Matrix (30) for a heat exchanger to exchange heat between a first fluid and a second fluid, the first fluid being for instance air and the second fluid being for instance oil. The matrix (30) comprises: a channel for the first fluid. an array of passages for the second fluid, the passages extending in the channel. The array supports at least two cooling fins. The matrix is made by a process of additive manufacturing. The fins are inclined with respect to each other along the direction of the flow of the first fluid.