A casing includes a metallic sheet and a screw. The metallic sheet is formed with an extruded tapped hole surrounded by an arcuate portion in the metallic sheet and a sleeve integrally with and depending from the arcuate portion. The sleeve has internal threads. On the other hand, the screw is configured to seal the extruded tapped hole of the metallic sheet. The screw has a threaded section configured to be mated with the internal threads of the sleeve of the metallic sheet, and a flat head capable of abutting against the arcuate portion and sitting flush with the metallic sheet when the screw is placed in the extruded tapped hole of the metallic sheet.