A heat sink assembly includes a first heat dissipating plate, a second heat dissipating plate, at least one flat heat pipe horizontally positioned between the first heat dissipating plate and the second heat dissipating plate. The at least one flat heat pipe surrounds or defines an enclosed-space. The enclosed-space is closed by the first heat dissipating plate and the second heat dissipating plate. At least one through hole communicates with the enclosed-space and is defined in the second heat dissipating plate.