I guess these boards only used Fly test. Flying needle test is one of the methods to check the electrical function of PCB (short circuit test). The flight test machine is a system for testing PCB in a manufacturing environment. Instead of using the bed-of-nails interface, which is used in all conventional online test machines, the fly pin test USES four to eight independently controlled probes that move to the components in the test. The unit (UUT,unitundertest) is transported to the test machine through a belt or other UUT transfer system. Then fix the probe contact test pad of the tester. That is not enough for inner layer, you should be choose resistance testing. That is not normal in PCB prototype manufacturer in China. Focob pcb and ALLPCB is a good choice. They will use resistance test in 4plus layers boards.
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