An electronic device assembly includes a PCB (10), a heat sink (30) mounted on the PCB, a back plate (20) attached below the PCB, a fan holder (40) mounted on a top of the heat sink and a fan mounted on the fan holder. The fan holder has a plurality of sleeves (46). A plurality of clips (50) is assembled on the fan holder. Each clip includes a locking column (52) received in a corresponding sleeve of the fan holder, a shank (56) extending through the column, a spring (54) mounted around the column and sandwiched between a top of the column and a bottom of the shank, and an operating member (58) pivotably connected to the shank. The back plate forms a plurality of studs snappingly engaging with the locking columns to securely mount the heat sink on the PCB.