Silver plating bath control and purification
Silver plating is a very important process in various industries, such as electrical and electronic industry, automotive industry and aerospace industry. In these industries one of the key problems is to ensure that the bath contents do not wander outside their normal operational limits. Poorly maintained and uncontrolled electrolytic baths will reduce the quality of their deposited finish, and may induce shorting of the electroplated drums or printed circuit boards.
The silver plating bath is a two component system consisting of a salt and distilled water. The solution is prepared by diluting the solid silver salt, usually silver nitrate (AgNO₃) or silver sulfate (Ag2SO₄) with distilled, deionized water to obtain a concentration in the range of 0.01M-0.5M silver ions. The solution is then heated to the recommended operational temperature.
The control and purification of the silver plating baths depends on the silver salt used and the method of operation. The most important aspect of silver plating bath management is to maintain the concentration of the silver ions within the operating range. This is achieved by monitoring the silver concentration during the deposition cycle, and making appropriate corrections to the silver salt concentration by means of either chemical addition or removal.
The chemical addition or subtraction of silver ions to the bath is most commonly done by adding a known amount of silver salt solution of predetermined strength to the bath or by adding an excess of the silver salt grain and subsequently firing off the excess ions in a pulse plating process. It is also possible to add a source of chloride ions, such as hydrochloric acid (HCl) or sodium chloride (NaCl) to the silver plating bath in order to increase the silver salt concentration.
The purity of the silver plating bath can also be maintained by removing impurities created during the plating process by periodic regeneration of the bath. The regeneration of the silver plating bath may involve the addition of household bleach (sodium hypochlorite (NaOCl))to the bath followed by filtration and/or the addition of ion exchangers to the bath.
One of the methods most commonly used for bath purification is the addition of acids such as phosphoric or hydrochloric acid to the bath before filtration. The acids will react with and reduce the concentration of carbonates and bicarbonates in the bath, which are formed during the plating process when hydrogen and bicarbonate ions enter the bath from the deposition of silver from the deposition plates.
The addition of an excess of the silver salt grains, followed by the pulse plating, is also a common method of purifying the silver plating bath. The pulse plating process involves the electrochemically removal of silver ions from the bath by interspersing a burst of current through the bath during the deposition cycle. This process will cause the impure silver ions created during the plating process to be electrochemically removed from the bath, leading to a purified bath with a higher concentration of silver ions.
Finally, the pH of the plating bath should also be monitored and maintained throughout the plating process. The pH plays a vital role in organizing the solution chemistry of the plating bath and should be kept at the optimum value for the silver salt being used. The pH meter or test strips should be used to monitor and correct the pH level of the plating bath.
In conclusion, the control and purification of the silver plating bath is essential for ensuring the quality of the plated parts. By monitoring the silver concentration and pH level, making chemical corrections to the bath, and periodically regenerating baths, it is possible to maintain the quality and integrity of the plated finish.
