Leicester Ionic Liquids Group
Green Solutions
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The main research interests of the group are firmly based around green chemistry with particular emphasis on electrochemical processes. It is active in developing novel solvent systems with industrial applications such as metal deposition and dissolution. It collaborates strongly with industry and much of the work to date has been in the development of novel processes using ionic liquids.

Immersion Coating

The electroless deposition of silver metal onto copper substrates is an important industrial process used primarily by the manufacturers of printed circuit boards to prevent degradation of exposed copper surfaces (conduction tracks) before the board is assembled into a finished device (this can be up to several months).

A printed circuit board and an SEM image of a failed solder joint.

Ag is more susceptible to aerobic oxidation than Cu, however, the silver oxide and sulphide (visible as surface tarnish), as well as the underlying silver metal, are very soluble in tin and tin/lead based molten solders that are used to bond the circuit components to the copper tracks of the circuit board. On the contrary copper oxides are poorly soluble in the molten solder which results in failure of the joint either through poor conductivity or because of low mechanical stress tolerance.

The current commercial electroless silver process typically uses an aqueous AgNO3 solution in the presence of HNO3. There are social and environmental concerns surrounding the use of strong inorganic acids, and an additional detrimental effect because of competitive etching of the copper tracks during silver plating. Many component failures arise because of copper etching (by the HNO3) before the surfaces are silver plated. In addition, the aqueous process requires the use of a colloidal catalyst (usually palladium metal) to sustain silver plating.

Scanning electron micrograph of a polished Cu sample exposed to AgCl solution of ChCl:2EG IL for 5 minutes and an SEM of a solder joint onto a PCB which has been coated with immersion Ag from the IL process

An immersion Ag process has been developed using an IL based on choline chloride and ethylene glycol eutectic mix, which gives a high quality solderable Ag finish for PCB applications where electroless silver deposits of up to several microns have been obtained by dip coating in ionic liquids without the use of catalysts of strong inorganic acids.

Acoustic impedance data (QCM) for a silver deposition experiment.

Crucially, the nature of the morphology of silver deposition facilitates sustained growth of silver coatings. This is in contrast the aqueous dip-coating reaction. We have used acoustic impedance spectroscopy performed with a Quartz Crystal Microbalance (QCM) together with AFM and SEM to probe the mechanism of deposition and the structure of the silver deposits. Electroless silver deposits of up to several microns have been obtained by dip coating in ionic liquids without the use of catalysts of strong inorganic acids.

AFM image (resonant mode) of a Ag plated Cu sample (SEM below); (a) xyz surface projection, (b) height contrast image of the same data, (c) height/distance data for the diagonal line trace sown in (b).