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37.3.1.6 Continuity Test False Opens. A common problem with continuity testing is a high incidence of false failures due to fixture and product contamination, poor product registration, or fixture damage. Dramatic improvement is often possible with the addition of product and fixture cleaning methods. Separation of the board-testing environment from such dust-producing processes as drilling/routing can be invaluable in increasing throughput. Highvoltage pulses are sometimes used to overcome thin-film contaminants or oxides coating the surface of the board and preventing good contact with the test probe. As no current is initially flowing through the oxide, some test systems offer a feature delivering a high-voltage pulse of strictly limited current and duration, and therefore limited total energy. While brief, the energy level is higher than for a normal continuity test and a small risk of damage at defect sites remains. Adding test time for automatic or manual retesting of the product yields additional boards, but this common approach becomes expensive when large numbers of false opens occur. It may be advisable to correct the root cause. 37.3.2 DC Isolation Test Method Isolation testing verifies the presence of adequate electrical isolation between networks that are not intended to be connected to one another. Typically a resistance measurement is made from a given network to another net (or group of nets). If the measured value exceeds the specified isolation resistance threshold while the specified voltage is applied, then the measurement is considered to have passed. Otherwise, a fault report is generated. So long as contact with the net has been ensured (during the continuity test), only a single test point per network is required to perform the isolation test. The actual number of isolation measurements required to test a given board can vary substantially with details of the algorithm employed, with subtle impacts upon fault coverage. These issues are discussed in a separate section. 37.3.2.1 Isolation Resistance and Voltage. As isolation testing is a means of evaluating the ability of product insulation to withstand voltage and prevent current flow, it is common for the isolation test specification to include not only a statement of minimum resistance, but also an applied voltage. This is the voltage that the insulator must withstand while exhibiting at least the minimum isolation resistance. Given the relationship R = E/I, increasing the applied voltage is also a means of increasing the measurement current level up out of the noise floor internal to the test system. Thus, high-voltage-capable bare board test systems usually are able to verify isolation at higher resistance levels and do so at reasonable speed. Typical values for isolation resistance can range widely, from as little as 1 k. to as much as 1000 M.Values of 2 to 10 M are common, but higher values are useful in detecting trace contamination. Excessive humidity in the test area may preclude the use of very high thresholds and may affect accuracy at lower thresholds. Values below 50 to 55 percent relative humidity are desirable. Common isolation test standards (see Table 37.2) have not always kept pace with the capabilities of new equipment to test at elevated and sensitive thresholds. In any event, assignment of specific test thresholds would ideally be based upon a competent analysis of the intended application of the specific circuits on the product to be tested. There is somewhat less emphasis on test voltage than was true previously, probably as a result of increasingly fine geometry in the product. Older or simpler test systems may be limited to 10 to 40 volts, but most are able to apply 100 to 250 volts during the isolation test.
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TABLE 37.2 Examples of Isolation Resistance Test Threshold Standards IPC-ET-652 (obsolete) > 500K Ohms > 2 M Ohms > 2 M Ohms MIL-55110D (obsolete) > 2 M Ohms > 2 M Ohms > 2 M Ohms
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