A securing mechanism for retaining a CPU to an electrical connector mounted upon a top surface of the PCB, includes a fastening device mounted upon the top surface of the PCB beside the electrical connector, and a back plate mounted upon the undersurface of the PCB and located between the PCB and a metallic case. The fastening device cooperates with the back plate to be secured to the PCB via a first set of fixing units. The case forms an opening/recessed region to receive a thickened central region of the back plate and is secured to the back plate via a second set of fixing units surrounding said opening. The thickened central region is fully received within the opening in the vertical direction. The first set of fixing units are located in the central region of the back plate.