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May 2017 • The PCB Magazine 95 this may be direct capacitive measurements or capacitive "counts." When the capacitive gather is complete on the first board it will perform a full direct mode test to validate the first board is actually con- forming and does not have any defects. If no de- fects are found the capacitive master is written as "golden." If any defects are found, the master still will be written but the defective nets will be discounted from the master as they were non- conforming. When the second board is tested, the capacitive gather is done again. When it is complete the machine will compare the values from board number two to the master. If any of the readings are not within tolerance of the master, those nets will be placed in a retest file for direct mode probing. This could be for either possible opens or shorts. The amount of direct mode probing will be controlled by the anoma- lies found during the capacitive gather. The less discrepancies found against the master the less direct mode probing will be done. This is how the speed is gained during indirect testing. Test Methods vs. IPC-9252B Test Level C Now that we understand the two basic meth- ods used in flying probe testing, what impact does this have on product in Performance Class 3? This can have a large effect based on whether indirect testing can be done or not. The 9252B specification does allow indirect testing for Per- formance Class 3 (Test Level C), but has the ca- veat of AABUS as defined previously. So as one reads the specification, if the allowance for indi- rect testing has not been stipulated in the flow- down, P.O., or customer specification, the de- fault flying probe test method for Performance Class 3 (Test Level C) is direct mode. How much of an impact can this be? It can be substantial based on the amount of test points, nets and adjacency pairs. We performed an experiment across five different PCBs with different amounts of test points, nets and adja- cency pairs. In Table 1 we show the five differ- ent boards with their individual attributes. Each of the PCBs 1−5 were tested in both di- rect mode and indirect mode. For our discus- sion, we will be doing a comparison based on the amount of measurements taken to test the individual boards. The reason we have used measurements instead of time required is that PCB topography is a variable and mechanical travel will not be the same nor can be correlat- ed to test points. Measurements more directly show the contrasting between test methods. For each of our PCBs in our experiment all went through direct mode test, indirect master generation, and nidirect subsequent board test. In Table 2 we see how many measurements are taken for each type of test. As we discussed earlier, the amount of measurements required to perform direct mode test will not change. How - ever, for indirect mode testing we see significant differences in the amount of measurements re- quired. The indirect column is the amount of measurements required for subsequent boards prior to direct mode retest, as this can vary from board to board. What sticks out to us immediate - ly is the advantage we see using Indirect testing vs. direct testing. In just our control group we Table 1: PCB group. Table 2: Indirect and direct test method measurements. FLYING PROBE TESTING VS. IPC-9252B