Feasibility and characterization study of Al and Au wire bonding on immersion Ag and Sn coatings

Immersion Ag and immersion Sn coatings obtained through the electroless process have emerged as an alternative to Ni/Au metallisation layers for more uniform thickness and lower material cost. Microscopic studies by SEM, AFM and TEM indicated that the Ag coating had a finer surface morphology and lower thickness than the Sn coating. Composition study by XPS indicated that copper and sulphur were present in the Ag coating surface, indicating migration of the underlying copper layer and tarnishing of the silver surface. Further study based on the D-SIM revealed that both the Ag and Sn immersion coatings were organometallic in nature. Wire bond mechanical tests showed that both the Al and Au wires were bondable on immersion Ag finish. However, the bondability of these two wires on immersion Sn was poor due likely to the low melting point of the Sn intermetallic. Further study is required to verify whether grain size and hardness of Sn finish would affect wire bondability. Plasma treatment and UV/ozone treatment were applied to these plates with no appreciable improvement in bondability. Fracture analysis of the bonded Ag coating indicated that the wire broke at the neck after pull test, a reflection of lower wire strength at the necked region than at the bonded interface, as also confirmed by the lower pull strength than the shear strength.