Fastener

A fastener comprising:

• a head;
• a shank unit extending longitudinally from said head and defining a central axis, wherein said shank unit includes a drilling portion and a shank portion formed between said head and said drilling portion, said drilling portion having a tip located in opposing relationship to said head;
• and
• a thread unit spirally disposed on said shank unit;
• wherein said thread unit includes a first thread spirally disposed on an outer periphery of said shank portion and a second thread and a third thread spirally disposed on an outer periphery of said drilling portion, respectively, with said third thread located between said first thread and said second thread, said third thread having a first end facing said head and a second end opposite to said first end, with said first end of said third thread connected to said first thread, said first thread defining a first path while spirally winding around said shank portion, said second thread and said third thread defining a second path and a third path respectively while spirally winding around said drilling portion, a first included angle being defined between said central axis and said first path of said first thread, a second included angle being defined between said central axis and said second path of said second thread, a third included angle being defined between said central axis and said third path of said third thread, said first included angle being greater than said third included angle, said third included angle being greater than said second included angle.