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IPC-9252 > 500 K Ohms > 2 M Ohms > 10 M Ohms
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TABLE 37.3 Examples of Isolation Test Voltage Standards IPC-9252 IPC-ET-652 (obsolete) MIL-55110D (obsolete) All least 40 volts, or twice the rated voltage of the board, whichever is greater
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Minimum isolation High enough High enough to test voltage to provide sufficient provide sufficient current, current, but avoid arcover but avoid arcover
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A higher voltage increases the test current, providing a better signal-to-noise ratio and generally improving test speed at higher isolation thresholds. Excessive voltage is inappropriate for very fine-pitch substrates and may result in damage from arcing in normal environments (see Table 37.3). The insulating properties typical of modern board material suggest that it may be of little value in specifying elevated voltages while using low (relaxed) threshold resistances. Raise the resistance threshold first, then use enough voltage to get adequate speed and accuracy. 37.3.2.2 True Isolation Test Method. Several different algorithms have been developed for sequencing the switching state during the isolation test, and the choice may affect test coverage. In the most rigorous method, each network is individually tested to determine the total parallel leakage resistance to all other networks on the product.This requires one measurement per network, as illustrated in Fig. 37.4 for a board with three networks. Each network is, in turn, given a chance to charge to an elevated voltage. All other networks are connected together and to 0 volts at this same moment. Notice that only one test point is needed per network.
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FIGURE 37.4
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True isolation test of three-network board.
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If the network under test is shorted or leaking to any of the other networks, or to any combination of them, it will fail to charge adequately and the test will fail. The pattern continues until each network has been tested. Figure 37.5 illustrates the status of the measurement system during the three tests. Notice that test points A and C are used to access their respective networks, and that test points B and D are not needed for the isolation test. These test point switches remain open through all measurements. A key feature of this method is the ability to answer the rigorous question: How well isolated is this network from the rest of the board Consider the example illustrated in Fig. 37.6.
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BARE BOARD TEST METHODS
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FIGURE 37.5
Measurement sequence for three isolations.
FIGURE 37.6 Parallel leakage detection.
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Here the isolation threshold is 100 M. Four leakage paths exit network A, one each to networks B, C, D, and E. Each path, measured separately, is well above the 100-M. pass/fail threshold. But electrically we can only guarantee that network A is isolated by the parallel combination of these four resistances, given by: RA = 1 = 68 Mohms ( 1/RAB) + ( 1/RAC ) + ( 1/RAD) + ( 1/RAE )
Thus, the true isolation test method will correctly fail this measurement. A not unlikely real-world situation would be a smear of contamination that touched A, B, and C. 37.3.2.3 Log of (N) Isolation Test Method. Table 37.4 illustrates the log of N isolation test method, and compares it to the true isolation method described earlier. The log method offers the powerful advantage that only a small number of measurements are required to test a complex board. That number is given by log2(N) rounded up to the next whole number, where N is the number of networks. Thus, for a board with eight networks, we need only three measurements. The gain is even greater for large boards. For a realistic board with 4,000 networks, only 12 measurements would be needed. This method involves a compromise of fault coverage for parallel leakage detection. Consider the example shown in Table 37.4.The top half of the table considers a very simple board with eight networks and illustrates the pattern of measurements required by the log method.
TABLE 37.4 Isolation Methods Compared Network name Measurement number Log method 1 2 3 True method 1 2 3 4 5 6 7 8 A + + + + B + + + C + + + D + + E + + + F + + G + + H +
Networks noted with a + in a given measurement are connected to the upper side of the measurement system. Networks with a are connected to the lower side. Notice that in each measurement, about half the networks are positive and half are negative. A hard short between any two networks in Table 37.4 will be detected by either method. In at least one measurement, a + will be at one end of the short, and a at the other. Both methods work well for hard shorts, and parallel resistance effects play no significant role. But consider the effect of two 15 M. leaks, one from network A to network B and the other from network A to network C. If the threshold is 10 M., this fault will not be detected by the log method, but the true
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