A heat sink assembly includes a mother board (70), a CPU (50), a pair of retention modules (40), a heat sink (30), and a fastener (20). The retention modules are secured to the mother board at opposite sides of the CPU. The heat sink is attached to the CPU. The fastener includes a spring (26), and a pair of wedges (22) engaged at opposite ends of the spring. The fastener engagingly spans across the heat sink. The spring provides the fastener with resiliency, and presses the wedges against the retention modules. The wedges are pressed by the retention modules toward the CPU, thereby pressing the heat sink against the CPU.