A header plate (20) is provided for receiving the flattened tubes (14) of a heat exchanger. The header plate (20) includes a plurality of tube receiving openings (26), with each of the openings being surrounded by a flange or collar (28) that extends from a base wall (24) of the header plate (20) towards the heat exchanger core (12) to surround the exterior cross section of the associated tube (14). Each of the flanges (28) is shaped to conform to the exterior of the associated tube (14), and a blend radius R is provided at the interface between the base wall (24) and the flange (28). The blend radius R has a size that increases from a radius R1 central to the length of a long side wall (30) of the flange (28) to a radius R2 central to the length of a short side wall (32) of the flange (28).