An in-situ Study of Buried Metal Interfaces Using Electrochemical and Neutron Scattering Techniques

Metal coatings form an integral part of a wide range of industries, from aerospace and automotive through to electronics. New legislation in recent years has rendered many traditional aqueous plating processes too environmentally damaging, driving the development of environmentally benign, sustainable processes. This thesis studies the growth and dissolution of common PCB metals in DES media. The electrochemical growth of two ‘model’ metals was monitored using event mode neutron reflectivity, providing access to thickness, roughness and porosity (solvent content) of the films as they grew. These data show that the resulting Ag and Cu films were of high quality (smooth, dense, non-porous) therefore representative of those found in the PCB industry. Ag and Cu single metal and bi-layers on Au substrates were used to demonstrate the capabilities of the STEP stripping technique; it was found that metal layers and mixing between them are qualitatively identifiable from characteristic stripping potentials for each metal layer. The presence of each stripping species was confirmed by NR and EQCM studies. Moreover, quantitative thickness information could be extracted for systems containing discrete layers. Interestingly, both configurations of Ag/Cu bilayer produced identical STEP traces, indicating that the order of stripping is dictated by thermodynamics rather than spatial accessibility. The STEP technique was used to monitor progressive interdiffusion at Au-Ag and Au-Cu interfaces and showed that a degree of mixing occurs at these interfaces on the timescale of PCB manufacture. Qualitative information could be drawn from STEP on systems involving more complex, reactive metals (Sn,Ni) and it was found that the addition of a more noble ‘barrier layer’ could prevent rapid interdiffusion at the Au-Sn interface. The novel use of STEP described in this thesis provides an accessible method for characterising multi-layer metal systems which can be used to guide and save time/cost on future NR studies.