The development of active antennas for space applications, and more particularly for constellations, has reinforced the need for R.F. BFNs - Beam Forming Networks. A transmit BFN connects M R.F. inputs to N R.F. Outputs to form N independent and simultaneous beams. The resulting matrix has MxN internal connections. Typically, this number runs into the hundreds that make it impossible to implement with classical connectors. A configuration using a full ceramic MCM-C module on a multilayer printed circuit board, with more than a hundred of resistors in the internal layers, has been studied and manufactured. This gives a mass of 16 grams for each point of connection in comparison with the 35 grams obtained with classical single layer structures. All the parts are working up to 14 GHz. One of the most important innovations was to implement all the DC and command signal (L.F.) connections on the backside of the MCM. A global assembly system has also been developed to bond the MCMs (12 on each side) to the board. In a single operation, the mechanical attachment and the L.F. connections are achieved. Then, only the high frequency connections are ribbon bonded; thus minimising the number of operations. This configuration is going to be qualified for space applications.