Sodium Hypophosphite - Nitrilotriacetic Acid-free Formaldehyde Electroless Copper Plating System

In recent years, environmental protection was received more and more attention. Electroless copper plating technology used formaldehyde as reduction agent was replaced by the plating bath of the sodium hypophosphiteas, and the electroless copper plating system became the environment-friendly. Because of the poor catalytic ability of the copper ion, the Co(Ⅱ) or Ni(Ⅱ) ions were uaually added to the electroless plating bath, which the resistance of deposited copper film rise and the color of copper film becam wine color.In order to improve the quality of deposited Cu film and reduce the Ni and P content of the deposited copper film, a new environment-friendly electroless plating bath using sodium hypophosphite as the reductant and nitrilotriacetic acid(NTA) as the complexing agent was investigated under the acid condition. The stable constant of complex [CuNTA]- was same as the complex of [NiNTA]-, so the copper ions were easier to deposit from the bath than nickel ions, and a pure copper deposited film can be obtained from the bath. On the other hand, electroless plate deposition rate was increased by the low approximate stability of [CuNTA]- and activation energy of reaction. The effects of bath temperature, pH, CuSO4·5H2O concentration, NTA concentration and NaH2PO2·H2O concentration upon the deposition rate and the bath stability were studied. The electrochemical results indicated that the deposition rate of the electroless copper increased with the decrease of the pH, and it reached 2.4μm h-1 when the bath pH was 6.0. The as-deposited Cu films were characterized by energy dispersive X-ray analyzer(EDX), X-ray diffraction(XRD), atomic force microscopy(AFM)and resistivity measurement. A uniform copper film with a low resistivity (2.60μΩcm) was obtained with an addition of polyethylene glycol(PEG 600), which can be used as seed layer for electroplating copper.In this study, additive PEG-600 was used to improve the properties and microstructure of the as-deposited copper film. The influence of PEG-600 on the deposition rate, structure, composition and properties of the electroless copper were investigated. The result indicated that an addition of PEG-600 with a concentration of 0.3 mg L-1 increased the deposition rate to 2.6μm h-1. The AFM measurements showed the surface of electroless Cu film deposited without additive was very rough, and the average surface roughness(Ra)and root mean square roughness(Rms)were 0.65μm and 0.81μm, respectively. When PEG-600 concentration was 0.3 mg L-1 in the plating bath, Ra and Rm became to 0.45μm and 0.58μm. The XRD results indicated, for electroless-plated Cu film without additive, the peak intensity ratio 1(111)/I(200) was 2.33 and the FWHM(term Full-Width Half-Maximum)of(111)for 2.4μm thick Cu film was 0.541°. When PEG-600 concentration in the plating bath was 0.3 mg L-1, the peak intensity ratio I(111)/I(200) was 3.47, and the FWHM of(111)reduced 0.293°. These results indicated that the peak intensity ratio 1(111)/I(200) increased obviously with an addition of PEG-600 and was beneficial for subsequent electroplating of copper.A new environmentally friendly electroless copper plating bath with a high deposition rate and stability was investigated. The deposited copper film had a better crystalloid and lower resistivity. It was very different from the former deposited copper seed layer with relative high Ni and P contents and a weak quality. This was beneficial to the future study of electroless copper plating using sodium hypophosphite as the reducing agent.In this paper, another important research was a new technique of electroless copper deposition on ABS plastics with a palladium-free catalyzer. Palladium-free activation using a sodium boronydride (NaBH4) as reducing agent, Ni(II) ions were deposited onto an ABS resin substrate that served as a substitute for the palladium catalyst customarily employed in electroless copper plating. The effects of Ni(II) ions concentration, NaBH4 concentration, NaBH4 reduction time on the adhesion strength were studied. And the average adhesion strength between the ABS surface and the electroless copper film reached 0.72 kN m-1, which was near the values(0.87 kN m-1)obtained by SnCl2/PdCl2 colloid. The surface topographies before and after the electroless copper were evaluated by scanning electron microscope(SEM), and X-ray photoelectron spectroscopy(XPS)were employed to detect the quantity of the groups on the surface. The results of experiment showed that the plating layer was even and bright and has good adhesion. This explains the new technique of electroless copper deposition on ABS plastics under palladium-free catalyzer is feasible.