 Ohms < 50 Ohms
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37.3.1.5 Continuity Test Current. Continuity test current is not addressed in IPC-9252 or in most other publications. Use of high current has been proposed as a means of burning out weak traces or mouse bites. But such currents may also damage good traces. If this occurs after the test system has already determined that there is a good connection, the result is a board that once tested as good but is now bad. It is preferred that the continuity test not be invasive or destructive. Typical test currents today are in the range of 5 to 50 mA.
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