Estimated reading time: 8 minutes

EuroTech: Institute of Circuit Technology Northern Seminar 2016, Harrogate

Professor Karl Ryder from the University of Leicester gave the final presentation, on next-generation surface finishes for PCBs. It was 150 years since Michael Faraday’s statement of the laws of electrolysis and his first experiments in metal finishing. But all of his work related to aqueous solutions. Since the late 1990s, a research group at the University of Leicester had been exploring an alternative approach to metal finishing using non-aqueous chemistry based on a series of ionic liquids consisting of eutectic mixtures of quaternary ammonium salts with either metal salts or hydrogen bond donors. These ionic liquids were effectively room-temperature molten salts which had unusual solvent properties, low vapour pressure and good thermal stability, and many had wide electrochemical windows which enabled results that could not be achieved with aqueous systems. However, they were very expensive, and the team at Leicester had sought to develop liquids that had similar properties but were practicable and cost effective. Mixing ethylene glycol and choline chloride, both readily available in industrial quantities, in 2:1 molar ratio produced a type of ionic liquid classified as a deep eutectic solvent, which had been named Ethaline 200, and this had been used as the basis of a series of metal finishing formulations. Because of its ability to dissolve metal oxides, it had also been demonstrated to have great potential as a soldering flux. 

Professor Ryder described applications in electroplating, electropolishing, metal recycling and energy storage before focusing on immersion coatings, in particular metals such as silver, palladium and gold used as solderable finishes on PCBs. Immersion deposition chemistries based on deep eutectic solvents were more environmentally friendly, less toxic and more energy efficient than traditional aqueous processes. The Ionmet project had demonstrated benefits with immersion silver and the Aspis project had shown that the corrosive effects of aqueous immersion gold on electroless nickel could be avoided using deep eutectic solvent chemistry. Moreover, the use of cyanide salts as the gold source was not necessary. 

The current multi-partner project, with the acronym Macfest, was nearing completion and the outcome was a nickel-palladium-gold universal soldering and wire bonding finish for PCBs. The base layer was a commercial electroless nickel-phosphorus three to six microns thick, onto which was deposited a layer of palladium 50–150 nanometres thick by a novel immersion process based on palladium chloride in Ethaline. The final layer was 25 nanometres of immersion gold, deposited from an acid-free and cyanide-free formulation where the source of gold could be either gold chloride or sodium gold thiosulphate, again using Ethaline 200 as the solvent. This finish, named ENIPIG had been independently tested for solderability and had shown results as good as or better than standard ENIG, without any evidence of the grain boundary attack which had been shown to cause “black pad” failures with the latter. 

Bill Wilkie wrapped up the proceedings, acknowledged the generous support of GSPK Circuits and thanked presenters for sharing their experience and delegates, some of whom had travelled from afar, for their attention and their participation in the question and answer sessions. The formal business over, the informal networking session characteristic of ICT events carried on well into the evening. 

Pete Starkey is based in the UK, and joined I-Connect007 as technical editor in 2008. Starkey has more than 30 years’ experience in the PCB industry, with a background in process development, technical service and technical sales. To contact Starkey, click here.

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