AuSn20 eutectic electrodeposition through alternative complexing of pyrophosphoric acid: Insights from electrochemical and DFT methods

Eutectic AuSn20 solder is an important material for electronic packaging technology due to its superior mechanical and thermal conductive properties. In this work, AuSn20 alloy films are prepared via the electrodeposition method for the first time. The electrodeposition is cost-effective with improved control over the alloy content when compared to traditional powdered metallurgy methods. Pyrophosphoric acid was found to be an effective complexing agent to minimize the difference of the deposition potentials between Au and Sn, making the codeposition of AuSn alloys possible. Importantly, electrochemical characterization was combined with density functional theory (DFT) calculations to provide insight into the mechanism of the alloy codeposition when pyrophosphoric acid was used as the complexing agent. In particular, natural bond orbital (NBO) charge distribution and the lowest unoccupied molecular orbital (LUMO) characteristics of [P₂O₇] ^4--Sn(II) and [P₂O₇]^4--Au(I) complexes are calculated, suggesting that [P₂O₇] ^4- is able to coordinate more strongly with Sn(II) than Au(I). As a result, it can thus shift the deposition potentials of Au(I) and Sn(II) much closer. As the DFT predicted, the role of pyrophosphoric acid as a complexing agent has been experimentally verified, making codeposition of Au and Sn realistic. The structures of the obtained AuSn20 films are determined using scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX) and found to be consistent with AuSn/Au₅Sn eutectic as predicted by the Au-Sn phase diagram. Additionally, the measured melting point is in good agreement with the theoretically determined one. Relevant tests demonstrated in this work indicate that the newly developed electrodeposited AuSn20 alloy coatings are suitable for microelectronic soldering applications.