To prevent the substrate attitude and positional relation from dispersing and effective area on the substrate from reducing by providing a recess at the side wall of a metal cover and adhering the lower end edge of the recess to corresponding substrate top surface; the recess being sunk inwards and decreasing the deformation upwards.

A metal cover 21 has a recess 25 at the middle of the side wall. The recess is formed by cutting two parallel cuts 22 of specified lengths extending parallel to the substrate surface and pushing a resulting square part 24 until deforming inwards. The recess 25 is soldered to a metal pad 26 on a substrate 20, thus mechanically coupling the substrate 20 to the metal cover 21. If the inward protrusion lengths of the part 24 of the recess 25 disperse, the vertical position of the lower face of the recess 25 is always const. and hence no mismatching of the substrate top face with the metal cover lower face occurs to avoid enlarging in the lateral direction.