Mechanism for the development of Sn- Cu alloy coatings produced by pulsed current electrodeposition

Pulsed current (PC) electrodeposition has become an essential tool for producing coatings in a wide range of industries. The pulsed current can have a significant influence on the composition, morphology and properties of electrodeposited coatings and this is particularly true when plating an alloy. In this study, the mechanism for the development of Sn-Cu alloy coatings produced by PC electrodeposition was investigated. Sn-Cu alloy coatings produced by PC electrodeposition were essentially composed of a dual layer of Sn-Cu electrodeposits and Cu6Sn5 intermetallic compounds (IMCs). In addition, it was observed that pulsed Sn-Cu electrodeposits exhibited an increased Cu content. Experiments were carefully designed to elucidate the mechanism for this important finding and based on their results a theory is proposed which explains the increased Cu content in terms of a metal displacement reaction occurring during the ‘relaxation’ time of PC electrodeposition.


INTRODUCTION
Tin is an important metal in industry as it can be alloyed with various metals (e.g. Cu, Pb, Bi, Cd, Co, Ni and Zn) to form binary alloy systems. Alloys of Sn-Cu of various compositions are of high interest due to their relatively low cost and excellent corrosion resistance and have found a range of applications in industrial sectors such as electronics, automotive and aerospace depending on their mechanical, thermal and electrical properties [1][2][3]. A thorough overview of electrodeposition of Sn-Cu alloy coatings can be found in a recent review paper [4]. Pulsed current (PC) electrodeposition exhibits advantages over direct current (DC) electrodeposition in the control of deposit composition, grain size, surface morphology and preferred orientation [5]. The present study was undertaken to investigate the effect of pulsed current (PC) on the characteristics of tin-rich Sn-Cu alloy electrodeposits. To the best of our knowledge, it is the first time that the mechanism of PC electrodeposition on Sn-Cu alloy coatings is thoroughly understood and reported. This mechanism is probably also applicable to electrodeposition of other metal alloys using a PC electrodeposition method.

EXPERIMENTAL
Pure Cu panels (Yamamoto B-60-P05) of standard Hull cell size (100 mm × 75 mm × 0.3 mm) were used as cathodes, which first had a 2 µm Ni interlayer electrodeposited on it from a standard Watts bath, followed by a 10 µm Sn-Cu alloy electrodeposit from a commercial electrolyte based on fluoroboric acids. The anode materials used for electrodeposition of Ni and Sn-Cu alloy coatings were a pure Ni and a pure Sn metal panel, respectively. Prior to electrodeposition, the Cu panels were washed with detergent (Decon 90), pickled in a 30 vol. % solution of hydrochloric acid (specific gravity (SG) 1.16) for 1 min, rinsed with deionised water and dried using hot air. The anode materials underwent the same pre-treatment process. Electrodeposition processes were carried out at various cathode current densities ranged from 5 to 50 mA/cm 2 for different deposition times to achieve a consistent thickness of 10 µm. PC electrodeposition was applied with a frequency of 10 Hz and a duty cycle of 10% using a benchtop pulse reverse rectifier (Plating Electronic GmbH pe86CB-20-5-25-S/GD). No agitation was applied during both direct current (DC) and pulsed current (PC) electroplating.
Scanning electron microscope (SEM) analysis, using a Carl Zeis (Leo) 1530 VP field emission gun scanning electron microscope (FEGSEM), was carried out to investigate the surface morphology of electroplated Sn-Cu alloy coatings. Energy dispersive x-ray (EDX) analysis of the chemical composition of electroplated Sn-Cu coatings was carried out using an Oxford Instruments X-Max 80 mm 2 EDX detector equipped in the FEGSEM.
The cross-sectional microstructure of the Sn-Cu alloy electrodeposits were investigated using a FEI Nova 600 Nanolab dual beam focused ion beam (FIB).

