Test system and test method for printed circuit board assembly

A test system is provided. A printed circuit board (PCB) includes a plurality of traces and at least one test point. A central processing unit (CPU) socket including a plurality of first pins and a memory module slot including a plurality of second pins are disposed on the PCB. Each of the second pins is coupled to the corresponding first pin of the CPU socket via the corresponding trace. A CPU interposer board is inserted into the CPU socket, and a memory interposer board is inserted into the memory module slot. The traces form a test loop via the CPU interposer board and the memory interposer board. When automatic test equipment (ATE) provides a test signal to the test loop via the test point, the ATE determines whether the test loop is normal according to a reflectometry result of the test signal.