A heat sink assembly includes a heat spreader (10), a plurality of fins (12) extending from the spreader, fasteners (20) and securement sleeves (30) fixed to the fasteners. The fastener includes a head (200) and a shaft (204) having a threaded bottom end (208). The sleeve comprises a cylindrical wall (302), an upper open end (304) and a lower open end (306). The sleeve further has an annular pedestal (308) perpendicularly connected with the wall and extending in the lower open end. The head is received in the wall and contacts the pedestal. The shaft extends through the spreader. A compressed spring (22) is providing between the head and the spreader for urging the spreader downwardly to engage with a heat generating electronic device.