Heat sinks with C-shaped manifolds and millichannel cooling

A heat sink for cooling at least one electronic device package includes a lower lid, an upper lid, and a body formed of at least one thermally conductive material. The body is disposed between and sealed to the lower and upper lids and defines a tapered inlet distribution chamber configured to receive a coolant, C-shaped inlet manifolds configured to receive the coolant from the tapered inlet distribution chamber, inverted C-shaped outlet manifolds configured to exhaust the coolant. The inlet and outlet manifolds are interleaved and disposed in a circular arrangement. The outlet manifolds extend around only a portion of the body and terminate adjacent to opposing sides of the inlet chamber. The body further defines a tapered outlet chamber configured to receive the coolant from the outlet manifolds, where the inlet manifolds extend around only a portion of the body and terminate adjacent to opposing sides of the tapered outlet chamber. Millichannels are formed in the body or are formed in at least one of the lids and are configured to receive the coolant from the inlet manifolds and to deliver the coolant to the outlet manifolds. The millichannels are disposed in a radial arrangement, and the millichannels and the inlet and outlet manifolds are further configured to cool one of the upper and lower contact surfaces of the electronic device package. A lidless heat sink is also provided.