A plug assembly includes a lower cover, an upper cover mounted on the lower cover, and a circuit board. The upper cover is equipped with a heat-conducting element. The circuit board is mounted between the upper cover and the lower cover. The circuit board has two first contact portions contacting with the lower cover and the upper cover, and a plurality of second contact portions. The circuit board is equipped with a chip. The chip is mounted on and covers the plurality of the second contact portions. The chip contacts with the heat-conducting element. The circuit board opens at least one perforation. At least one of the lower cover and the upper cover protrudes towards the circuit board to form at least one fastening portion passing through the at least one perforation and contacting with the at least one of the upper cover and the lower cover.