Study on Co-Electrodeposition Mechanism of Au-30at.%Sn Eutectic in Non-Cyanide Bath by Electrochemical Methods

Abstract

The co-deposition of Au-30at.%Sn alloy in a non-cyanide alkaline Au + -Sn 2+ bath was successfully achieved with sodium sulfite (Na 2 SO 3 ) and potassium pyrophosphate (K 4 P 2 O 7 ) as green complexing agents, ethylenediamine tetraacetic acid (EDTA) as bath stabilizer and catechol as additive. The Au(I) is mainly complexed with sulfite in the form of [Au(SO 3 ) 2 ] 3− , while the Sn(II) is mainly complexed in the form of [Sn(P 2 O 7 ) 2 ] 6− . The reduction potentials of both Sn(II) and Au(I) shift to a negative value (more negative than −0.70 V vs. SCE) in the presence of sulfite and pyrophosphate complexing agents, resulting in the onset reduction potential gap between Au(I) and Sn(II) decreased from 1.828 V (for standard deposition potential) to 100 mV, and consequently the co-deposition of Au and Sn is realized. The cyclic voltammetry and chronoamperometry characterizations revealed that the co-deposition of Au-Sn alloy is an irreversible and diffusion-controlled process and the nucleation mechanism is represented by the progressive model. The addition of catechol, which can be adsorbed on the cathode surface, increased the co-deposition overpotential, inhibited the co-deposition process and refined the grain size of the electrodeposits. Au-30at.%Sn eutectic solder has been extensively used in microelectronic and optoelectronic packaging, due to the superior mechanical and thermal conductive properties as well as the high long-term reliability. [1][2][3][4] As one of the hard solders, the relatively lower melting temperature (278 • C) of Au-Sn eutectic solder compared with those of Au-Ge (356 • C) and Au-Si (363 • C) eutectic solders, makes it ideally suitable for packaging of bonding devices those are sensitive to high processing temperatures, such as GaAs or large Si dies on alumina. Moreover, the high thermal conductivity of Au-Sn makes it suitable for bonding high-powered devices, such as high-powered LED and laser diode packaging with the demand of good heat dissipation. 5 Conventionally, Au-Sn eutectic solder is prepared by solder preform, 6 evaporation 7 and sputtering. 8 However, the Au-30at.%Sn solder fabricated using these techniques cannot meet the demand for microscale solder preparation due to the limitations of precise pattern formation and deposits thickness. As an alternative method, electrodeposition technique 9 can overcome such limitations and would be an attractive way in Au-Sn solder manufacturing.In the past decades, due to the high stability of cyanide with metallic ions, cyanide containing baths were primarily used to prepare Au-Sn alloy deposits. 10,11 However, cyanide is one of the most toxic chemicals, which is extremely harmful to human health and the environment. Furthermore, cyanide also destroys the photoresist in the process of flip chip bump preparation. To solve this problem, a number of attempts have been made to develop non-cyanide Au-Sn electroplating baths. [12][13][14] Up to now, the electrodeposition technology of Au-Sn alloys has not yet fully developed due to the l...

Complexation Behavior and Co-Electrodeposition Mechanism of Au-Sn Alloy in Highly Stable Non-Cyanide Bath

“…Cyclic voltammetry (CV) and density functional theory (DFT) calculation were carried out to investigate the coordination effect between metal ions and selected multiple complexing agents, and the reason for the high stability of Au-Sn bath was clearly clarified. In our previous study, 22 the Au-Sn bath without the DMH addition would decompose with the formation of black powered gold after sealed strorage for 20 days. The investigation in the present work is the effect of DMH addition.…”