Impedance and Morphological Properties of Electroless Gold on Industrial Metal Coupons

An approach for correlating the bath chemistry during electroless gold deposition with the overall plating quality on industrial wirebond samples is presented. Electrochemical impedance spectroscopy of the reducing agent dimethylamine borane (DMAB) on gold in alkaline solution was used to provide qualitative information on the interfacial adsorption/desorption processes occurring during deposition. Depending on hydroxide and DMAB concentration, impedance data obtained under hydrodynamic conditions at the gold oxide reduction potential (0.184 V vs Ag/AgCl) follow two different equivalent circuit models that do not exhibit contribution from diffusion. Gold deposit quality was assessed with scanning electron microscopy and energy-dispersive spectrometry under a variety of bath compositions. Finally, bath efficiency was evaluated in terms of plating rate obtained from X-ray fluorescence data. The substrate surface/solution interactions are linked to the final quality of the resulting gold layer. This work shows that substrate incompatibility and the ratio imbalance between gold and the reducing agent concentrations produce thin, dull deposits. Maintaining the optimum ratio of DMAB/KOH provides balance in the bath chemistry necessary for efficient plating.