Study On The Stability Of A Non-cyanide Au-Sn Co-electrodeposition Plating Solution

Au-30at.%Sn eutectic alloy is widely used in electronic packaging industry as solder material for its excellent mechanical properties, outstanding thermal conductivity, and can be used without flux. The high melting point, excellent creep resistance and thermal conductivity made Au-30at.%Sn alloy particularly suitable for power device packaging, such as light emitting diode(LED) packaging. However, production cost of the Au-30at.%Sn eutectic alloy is high, and it is difficult to produce using conventional preparation method. Instead, plating is a promising low coast preparation method which can deposit Au-30at.%Sn eutectic solderable coatings onto complex surface. As the development toward green manufacturing is an inevitable trend for microelectronics manufacturing industry, the preparation of Au-30at.%Sn eutectic solderable coatings using non-cyanide plating solution is of great concern by the industry. However, the stability issue of the plating solution has been the main concern and bottleneck to put the non-cyanide Au-Sn co-deposition technology into practical production.The main conclusions of the present work are as follows(1) Several samples were plated in the same non-cyanide for a long time. It is found that with the increase of the total usage time of the plating solution, the efficiency of electroplating current is gradually reduced, the composition of Sn in the coatings decreases. Calculation shows that the actual consumption of Sn in the plating solution significantly deviated from the ideal situation, which means that the concentration of the Sn ions changed during the long time plating. As a result, the composition of the deposited Au-Sn coatings was not stable to be Au-30at.%Sn eutectic alloy.(2) Linear sweep voltammetry method was further used to investigate the stability of the alkaline pyrophosphate tin plating solution, the alkaline sulfite gold plating solution and the non-cyanide Au-Sn co-electrodeposition plating solution. Experimental results reveal that the Sn main salt was not stable in the solution after long time storage, while Au main salt can be kept stable within5days in the solution. The stability of the Sn main salt is the main factor of the stability of the Au-Sn plating solution.(3) The citric acid-divalent tin (Sn(II)) system was studied to find a stable Sn plating system. The Sn (II) complexes species in citric acid solution are SnCit2-、SnCitH-、SnCitH2-SnCitH3+. The concentrations of different complexes species vary with the total concentration of citric acid, the total concentration of divalent Sn (Sn(Ⅱ)) main salt and the pH value. Calculation shows that when the pH value is5, almost97%of the Sn(Ⅱ) exists in the form of SnCitH’complex. The oxidation of Sn(Ⅱ) is responsible for the deterioration of the plating solution. Therefore, the stability of citrate-Sn(Ⅱ) system is significantly improved with the addition of antioxidant. The ascorbic acid is a stronger antioxidant than catechol.