An LED based light source assembly (10) comprises a circuit board (11), a plurality of LEDs (13), a heat sink (15) and a plurality of fasteners (17). The circuit board comprises a first surface (12) and a second surface (14) opposite to the first surface. The first surface is provided with electrical conductive traces (118). The plurality of LEDs are arranged on the first surface of the circuit board and electrically connecting with the electrical wires of the circuit board. The heat sink thermally contacts the second surface of the circuit board. The plurality of fasteners each comprise a first end (172) and a second end (174) opposite to the first end. The first end of each of the fasteners abuts on the first surface of the circuit board. The second end of each of the fasteners extends through the circuit board to be secured to the heat sink.