barcode scanner code in asp.net BASIC ELECTRICAL TESTING METHODS in Software

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BASIC ELECTRICAL TESTING METHODS
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Electrical testing is the final test method frequently used to determine whether a board should be shipped. Electrical testing emulates the intended function of the board conductor and insulator patterns by passing currents through conductors and applying voltages across insulators. Such direct electrical measurement requires that the board come into physical contact with a measurement system. Two test types are almost universally performed: continuity and isolation testing. Some other tests may be applied selectively, depending upon the product and customer requirements. Test order is usually such that the continuity test is performed first. This verifies that each network is intact within itself, and that contact is established between any test fixture and the product. The isolation test can then be performed using only a single test point per network. Some test methods attempt indirect inference of continuity and isolation without making direct current (DC) measurements. These methods are commonly employed in flying probe systems.
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DC Continuity Test Method Continuity testing checks for the expected continuous path within each electrical network. This is done in a series of point-to-point measurements within each network. The resistance found in each measurement is compared to the selected continuity resistance threshold. If the measured value is higher than this threshold, then a fault report is generated. For complex networks, multiple measurements are required in order to ensure that all extremities of the network are interconnected. For example, the network with test points labeled A through D, shown in Fig. 37.1, could be tested in the sequence illustrated in Fig. 37.2. For a network with four test points, the minimum number of tests would be three to determine whether all points are connected. If a board contains a total of N isolated networks, and contains a total of X test points, we may calculate the number of continuity measurements C as C = X N. In assigning test points, the software system often is programmed to delete those that are unnecessary. This is referred to as test point optimization. In Fig. 37.1, a test point located along the path between D and the branch to C would not be useful. Various optimization rules may be applied, but should be applied with care to ensure that adequate test coverage remains in place.
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FIGURE 37.1
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Sample network.
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37.3.1.1 Continuity Test: Two-Wire versus Four-Wire Switching. Switch matrixes are constructed using either two-wire or four-wire circuits. A diagram
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BARE BOARD TEST METHODS
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comparing a single continuity measurement being performed on both two-wire and four-wire matrixes is shown in Fig. 37.3. Grid test systems use solid-state switches to connect appropriate test points to an internal measurement system (which is effectively an ohmmeter). Current I is driven from the upper test point through the product network and returned to the measurement system through the lower test point. The resultant voltage V across the network is measured. The continuity resistance R is then determined using the relationship R = V/I. 37.3.1.2 Two-Wire Switch Matrix Construction. The simplest, most affordable, and most common construction of the switch matrix is to have a pair of switches for each test point that can connect the test point to either the high side or low side of the ohmmeter. This technique is illustrated on the right side of Fig. 37.3. However, as the solid-state switches exhibit a certain amount of on-resistance, they contribute error to the measurement. The resistance of the switches is added to the resistance we measure, increasing the likelihood of failing the test. These errors can be reduced in various ways:
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Use switches with low on-state resistance (and thus a FIGURE 37.2 Continuity test algorithm. small amount of variation in on-resistance). This means using physically larger transistor dies. Such devices are not expensive, but are harder to integrate into integrated circuits. Thus, most two-wire designs rely upon discrete output transistors as switch elements. Use software to subtract the estimated typical on-resistance. As switch resistance effects vary from device to device, and change with temperature, some error remains. Make multiple measurements for each continuity to be inspected, and mathematically subtract most of the switch resistance. However, this slows the measurement process somewhat by requiring extra measurements and requires high-quality fixture construction such that contact resistances do not vary during the required multimeasurement sequence.
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