Fixing apparatus for mounting a heat sink to a printed circuit board

A method for assembling a retention module and a back plate to a printed circuit board, the retention module being for carrying a heat sink thereon and having a plurality of downwardly extending posts therein, the printed circuit board having a plurality of apertures, and the back plate having a plurality of upwardly extending poles each having a hook on a top thereof, the assembling method comprising inserting the poles from a bottom surface the printed circuit board through the apertures into the posts until the hooks engage with the posts at insides of the posts so that the poles are locked with the posts, the retention module is securely mounted on a top surface of the printed circuit board and the back plate is securely attached to the bottom surface of the printed circuit board;

• wherein each post forms at least a support portion therein, the at least a support portion having a lower inclined plane and a horizontal top platform, during the insertion of the poles in the posts, the hooks first sliding on the inclined planes, respectively, and then engaging the horizontal platforms when the poles are locked with the posts.