A heat exchanger apparatus 10 that has one or more tubes 12 for carrying a first heat transfer fluid, such as a refrigerant. Fins are provided in thermal communication with the tubes. Some of the fins have fin collar bases 16 that are positioned around the outside perimeters of the tubes 12. One or more bumps 20 protrude from at least some of the fin collar bases 16. The bumps disturb a second heat transfer fluid, such as air, that passes over the fins 14 and the tubes 12. Also disclosed is a method for improving the efficiency of heat exchangers.