A circuit unit such as performing the function of a chip carrier. In one example, it is in the form of a thin square of insulating material having contact pads arranged side by side along the four edges of both major surfaces. The chip is secured substantially centrally of the insulating material and connections are made from it to the contacts on both of the major surfaces. The carrier has a lower insulating layer (5) having contacts (6) extending over its under surface and carrying the chip (14) on its upper surface. An insulating spacer (8) carries further contacts (12). Electrical connections (16 and 18) are made to the contacts (6 and 12). An insulating cover (not shown) then closes off the hollow interior, locating on a shoulder (10). The contacts (12) thus provide the contacts on the upper surface of the finished construction. A single layer construction is also disclosed. Double-sided chip carriers formed in this way are particularly suited for side-by-side mounting in racked manner, enabling maximum use to be made of the contacts on both of the major surfaces and facilitating flow of cooling air.