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52 The PCB Magazine • July 2014 must have a minimum resistance between them or a failure is reported. For example, IPC-9252A Class III specifies greater than or equal to 10M ohms. This measurement is taken very quickly and due to the capacitive charge time the test machine may report this momentary leak as a false short. Testing Grid Test Unfortunately one finds that many of the older grid test machines have no solution for negating this capacitive charge component. Newer machines have the ability to delay the measurement for a specified time to allow for ELECTRICAL TESTING OF PASSIvE COMPONENTS continues the capacitive charge to stabilize before the reading is taken. This is relatively transparent to the operator. Once the test is initiated the continuity test is performed as normal. When the isolation (shorts) test begins the machine applies the test voltage to the PCB and then waits during the input delay time before taking measurements. This allows the capacitive core to charge and stabilize. The end result is a pass rather than a false error reported. Flying Probe These machines have the same dilemma when testing PCBs with buried capacitance. Fly- ing probes do not by default test every net to every net for shorts as the grid test machines Figure 6: Various capacitor evolution, clockwise from upper left: ceramic paper, SMt and buried capacitive cores.