Electrodeposition of Soft Gold from a Thiosulfate‐Sulfite Bath for Electronics Applications

Abstract

The possibility of electrodeposition of soft gold from a thiosulf ate-sulfite bath was explored for electronics applications. The bath does not contain cyanide, and it is operated at a near neutral pH and a mildly elevated temperature. The bath is stable, does not undergo spontaneous decomposition without the addition of any stabilizer, and yields gold deposits with a hardness sufficiently low for use as gold bumps on semiconductor devices. Factors affecting the hardness were investigated in detail. It is shown that the use of high concentrations of the complexing agents and/or the addition of thallium(I) ions decreases both hardness and sulfur content of the deposit. The lowest Vickers hardness values achieved were approximately 80 kg mm2 in the as-deposited state and 50 kg mm2 after annealing at 350°C for 30 mm in air. The relationship between hardness and microstructure of the deposit was also examined.* Electrochemical Society Active Member.

Development of new electrolytic and electroless gold plating processes for electronics applications

“…Fig. 1 demonstrates the effect of thallium concentration on the hardness of the gold deposit formed in the thiosulfate-sulfite bath [1]. The second approach we found effective for decreasing the hardness is to increase the thiosulfate concentration.…”

“…In contrast, the Au(I) thiosulfate-sulfite mixed ligand bath we developed is highly stable and requires no stabilizing additive [1,2]. We selected this system in view of the fact that it is used successfully for formulating noncyanide, autocatalytic electroless gold plating baths yielding good bath stability and deposit properties [10][11][12].…”

“…Section 2 summarizes results of our investigation carried out to formulate a new non-cyanide, soft-gold electroplating bath containing both thiosulfate and sulfite as ligands for Au(I) [1,2]. This bath is highly stable, and unlike the conventional sulfite bath, it does not require the addition of a stabilizing agent.…”

Some recent developments in non-cyanide gold plating for electronics applications

“…In view of the prior successful development of electroless gold plating baths containing both thiosulfate and sulfite (17,18), which will be described in the subsequent section, the present authors and collaborators investigated the possibility of electroplating soft gold from a bath containing the two ligands (19). It was found that the mixed ligand bath is highly stable even without the addition of any stabilizer.…”

Significance of inclusions in electroplated gold films for electronics applications

“…For example, the hardness of pure, soft gold films plated from the classical cyanide bath containing KAu(CN) 2 as the source of gold and a phosphate or citrate buffer to control pH is known to increase with an increase in carbon content (2). In the case of the soft gold plated from the recently developed non-cyanide bath containing sulfite and thiosulfate, the amount of included sulfur-containing species has been shown to be closely related to the hardness (3). Hardness of the soft gold used for fabricating microbumps on silicon IC chips to be mounted on circuit boards in modern electronic packaging technology, must be sufficiently low and hence easily deformable to allow uniform bonding of a large number of bumps with slightly varied thicknesses.…”