Abstract: A fabricated, B type heat exchanger tube is folded with a distinctive internal enclosed seam of substantially triangular cross section, with an enlarged cross sectional area and longer, sharper corners. The uniquely shaped internal seam prevents liquid braze material from bridging and filling the cross sectional area of the seam, and thereby causes a lengthwise, erosive flow of braze material to move along the internal seam, rather than along the external seam.