PASS BOX is one kind of facilities, which located on partition walls of clean room and its function shall be a buffer area for transferring goods between inside and outside of clean room. So that, it can prevent any disturbance of airflow and air pressure in clean room, which caused by the door opening for goods transferring. Furthermore, it can reduce the time for operators¡¯ moving in and out, to prevent any dust coming from the out regions. That¡¯s why there is one interlock guard system controlled the inlet and the outlet of PASS BOX not be opened at the same time, in order to prevent the cross contamination.