Abstract: A flat plate heat exchanger module for use in aerospace applications, automotive applications, industrial applications or similar. The flat plate heat exchanger module comprises a stack of heat exchanger plates, where at least one of the heat exchanger plates further comprises at least one elongated aperture extending across the surface of the heat exchanger plate. This elongated aperture is in fluid isolation from the fluid flowing across the surface of the heat exchanger plate. The use of at least one elongated aperture throughout the stack of heat exchanger plates minimises the overall effect of the expansion and contraction of the metal due to exposure to high temperature gradients. A method of manufacturing such a heat exchanger module is also provided.