Seam welding (seam sealing) is an equipment to put a lid on a package to seal electronic components. The purpose is to isolate the components inside the package from outside air, and to keep them stable for a long period of time.
This is called hermetic sealing.
The type of components placed inside the package includes crystal devices, MEMS, sensors and optical devices. Same as the case of resistance welding, seam sealing also welds the package and the lid using pressure force and electricity.