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.
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.
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.