Research Of Catalytic Oxidation Of Formaldehyde On Copper And Its Application In PCB

The future development of printed circuit board lies in the unmanned factory and the additive process.The unmanned factory frees us from the prohibition of the use of formaldehyde,and there is pressing demand to improve the quality of electroless copper to satisfy the industrial production.The research of the formaldehyde oxidation on the copper surface is still in 1990s,which is,however,far behind the industrial progress.It is,therefore,necessary to promote its research development again to catch up with industrial progress.Here the mechanism of formaldehyde oxidation on copper surface was studied by density functional theory and electrochemical methods,and it was applied in the printed circuit board industry for electroless nickel/immersion gold,electroless copper and conductive pattern made by an additive process.The results are as follows:?1?The electro-oxidation of formaldehyde on copper was studied by density functional theory.It is found that OH^-does not participate in the rate-determining step of the formaldehyde oxidation on the copper surface.The activation energy of the reaction is 95 KJ/mol by DFT calculation.The phenomenon of excess hydrogen evolution in the process of formaldehyde oxidation on copper in the alkaline solution was also studied by DFT.It is found that the superfluous hydrogen evolution is invoked by Heyrovsky Reaction whose occurrence needs not any activation.It should be noted that Heyrovsky Reaction is one step of water electrolysis for preparing H₂.?2?Before electrochemical tests,the polymerization,hydration,dissociation,and cannizzaro reaction of formaldehyde in the alkaline solution were studied.It is found that the electrochemical results need to be rectified or transient electrochemical method should be adopted.Firstly,the DFT result that OH^-does not participate in the rate-determining step,was verified by chronoamperometry.The increase of anodic current with the growth of the pH was caused by the increase of methylene glycol ion concentration.Then we employed potentiostatic step method to study the reaction kinetics.The reaction rate,reaction rate constant,activation energy and charge transfer coefficient were worked out.It is found that the reaction rate near the open circuit potential is lower than the other normal reaction rate.This phenomenon can be explained by Heyrovsky Reaction.We found that the charge transfer coefficient decreased as temperature increased.Its value lies between 0.24 and 0.12,which are different from 0.5 reported by other literature.The activation energy acquired by electrochemical tests is about 125KJ/mol.This value is much higher than the DFT result,which may be caused by the DFT simulation not considering hydrogen bond.When methylene glycol ion attained 0.007mol/L,the electrode potential changed from-0.1 V?vs.Ag/AgCl?to-0.8 V.Heyrovsky Reaction could interpret this phenomenon.?3?Electroless nickel/immersion gold as one of the most important final surface finshing technologies in printed circuit bord industry usually employs PdCl₂ or PdSO₄ to initiate electroless nickel plating on copper surface.Pd as activation agent,howerver,has some disadvantages.For instance,the Pd is too expensive,it can cause short circuit between copper lines and lead to the instability of electroless solution.It is found that the atomic hydrogen produced by formaldehyde oxidation on the copper surface can initiate electroless nickel plating.The Raman spectrum indicated that there indeed exists the hydrogen adsorbed on the copper surface.The permeate hydrogen test indicated that the hydrogen could diffuse in the copper lattice.Once the activated electrode was exposure to air,its electrode potential would rise up to-0.443 V?vs.Ag/AgCl?which indicated that the Cu₂O was formed on the copper surface,and the electrode lost its ability to initiation electroless nickel plating.By comparing the formaldehyde-activation and Pd-activation methods,we found that these two methods reached an agreement in morphology,element composition and hardness.In the induction stage,corrosion resistance,and black pad tests,the formaldehyde-activation method is superior to the Pd-activation method.Especially the formaldehyde-activation method does not cause extraneous deposition.?4?Hole metalizaiton as a key process of printed circuit borad needs the high quality of electroless copper layer.However,the quality is influenced by the number and size of hydrogen bubbles in the copper layers.A model was proposed to demonstrate the process that microvoid induces co-deposition of hydrogen bubbles and copper layers.It includes three stages:1)the hydrogen bubbles adsorb on the bottom and two sides of microvoid;2)microvoid develops into the pyriform void;3)pyriform void develops into closed void.Nickel ion has three roles in electroless copper:1)it can prohibit the production of hydrogen and pass the electron from atomic hydrogen to copper ion;2)it can accelerate the electroless copper;3)it can hinder the hydrogen bubbles growing up.Potassium titanium hexacyanoferrate?K₄FeCN₆?could dissociate CN^-in the electroless solution.By DFT simulation,we found that CN^-preferred adsorbing on the copper surface without no negative charge.Therefore,we infer that CN^-is not inclined to adsorb on the bottom of microvoid,but on the top.Because of CN^-prohibiting the formaldehyde oxidation on the copper surface,it could prohibit the growth of top layer.We found that the optimum concentration of K₄FeCN₆ for prohibiting the formaldehyde oxidation is 10 ppm.Combing Ni²⁺with K₄FeCN₆,we could avoid the co-deposition of the hydrogen bubbles with copper layer.The optimum concentration of Ni²⁺is 70 ppm.Under this condition the fractal dimension of the prepared copper layer is 2.51 and its ductility is 8%.?5?A simple method was proposed to make a conductive pattern on polyimide film by a machine which is consist of three dimensional console and extrusion system.It mainly includes three steps:1)activation solution was dispensed on the polyimide film to form a pattern;2)the activation solution was washed away by water;3)the polyimide film was immersed into electroless solution to form a conductive pattern.