Development of a non-toxic electrolyte for soft gold electrodeposition: an overview of work at University of Newcastle upon Tyne

Electroplated soft gold is widely used in the growing fields of micro- and opto-electronics as a conducting material for interconnects and devices. Due to problems related to resist compatibility, safety and disposal concerns, cyanide-free plating baths are now strongly in demand. The interest in developing non-toxic gold electrolytes, such as those based on sulfite complexes, has grown rapidly in recent years. The most common non-cyanide gold electrolyte is based on a gold-sulfite complex, which has problems related to stability and resist compatibility. Recently, a novel electrolyte that can be used for soft gold deposition, and is suitable for the formation of microbumps on wafers for electronic applications has been proposed. This bath, containing both thiosulfate and sulfite as complexing agents, is non-toxic, stable on storage and operation, and does not contain any chemical additives or stabilisers. At Newcastle University, we have tested this electrolyte for long term stability, suitability for large scale production, and recyclability (by electrowinning). We have reported the performance of this electrolyte here in this review.