The fabrication and assembly of printed circuit boards (PCBs) involves the layering of metals onto an insulating substrate prior to the addition of electronic components. The interfaces between these metal layers are often subject to metal-metal interdiffusion, formation of brittle intermetallics and corrosion, compromising the integrity of the PCB-component solder joints. We have recently developed environmentally sustainable, non-toxic methodologies for application of metal PCB finishes using Deep Eutectic Solvents (DESs) as a novel solvent medium.^1-3

Here we focus on the characterisation of metal-metal interfaces in DES media quantifying interfacial structure, layer thickness and morphology using a range of electrochemical and microscopy techniques, including chronoamperometry, cyclic voltammetry (CV) and novel simultaneous thickness electrochemical potential (STEP) measurements coupled with neutron reflectivity (NR.) We have developed the STEP methodology to ascertain some, if not all, of the same information that NR provides.

Deposition of Ag and Cu layers has been carried out in different configurations onto Au substrates and characteristic STEP traces are presented for each upon galvanostatic stripping. The STEP methodology facilitates the identification and quantification of each metal layer by galvanostatic dissolution, which is comparable with time resolved NR measurements. The characteristic features of the STEP measurements are discussed including the shape and potential of the dissolution trace. The degree of mixing between the layers is qualitatively determined from E vs. t traces, along with the layer composition and thickness for systems in which layers remain discrete.

In summary, we present a new, accessible and inexpensive technique, which probes the thickness, composition and interfacial mixing between layers in PCB metal systems using DES electrolytes and galvanostatic stripping methodologies.

[1] A. D. Ballantyne, G. Forrest, M. Goosey, A. Griguceviciene, J. Juodkazyte, R. Kellner, A. Kosenko, R. Ramanauskas, K. S. Ryder, A. Selskis, R. Tarozaite and E. Veninga, Circuit World, 2012, 38(1), 21-29.

[2] K. S. Ryder, A. D. Ballantyne, D. Price and T. Perrett, The PCB Magazine, February 2012, 22-30.

[3] A. P. Abbott, S. Nandhra, S. Postlethwaite, E. L. Smith, and K. S. Ryder, Phys. Chem. Chem. Phys., 2007, 9, 3735–3743.