I came across to the abr. "NBEG" which is supposed to be "Nickel Barrier Electroless Gold". I can't find a simple explanation of this plating. Is it cheaper than ENIG? What are the advantages of NBEG w.r.t. ENIG?

 

That sounds a lot like what we use for microelectronic hybrids, though I won't swear to it. Basically a process designed to plate a alumina ceramic substrate, which is used to make the hybrids.

They use conductive and non conductive epoxies to glue the chip die (transistors, ICs, capacitors) onto a small circuit board made with gold and ceramic. A gold or aluminum wire bonder (the wire typically measures 0.0007" or 0.001") to connect the die to the alumina substrate.