A housing includes a first surrounding wall connected around a base wall. The first surrounding wall has a first joint portion opposite to the base wall, a first outer surface and a first inner surface. The first outer and inner surfaces extend from the first joint portion toward the base wall. The first joint portion is stepped and has a first projecting portion adjoining the first outer surface, and a first shoulder portion indented from the first projecting portion and proximal to the first inner surface. The first shoulder portion has a plurality of parallel spaced-apart ribs protruding therefrom in a same direction as the first projecting portion. Each of the ribs has a tip that does not extend beyond a top end of the first projecting portion.