Study On Electroless Gold In Sulfite Bath On The Ni-P Alloy Substrate

ENIG flim has been widely used in microelectronic field for its excellent corrosion resistance, weldability. However, a hyperactive corrosion of Ni-P coating taking place during the IG process could affect the subsequent welding performance. In this paper, a cyanide-free electroless gold process was developed by adding corrosion inhibitors and reducing agents or exploring electroless nickel/electroless palladium/immersion gold（ENEPIG） to alleviate the hyperactive corrosion of Ni-P coating, so as to reduce the occurrence of the subsequent solde point failure. The experimental results were as follows:The effects of three organic additives, such as polyethylenimine（PEI）, hexamethylene tetramine（HET） and benzotriazole（BTA） on the depositing process and the performance of Au coating were investigated. The study was performed by electrochemical tests in combination with X-ray fluorescence spectrometry（XRF）, scanning electron microscope（SEM） and X-ray diffraction（XRD） analysis techniques. The results showed that PEI, HET and BTA could adsorb on the surface of gold film and they had the similar influences on the open circuit potential. These three additives changed the morphology of Au deposits, decreased the initial deposition rate, and thus decreased the size of Au particles, but had no effect on crystal structure and composition. Tafel studies demonstrated that the corrosion resistance of coating could be enhanced with the addition of additives. Comparision of different additives and found the BTA had better corrosion effect. Furthermore, on the basis of non-cyanide immersion gold, BTA was chosed as the optimal additive in electroless gold bath and the quality of gold film was better when the concentration of BTA was 50 mg/L.M was chosed as the optimal complexing agent in electroless gold ba th by comparing the different complexing agents on the influence of the performance of gold coating and plating solution. The effect of gold bath composition and process parameters on gold coating and bath was studied by single factor experiments. The optimum electroless gold composition and process is: Na₃Au（SO₃）₂ 2g/L, Na₂SO₃ 25.2g/L, Na₂S₂O₃ 15.8 g/L, M 3 g/L, CN2H4 S 2g/L, temperature 80 ?C, pH=7. At the same time, the effect of gold bath composition and process parameters on the process of gold deposition was investigated by the open circuit potential test. Furthermore, the process of electroless gold of ENIG, ENEPIG and ENEG technologies was studied by electrochemical tests in combination with XRF, SEM, wetting angle and XRD analysis techniques. The results showed that the plateau potential of ENEG was the most positive and the time reaching the plateau potential value was the shortest. Moreover, the gold deposition rate of ENEG technology was the fastest. A nodular structure of IG deposit was observed on the surface of these three technologies and the gold coating of ENEG was more compact. In addition, the Ni-P film of these three deposits was amorphous and the Pd-P film of ENEPIG plating was also amorphous. Furthermore, ENEPIG film had the better corrosion resistance and lower porosity. Comparing three kinds of coatings, the performance of ENEPIG coating was more superior.The cost of ENEPIG process was fewer than ENIG by comparing the economic cost of the two kinds of process. At the same time, two kinds of process didn’t contain cyanide and can reduce the harm to the body and the environment.