When the "Micro Cooling Fan" operating conditions deviate the flow direction changes resulting in various blade inlet angle of attack can not be the same. If a blade angle of attack at the inlet reaches a critical value, it is the first place on the blade stall, but not all "Micro Cooling Fan" blade stall occurs simultaneously, this phenomenon continue in that stall caused clogged area along with promoting the impeller rotates in the opposite direction, that is, a so-called "rotating stall" phenomenon. "Micro Cooling Fan" fan into an unstable operating zone run, will produce a number of rotating stall area to the impeller. Each time through the blade stall area would be the role of a exciting force, thereby allowing the blade to resonate.