and two opposite side edges of the gap 408 have grooves 410 and 412 respectively in this embodiment the first inner surface 406 is a surface surrounding the accommodation space 404 in the outer frame 402 and opposite to the air outlet surface 407 a main circuit board 401 comprises a first circuit board 416 and a second circuit board 418 the first circuit board 416 has a stator 420 and the second circuit board 418 has a driving circuit 422 and the stator 420 is electrically connected to the driving circuit 422 the first circuit board 416 is disposed on the first inner surface 406 and the second circuit board 418 is embedded into the grooves 410 and 412 and blocks the gap 408 in other words the second circuit board 418 becomes a part of the outer frame 402 the stator 420 may be but is not limited to three sets of stator coils the driving circuit 422 comprises a connecting terminal 424 and a driving element 426 fig7 a and fig7 b are respectively a three dimensional exploded view and a three dimensional assembled view of an outer frame and a main circuit board according to a fifth embodiment of the present invention referring to fig7 a and fig7 b a micro cooling fan 500 comprises a housing and a main circuit board 501 and the housing is an outer frame 502 the outer frame 502 has an accommodation space 504 a first inner surface 506 an air outlet surface 507 and a gap 508 the gap 508 is in communication with the accommodation space 504 and two opposite side edges of the gap 508 have grooves 510 and 512 respectively a main circuit board 501 comprises a first circuit board 516 and a second circuit.