length in the left to right direction substantially equal to the width of the lower flange portion 126 and a depth slightly smaller than a thickness of the lower flange portion 126 thus each of the cutouts 150 151 extends through left and right sides of the lower flange portion 126 the front cutouts 150 of the lower flange portions 126 of each two neighboring first fins 11 are communicated with each other more specifically for each two neighboring first fins 11 a right end of the front cutout 150 of a left first fin 11 connects a left end of the front cutout 150 of a right first fin 11 thus a first guiding line 15 is formed at a front end of the guiding surface 19 of the heat sink 10 by the front cutouts 150 of the lower flange portions 126 after the first fins 11 are stacked together similarly the rear cutouts 151 of the lower flange portions 126 of each two neighboring first fins 11 are communicated with each other and a second guiding line 15 is formed at a rear end of the guiding surface 19 of the heat sink 10 by the rear cutouts 151 of the lower flange portions 126 after the first fins 11 are stacked together when the heat sink 10 is assembled the first fins 11 are arranged at the central portion of the heat sink 10 side by side and the second fins 12 are arranged at the left and right sides of the first fins 11 respectively the first and the second guiding lines 15 are accordingly formed by the front and.