Abstract: The disclosure concerns a heat exchanger comprising a heat exchanger core having a first heat exchange passage for a first working fluid intended to be involved in a heat exchange process, and a second heat exchange passage for a second working fluid intended to be involved in the heat exchange process. The heat exchanger further has a header comprising a duct for conveying working fluid to or from at least one of the first and second heat exchange passages, wherein the header is welded to the heat exchanger core along a header-to-core joint. The heat exchanger core has a groove extending at least partly along the header-to-core joint, wherein a cross-section of the groove has a rounded shape with a depth of the groove larger than half the width of the groove. The disclosure also concerns a method for manufacturing such a heat exchanger.