A fastener comprising a shank defining a central axis, a head connected to said shank, a drilling portion disposed on said shank, opposite to said head, and a plurality of first threads spirally formed on said shank, each of said first threads includes a first upper surface facing said head and a first lower surface facing said drilling portion, said first upper surface and said first lower surface extending radially from said shank to converge at a first ridge; wherein a plurality of second threads are spirally formed on said shank and affixed to said first threads, each of said second threads including a second upper surface facing said head and a second lower surface facing said drilling portion, said second upper surface and said second lower surface extending radially from said shank to converge at a second ridge, a plurality of opening slots being defined on each of said second upper surfaces and each of said second lower surfaces, said plurality of opening slots exposing portions of said first threads therethrough, characterised in that said second ridge defines a line normal to said central axis of said shank, an upper included angle being defined between said second upper surface and said line of said second ridge, a lower included angle being defined between said second lower surface and said line of said second ridge, said upper included angle being smaller than said lower included angle to allow a slope of said second lower surface relative to said shank to be steeper than a slope of said second upper surface relative to said shank, said plurality of first threads dividing said shank into a plurality of shank segments, each of said shank segments having a base part and a transition part extending outward from said base part, said transition part being inclined to said base part by 5 to 10 degrees. wherein a plurality of second threads are spirally formed on said shank and affixed to said first threads, each of said second threads including a second upper surface facing said head and a second lower surface facing said drilling portion, said second upper surface and said second lower surface extending radially from said shank to converge at a second ridge, a plurality of opening slots being defined on each of said second upper surfaces and each of said second lower surfaces, said plurality of opening slots exposing portions of said first threads therethrough, characterised in that said second ridge defines a line normal to said central axis of said shank, an upper included angle being defined between said second upper surface and said line of said second ridge, a lower included angle being defined between said second lower surface and said line of said second ridge, said upper included angle being smaller than said lower included angle to allow a slope of said second lower surface relative to said shank to be steeper than a slope of said second upper surface relative to said shank, said plurality of first threads dividing said shank into a plurality of shank segments, each of said shank segments having a base part and a transition part extending outward from said base part, said transition part being inclined to said base part by 5 to 10 degrees.