Nickel-based electrodeposits for corrosion protection of smart card contacts

Abstract

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 Abstract ii (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...