A microchannel heat exchanger having: fin segments defined between lower and upper fin tips; at each fin tip, the fin segments have inner and outer facing surfaces; each fin segment having louvers having: an upper transition region at an upper louver end, adjacent an upper fin tip; a lower transition region at a lower louver end, adjacent a lower fin tip; and a straight region extending therebetween, wherein: the transition regions along the inner facing surface at each fin tip have a first transition surface having a first transition length, disposed at a first transition angle; the transition regions along the outer facing surface at each fin tip have a second transition surface having a second transition length, disposed at a second transition angle; wherein: the first transition length is longer than the second transition length; and/or the first transition angle is smaller than the second transition angle.