Abstract: Before semiconductor units are shipped to customers, each unit is electrically tested for ensuring its functionality. However, when a unit is stuck in a test area of a testing equipment, subsequent units are thereafter not properly tested. A reflective sensor is used to check for a stuck unit within the test area. The reflective sensor includes a transmitter and a receiver. In each test cycle, a signal is transmitted toward the test area. An alert indicating a stuck unit is generated when: (1) the reflectivity and/or color of the transmitted signal is different from a bare test area reflectivity and/or color, (2) a reflected signal is received outside a window of the receiver or is not received by the receiver, or (3) the reflected signal has the same light intensity as the transmitted signal. Embodiments of the present invention protect untested units from being inadvertently shipped to customers.