A stacked-plate heat exchanger for cooling a plurality of heat-generating electronic components arranged in a plurality of layers comprises a stack of flat tubes defining a plurality of parallel fluid flow passages, the tubes being separated by spaces for receiving the electronic components. One or more flow-restricting ribs is arranged within at least some of the fluid flow passages to partially block fluid flow between at least one the manifolds and the heat transfer area by reducing the height of the fluid flow passage outside the heat transfer area, along at least a portion of the width of the fluid flow passage, in order to improve the flow distribution of a heat transfer fluid between and within the fluid flow passages of the heat exchanger, and to minimize bypass flow at the outer edges of the fluid flow passage.