A method of creating a flat-round tube-to-header joint in a CuproBraze™ heat exchanger wherein the flat-round tube-to-header joint is disposed between a tube and a header having a generally circular opening having a first predetermined diameter formed on a first side thereof for receiving one end of a tube, and provides at least one generally circular end having a second predetermined diameter on the tube to fit into the generally circular opening of in the header. The method further provides a predetermined temper on at least one generally circular end which is at least sufficient to enable cold working of at least one generally circular end to prevent premature failures of the flat-round tube-to-header joint. The flat-round tube-to-header joint is formed by inserting one end of the tube into the first side of the header and forming the flat-round tube-to-header joint between one end of the tube and the header.