The present invention relates to heat control and cooling of an optoelectrical unit, which converts between electrical and optical signal formats. The optoelectrical unit contains at least one optoelectrical capsule positioned on a circuit board. A primary heat sink is adapted to receive heat energy dissipated from the capsule. The capsule is oriented on the circuit board, such that it presents a relatively small footprint thereon and, at the same time, rises relatively large area sides, which do not face directly towards the circuit board. The primary heat sink has at least one cavity, which is adapted to the shape and dimensions of the capsule, such that the cavity contains at least one capsule. The primary heat couples thermally well to the capsule. Furthermore, the capsules assist in aligning the primary heat sink in its intended position.