Abstract: A heat exchanger comprising at least one row of flattened tubes through which a heat exchange medium may pass. A serpentine fin 10 disposed between adjacent flattened tubes has relatively straight segments that are connected by bend radii 20. The fin 50 defines a plurality of louvers 32 therein. Each louver 32 forms an elongated slit that extends at least partially into the bend radii 20, through which a fluid to be heated or cooled by the medium may pass. The fin 50 is formed by a tool with fin blades that have tips that are configured to form the extended length louver that protracts into the bend radius that separates adjacent convolutions.