A reduction in tube to header joint failures in a heat exchanger having spaced headers (12,14), elongated, side-by-side parallel spaced tube slots (22) in the headers (12,14) along the length thereof and a plurality of flattened tubes (26) having ends (24) received in the tube slots (22) and metallurgically bonded to the header (12,14) thereat was achieved through the use of a reinforcing structure (38) having at least two projections (40) having a cross sectional shape complimentary to at least a part of the surface of the tubes (26) at their ends (24) and a length sufficient to extend along the tube ends (24) to a location past the metallurgical bonds between the tube ends (24) and a header (12,14), and a spine (44) extending transverse to the projection. Also disclosed is a reinforcing structure (38) and a method of reinforcing the tube to header joints in a heat exchanger.