Electrodeposition of Cu-Sn alloys from methanesulfonate electrolytes

Abstract

The most commonly used alloy in the electronics industry has been the ubiquitous tinlead alloy. As the demand for electronic devices continues to increase, there have been concerns about the continued use of lead and its long term environmental impact. In the last decade there has been a push to ban the use of lead in electronic products. Legislation from various governments around the world limiting the use of lead has given rise to the drive to find suitable lead-free alternatives. The aim of this research project was to establish a systematic approach for the selection of electrochemical parameters for the electrodeposition of tin-rich copper-tin alloys from a single electroplating bath. By studying and understanding a model system such as copper-tin, one can then use the information obtained as a basis to successfully deposit various other tin binary alloys in the future. Tin-rich deposits were enabled by employing various strategies such as maintaining a high Sn to Cu ratio in the electrolyte and by using surface active agents that have been known to facilitate alloy co-deposition. The effect of surfactants on the tin content in the deposit was initially examined with the aid of a rotating cylinder Hull cell. It was found that the presence of fluorosurfactant was crucial in eliminating metal oxide formation. Cyclic voltammetry at a rotating disk electrode showed that inclusion of surfactant in the electrolyte had no effect on the reduction potential of tin which remained at -0.45 V vs SCE. However, the reduction potential for copper shifted from approximately -0.13 to -0.18 V vs SCE, thereby facilitating alloy co-deposition. Chronoamperometry and anodic stripping voltammetry showed that current efficiency for copper-tin deposition ranged from 55-92% depending on the deposition time and deposit composition. Results from voltammetry experiments were used in the next galvanostatic electrodeposition experiments at vitreous carbon electrodes. Deposits containing up to 96 wt.% tin were obtained from both direct current and pulse plating modes. It was found that an optimal current density of 22 mA cm-2 was needed to obtain desirable deposits. For pulse plating the peak current density should be set to 100 mA cm-2 with a duty cycle of 0.2. Cu-Sn alloys obtained consisted of two phases, tetragonal tin and a hexagonal Cu6Sn5 intermetallic compound. Deposit annealing showed that the Cu3Sn intermetallic was not formed.