Study On The Corrosion Behavior Of Printed Circuit Board And Its Influencing Factors

Electronic material is the foundation and precursor of the information technologies. The electronic products have penetrated almost all aspects of society, such as scientific research, manufacture, national defence and daily life. However, the development of modern technology needs electronic equipment with higher precision and reliability. Assembly in higher density, higher impedance and larger magnifying ratio made the modern electronic equipment more sensitive to corrosion in their serves. The electronic material, or even the whole equipments may be destroyed by very slight corrosion, so it is important to study the corrosion mechanism and corrosion protections.Printed circuit board (PCB) is an important electronic material, as support and electric joint of electronic component, the surface of PCB is covered by a layer of copper. The PCB may be disabled if copper on its surface corroded. To meet the fast development of electronic equipment, the PCB is developed towards higher density, higher precision, thinner line, smaller hole, higher reliability, lower cost and automatic continuous manufacture. When working in a corrosive or humidity environmental, the corrosion will take place on the PCB, which may result in a complete failure of the electronic equipment. Therefore, the studies on the corrosion and protection of the PCB are of greatly practical significance. In this work, the corrosion behavior of PCB in neutral NaCl solution and its influencing factors were investigated by electrochemical techniques and surface analysis methods. The main results are as follows:1. Measurements of pH and Cl^- concentration distribution in the crevice during the crevice corrosion of PCBAlthough the crevice corrosion of metals had been extensively studied in the past years, it remains lack of direct experimental information of the micro chemical environments and electrochemical conditions inside the corroding crevice. The distribution of chloride concentration and local pH in the occluded area are the most crucial factors in development of crevice corrosion. The array probes in one dimension of Ag/AgCl and Ir/IrO_x array electrodes were developed in this work to in situ follow the distribution of chloride concentration and local pH in the occluded area during the crevice corrosion propagation. Based on a designed simulated crevice corrosion system in 0.5 mol/L NaCl solution, the distribution of chloride concentration and local pH inside corroding crevice for the electronic circuit board were imaged correlated with corrosion potential in the crevice. It is found that the chloride concentration increases and the local pH decreases inside the corroding crevice with the immersion time. And the Cl^- concentration and local pH at the deeper location inside the crevice are always higher and lower, respectively, than that near the mouth of crevice, due to the mass transfer difficulty by geometrical limitation of crevice. That is the reason why the crevice corrosion always develops in depth direction. All obtained experimental results provide a further understanding to the mechanism of crevice corrosion of electronic materials in the given environments.2. Study on localized corrosion of Cu/soldering tin interface by using scanning Kelvin probe and CCDLocalized corrosion in an early stage of Cu/soldering tin interface were observed by scanning Kelvin probe and CCD. The CCD observation indicated that the corrosion of Cu/soldering tin electrode did not occur immediately after exposing to 1.0 mol/L NaCl solution. The pitting corrosion took place after ten minutes immersion. The pitting corrosion occured on the surface of soldering tin near the interface, where exists defects. With the time increasing, the pitting corrosion developed to the interface, finally along the interface. The measurement of scanning Kelvin probe indicated that before immersion, there was large potential drop between copper and soldering tin. With the immersion prolonged, pitting corrosion initiated at the interface and developed along the interface. There is a good agreement between results for the two methods. Moreover, the influence of electrode potential, the ratio between the area of cathode and anode and the Cl^- concentration on the corrosion of Cu/soldering tin were studied by electrochemical techniques. The results indicated that corrosion of Cu/soldering tin is controlled by cathode, the change concentration mainly influences anodic reaction.3. Inhibition mechanism of several inhibitors for PCB in neutral NaCl solutionThe inhibition effect of Cetyl Trimethyl Ammonium Bromide (CTAB), Sodium Dodecyl Sulphate (SDS) and Sodium Tungstate (Na_WO₄) on the PCB in neutral NaCl solution were studied by electrochemical techniques. Moreover, Sodium Tungstate was mixed with CTAB and SDS and the effect of the inhibitor compounds were also studied. Attenuated total reflection fourier transform infrared (ATR-FTIR) was used to study the inhibition mechanism. The following conclusion can be drawn: (1) When the inhibitors were used separately, 350 mg/L Na₂WO₄ or 1.0×l0^-4 mol/L CTAB or 5.0×10^-3 mol/L SDS has best effect of inhibition. (2) The solution contained 250 mg/L Na₂WO₄ + l.0×10^-4 mol/L CTAB or 300 mg/L Na₂WO₄ + 5.0×10^-3 mol/L SDS shows a best inhibition function. ATR-FTIR results indicated that WO₄^2- and C₁₆H₃₃N(CH₃)₄⁺ or WO₄^2- and C₁₂H₂₅SO₄^- were both absorbed on the surface of the electrode. So the compounds shows a synergic effect. (3) The potential mapping by home-made array electrode measurement system indicated that the compounds of 250 mg/L Na₂WO₄ and l.0×l0^-4 mol/L CTAB have excellent inhibition effect on the PCB in 0.5 mol/L NaCl solution.