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96 The PCB Magazine • May 2017 saw 263−327% difference in required measure- ments. Even though we see significant advan- tages to use Indirect over direct mode if there is no AABUS or allowance via procurement docu- ment, customer specification or other means we are bound to IPC-9252B and direct mode. Hybrid Indirect Mode What if there was a way to incorporate some of both modes into a method where ca- pacitive gather is done, direct mode continuity is done and finally the isolation test via adja- cency is done based on the results of the capac- itive master? Well there is! The main require- ment of most Test Level C product is the conti- nuity resistance. In the IPC-9252B standard this is 10 ohms. The argument has been that in indi- rect mode you do not necessarily test the net for 10 ohms unless it happened to be captured in the retest file during capacitance gather. How- ever, with this hybrid test the capacitive gath- er is done as normal for indirect testing and the first board will receive the full direct mode val- idation as required. The change is how the sec- ond and subsequent boards are tested. They will receive the capacitive gather but this will only be used for retest during the isolation adjacen- cy test. With the hybrid test all nets will be test- ed for continuity as in direct mode. For isola- tion, only nets that were found possibly faulty during the capacitive gather and placed in the retest file will be tested. This will reduce the amount of isolation tests required. So how does this new mode compare against direct and indirect mode? In Table 3 we have added a column for hybrid test. We see from Table 3 that we still have a reduc- tion in measurements over our direct ode base- line. Of course, we did not expect the reduction as shown for indirect testing but the reduction to 160−210% is quite favorable. In fact, if we av- erage our results from our control group, we see that 297% more measurements are required in direct vs. indirect testing. We also see that there is an average of 188% more measurements re - quired in direct mode vs. hybrid test mode. Conclusion When flying probe testing under the re- quirements of Test Level C, we are bound by the default direct mode. We can see from our exper- iment that this could cause an average increase in measurements of 297% over indirect testing. However, without satisfying the AABUS require- ment the extra measurements are required. And yes, this will take extra time. PCB Todd Kolmodin is the vice presi- dent of quality for Gardien Services USA, and an expert in electrical test and reliability issues. To read past columns, or to contact Kolmodin, click here. Table 3: Indirect, direct and hybrid test measurements. FLYING PROBE TESTING VS. IPC-9252B

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