Abstract: A method of manufacturing a heat exchanger core, including providing a plurality of flat tubes having a plurality of flow channels therein, forming at least a portion of each of said flat tubes to define an arcuate tube cross-section, piercing a first and a second header to produce a series of slotted openings therein, assembling the first and second headers to first and second ends, respectively, of the plurality of tubes, and placing the assembled tubes and headers into an elevated temperature environment to cause a braze alloy cladding on either the tubes or the headers or the tubes and the headers to flow and form brazed joints between the tubes and headers. Forming the portion of each of said flat tubes to define the arcuate tube cross-section includes forming an arc extending between an air inlet side and an air outlet side of the tube.