Nickel-based electrodeposits for corrosion protection of smart card contacts
Vinod Kumar Murugan
Date of Issue2016-02-01
School of Materials Science and Engineering
Information Management Research Centre
Reliability of smart cards depends extensively on the corrosion resistance of the electrical contacts they contain as these contacts are primarily exposed to the environment. Although corrosion as an entirety is a century old problem, the corrosion of thin electrical contacts has been rarely studied in a systematic manner. Cu is typically used as substrate material for electrical contacts and nickel based barrier layer coating have been essential to suppress copper diffusion through the gold topcoat. This thesis aims to bridge knowledge gaps in literature with regards to the corrosion behaviour of Ni and Ni-P barrier coated electrical contacts. Much work has been dedicated to the corrosion resistance of electroless Ni-P deposits but research on electrolytic deposits is limited. The impact of electrodepositing parameters on corrosion resistance has not been well documented. This thesis aids the understanding of the electrodepositing parameters and their implications on corrosion resistance. Moreover, corrosion behaviour due to different arrangement of thin Ni and Ni-P stacks (< 3 μm) is unknown. This thesis also explores different thin Ni and Ni-P stacks and its corrosion resistance to salt spray (SS) and mixed flowing gas (MFG) environments. The corrosion behaviour of these stacks, corrosion mechanism and causes of corrosion disparity have been also discussed in this report. The thesis is bounded by three main considerations viz., method and type of deposition, and corrosion testing methods. Electrodeposition was the choice of deposition technique, Ni and Ni-P electrodeposits were chosen barrier layers and lastly, accelerated tests which simulate atmospheric corrosion were employed. The research starts with a systematic study on the electrodepositing parameters and their influence on the corrosion resistance of electrical contacts. Corrosion products due to the neutral salt spray (NSS) and mixed flowing gas (MFG) tests were characterised through spectroscopic and diffractions studies. Electrical contacts exposed to NSS tests produced green corrosion residues which consist of CuCl (nantokite) and CuCl2(OH)3 (clinoatacamite) and brown residues which consist of Cu2O (cuprite). Electrical contacts exposed to MFG test produces copper sulfides (major) and nickel sulfides (minor). Also increasing the thickness of Ni consistently improved the corrosion resistance in both neutral NSS and MFG corrosion tests. However, the influence of Ni thickness on corrosion was overshadowed by the Ni-P thickness. Increasing the thickness of Ni-P deposits resulted in higher corrosion in the NSS environment and decreased corrosion under the MFG environment. Although initial results pointed towards a disparity that Ni-P may not be suitable for corrosion protection in environments with heavy chlorine concentration but appropriate for sulfur-containing corrosion media, subsequent in-depth studies revealed that this disparity was due to the high edge porosity caused by fast plating. Subsequently, the arrangement of Ni and Ni-P on Cu was hypothesised to affect the eventual corrosion resistance of an electrical contact. The thickness of the overall barrier layer was kept constant and different stacks such as Ni/Au (NA), Ni-P/Ni/Au (PNA), Ni-P/Au (PA), Ni/Ni-P/Au (NPA) and Ni-P/Ni/Ni-P/Au (PNPA) stacks were examined. The discrete and coalesced pits formed due to MFG were explained with a proposed dominant pit concept. Results showed that multi-layer stacks, such as PNA, NPA, PNPA, displayed worse corrosion resistance than single-layered stacks (NA, PA). Removing Ni and Ni-P interfaces was effective in inhibiting corrosion in NSS and MFG environments as Ni/Ni-P interfaces accelerate corrosion due to galvanic coupling. Corrosion pits propagated horizontally and vertically through Ni and Ni-P films, depending on the way Ni and Ni-P were arranged within a stack. Tunnelling corrosion through Ni-P turned distinct with a reduction in Ni-P thickness. This was attributed to possibly higher percentages of discontinuities (i.e. porosity) with decreasing thickness. The reducing thickness coupled with the galvanic coupling between Ni and Ni-P layers further worsened the corrosion in stacks such as Ni-P/Ni/Ni-P/Au. Based on the two aforementioned studies, disparity in corrosion resistance of Ni and Ni-P barrier layers under NSS and MFG corrosion tests was a recurring observation. When corrosion resistance of Ni/Au and Ni-P/Au stacks was assessed using NSS and MFG corrosion tests, each stack emerged superior in only one test. To explain this disparity, factors that could influence corrosion resistance were examined first, namely: internal stress, surface wettability and sulfur co-deposition. None of these factors could explain for the disparity in corrosion performance. Corrosion product analysis was then performed to understand the corrosion product formation due to NSS and MFG. Detailed pits analysis revealed that the disparity was caused by the porosity and corrosion product migration. These concepts were supported experimentally. Based on the understandings generated from the three main studies, some guidelines are proposed to improve the corrosion resistance of electronic contacts under the MFG and NSS corrosion tests. Optimisation of various parameters to generate the current stacking arrangement (i.e. Ni/Ni-P/Au) offer important but limited solution to enhance corrosion protection as individuals layers behave differently when coupled. Different stacking arrangements studied in this thesis show that single-layered stacks such as Ni/Au or Ni-P/Au offer better corrosion protection. The failure of bi-layered and tri-layered stacks does not necessarily stem from the design of such stacks but stem from the industrial practice to layer very thin coatings (such as Ni-P at 500 nm) on Ni under-plates. The increased defects in such thin Ni-P coatings accelerate galvanic corrosion due to difference in corrosion potentials and the small-anode-large-cathode phenomenon. The study on single layered stacks points to the most important aspect of this thesis. Ni-P as a single barrier layer deposited at lower current densities (≤ 1 ASD) and at higher thicknesses (≥ 2 µm) can improve overall good corrosion protection in both NSS and MFG corrosion tests.