A fastener for fastening a heat sink to a printed circuit board via a back plate comprises a head and a shaft extending from the head. The shaft comprises a first and second engaging portions and a connecting portion therebetween. The first and second engaging portions have outer threads on exteriors thereof. Two cutouts are defined in the connecting portion. The back plate defines four thread holes machined with inner threads for engaging with the outer threads. In use of the fastener, an elastic force is produced to the fastener due to a deformation of the two cutouts during the threading. The elastic force helps the outer threads to have a secure engagement with the inner threads whereby the fastener can firmly engage with the back plate even when the fastener is subject to vibration or shock.