RESULTS AND DISCUSSIONS
SEM analysis of the surface morphology of Sn-Cu alloy coatings produced by DC electrodeposition is shown in Figure 1. The surface of the samples electrodeposited at 5 mA/cm 2 was populated with irregularly raised features ( Figure 1a). EDX analysis showed that these raised features were predominantly composed of Sn (~ 91 wt.%), while a higher Cu content (~ 54 wt.%) was detected from adjacent lower surfaces. The average Cu content over the entire coating electrodeposited at this current density was over 50 wt.%.
An increase in current density resulted in an increased number of raised features present on the surface, which gradually developed to cover the entire coating surface (Figure 1b).
When the current density was above 30 mA/cm 2 , the initial deposit surface was completely covered by the raised Sn-rich features (Figure 1d). EDX analysis showed that the electrodeposits having this surface morphology contained a high Sn content (> 95 wt.%). As the current density was further increased, the surface morphology remained very similar and the deposit surface became smoother and more compact (Figure 1 e and f). Based on these observations, it is thought that when a very low current density was applied, Cu (the more noble metal) was first electrodeposited on the cathode surface to form an initial layer of pure Cu. As the reduction potential of the Sn (the less noble metal) was obtained, Sn started to co-electrodeposit with Cu as an alloy. A further increase in the current density enhanced the electrodeposition of Sn and resulted in the development of Sn-rich grains (the raised features) from the Cu-rich coatings, which continued to grow to form a complete coverage.
For PC electrodeposited samples, it was found that the surface morphology remained very similar under the same current density, which suggested that pulsed current had little effect on the surface morphology. Deposition current density was the predominant factor that influenced the surface morphology of the electrodeposited Sn-Cu alloy coatings.
EDX analysis (shown in Figure 2) suggested that the Cu content in the Sn-Cu alloy coatings was inversely proportional to the electrodeposition current density, irrespective of electrodeposition conditions. An increase in current density resulted in a dramatic reduction in the Cu content. More importantly, it was found that a higher Cu content was obtained for PC electrodeposited samples over all the current densities. To understand the role of PC electrodeposition in the chemical composition of the Sn-Cu alloy coatings, immersion tests with the help of EDX and FIB analysis have been used in the current study. Sample I was prepared using DC electrodeposition at a current density of 40 mA/cm 2 for 5 min and the resultant Cu content in the alloy coatings was 1.7 wt.%. Sample II was PC electrodeposited at 10 Hz and 10% duty cycle for 5 min under the same electrodeposition conditions. The obtained Cu content (2.0 wt.%) of the PC electrodeposited sample was 0.3 wt.% higher than that of Sample I. Since the duty cycle used is 10%, Sample II was in essence subjected to 0.5 min of electrodeposition (Ton) and 4.5 min of 'relaxation' (Toff). Sample III was firstly DC electrodeposited at 40 mA/cm 2 for 0.5 min, which was the same electrodeposition time as that of Sample II. EDX analysis straight after electrodeposition showed that Sample III exhibited a Cu concentration of 1.5 wt.%. Sample III was subsequently immersed in the Sn-Cu electrolyte for 1 h with no current applied. It is interesting to find that the resultant Cu content of Sample III after immersion was dramatically increased to 45 wt.%. Sample IV went through the same process as Sample III, except that the immersion time was increased from 1 to 24 h, after which the Cu content was increased to 48 wt.%.
FIB analysis was carried out to investigate the cross-sectional microstructure of these samples. It is apparent that there is no major difference between the microstructure of Sample I and Sample II (shown in Figure 3a and b). Both of the DC and PC electrodeposited samples were composed of columnar grains of approximately 1 µm in width with vertical grain boundaries. More importantly, it was found that white features were present along the grain boundaries. EDX analysis of the cross-section showed a high Cu content (~ 36.7 wt%) in the white features, whilst no Cu was detected from the adjacent grains. These results suggested that the white features were in essence Cu6Sn5 intermetallic compounds (IMCs) and is consistent with previous studies that propose that, for Sn-rich Sn-Cu alloy electrodeposits, Cu is present in the form of Cu6Sn5 IMCs precipitated in the grain boundaries [6].
More importantly, it was found that the cross-sectional microstructure of Sample III was significantly different (shown in Figure 3c). A thin layer (~ 300 nm) was formed on the top surface with the presence of a few holes in the Sn-Cu layer. EDX analysis of the newly formed layer showed a Cu content of 35.4 wt.% and the rest was Sn, which is similar to that of Cu6Sn5 IMCs found in Sample I and II. Hence, it is proposed that the newly formed layer was in essence Cu6Sn5 IMCs. Based on these observations, it is conjectured that when the Sn-rich Sn-Cu alloy samples were immersed in the electrolyte with no current applied, since Sn is more reactive than Cu, a metal displacement reaction

CONCLUSIONS
Cu content in the Sn-Cu alloy coatings was inversely proportional to electrodeposition current density.
PC electrodeposition was more favourable for the co-electrodeposition of Cu, compared with DC electrodeposition.
The Sn-Cu alloy coatings produced by PC electrodeposition were in essential composed of a dual layer of Sn-Cu electrodeposits and Cu6Sn5 IMCs.
A mechanism for the increased Cu content for PC electrodeposited samples has been elucidated and was attributed to the occurrence of a metal displacement reaction between Sn and Cu occurring during the 'relaxation' time.