Study on the coordination agent system of Sn-Ag-Cu ternary alloy co-deposition

With the developing of Surface packaging technology, the requirement of electronic components on coating is getting more and more rigorous. Sn-Ag-Cu ternary alloy coating has the advantages of low melting point, good weldability and so on. It is considered to be the best choice to replace Sn-Pb coating. However, due to the large difference in deposition potentials of Sn
²⁺
, Ag
⁺
and Cu
²⁺
, it is difficult to co-deposit. It is necessary to add coordination agent in the plating solution to make the deposition potentials of Sn
²⁺
, Ag
⁺
and Cu
²⁺
close to each other and stabilize the plating solution. In this paper, the co-deposition of Sn-Ag-Cu was achieved by using EDTA-2Na, ammonium citrate and thiourea as additives. The effects of EDTA-2Na, ammonium citrate and thiourea on Sn, Ag and Cu Sheet metal and Sn-Ag-Cu alloys were analyzed by Potentiometric scanning curve. It was found that the three additives could make the deposition potentials of Sn
²⁺
, Ag
⁺
and Cu
²⁺
close to each other. The kinds and proportions of additives were changed in order to find a suitable coordination agent system. The bath with ammonium citrate and EDTA-2Na was more stable than that with thiourea. The surface morphology of the coating was observed by Scanning Electronic Microscopy (SEM). It is found that the coating with compact structure and smooth surface can be obtained by adding thiourea in the plating solution.