| It is easy to form the oxide layer on the surface of Printed circuit board (PCB) copper pad, which increase the resistance and lead to the formation of intermetallic compound (IMC), the IMC is prone to brittle fracture, which seriously affects the solderability. In order to limit or eliminate the presence of IMC, the barrier layer is added between the interface of copper and the solder, the electroless nickel/immersion gold surface treatment is used. However, The nickel layer has a high porosity and the deposition of gold particles is large, so the thicker nickel layer and gold layer is need to reduce the gap. Meanwhile, in the immersion gold process, gold over attack the nickel plating layer, which lead to the risk of "black pad". This paper deseribed a method for preparing amorphous Ni-Pd-P alloy coating by eleetroless Plating. This coating as barrier layer are studied by two methods: (1)Electroless nickel-palladium alloy on the copper, ENEP for short; (2)electroless nickel-palladium alloy on the copper covered with thin nickel coating, EN/ENEP for short. Due to the fine palladium particles, the coating obtained by two kinds of processes are compact, has low porosity, bright white surface, can be directly used for welding. At the same time the bottom nickel is not prone to oxidation, could avoid the "black pad".The effeets of nickel chloride, palladium chloride, ethylenediamine, ammonia, sodium hypophosphite, temperature and pH on the nature of alloy were studied by SEM, EDS, XRD, X-ray fluorescence thickness gauge and electrochemical workstation.In addition,the properties of alloy were characterized with adhesion test, neutral salt spray test and solderability test. Based on the orthogonal experiment,the optimal bath formulation and process conditions were obtained as follows:NiCl2·6H2O 12g/L, PdCl2 O.lmol/L, En 9mL/L, NH3H2O 7mL/L, NaH2PO2-H2O 6mg/L, NH4Cl 3g/L, temperature 65?70?, pH 8.5?9.0. Representative typical additive were selected to study its influence on the bath stability, plating porosity and deposition rate, through orthogonal experiments, the final combination of additives follows:SeO2 7mg/L, CuSO4·5H2O 15mg/L, PrCl3 7mg/L.The Ni-Pd-P alloy coating which consists 85.19 wt% nickel,6.25 wt% palladium and 8.56 wt% Phosphorus is smooth, has uniform particle size, fine crystal, no obvious pores and cracks.The elements are distributed evenly; A simple and effective method to control the alloy composition based on the initial mole ratio Ni(?):Pd(?) and varying pH in the solution has been found, It has been detected that the lowering of the mole ratio Ni(?):Pd(?) from 100:1 to 10:1 in pH 8.5 solutions provides enrichment of the alloy with palladium from 4 to 50 wt.%; The corrosion resistance and solderability of Ni-Pd-P alloy coating are better than that of electroless nickel coating, the more the content of palladium in the alloy, the smaller the corrosion tendency of the coating.The cathodic polarization curves of Ni-P, Pd-P and Ni-Pd-P alloy were studied through linear potential scanning with copper electrode and nickel phosphorus electrode as the work electrode, and the effect of various additives on the cathodic deposition process was discussed. It is found that palladium catalyst on copper, Ni-P and Pd-P may possesses catalytic activity in oxidation of hypophosphite ions in solutions, During the deposition of Ni-Pd-P alloy, the process of palladium cementation with nickel occurred simultaneously. palladium cementation with nickel. Palladium can promote the reduction of nickel, nickel inhibits the deposition of palladium... |
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