A Study On The Technology Of Environment-friendly Electroless Copper Plating

Electroless copper plating is an important surface treatment technology, which is widely used in the field of conductive treatment of nonmetal materials, the production of electronic components, the preparation of printed circuit boards and etc. Formaldehyde is usually used as the reducing agent in electroless copper plating solution. Formaldehyde has strong reduction while it has the drawback that it is toxic, carcinogenic, easy to volatilize, harmful to the environment and human body. Therefore, we must find new type of environment-friendly reductant. Glyoxylic acid is a non-pollution chemical substance that it has a strong reduction and has become one of the new reducing agent for electroless copper plating in the word for its broad application.In this paper, the process of electroless copper plating with glyoxylic acid as reducing agent was studied. Firstly, the effects of composition and process conditions on stability of bath, deposition rate and electrical conductivity of copper plating layer were investigated. Then we studied the influences of the main salt, complexing agents, additives to electroless copper plating with glyoxylic acid as reducing agent using orthogonal test method and univariate method. Their usages were optimized. The influences of temperature, pH, electromagnetic and other external conditions were discussed. The best formula obtained as follows:copper sulfate lOg/L, glyoxylic acid3g/L, EDTA40g/L, potassium sodium tartrate20g/L,2,2’-bipyridine5mg/L, potassium ferrocyanide4mg/L, polyethylene glycol50mg/L, the pH is12.5, the temperature is50℃. Under these conditions, the solution of electroless copper plating is stable and the plating rate reaches to4μm/h or more, the coating surface is smooth and bright, the surface resistance is less than50mΩ/sq and the binding force reaches to4.The detection methods of copper sulfate and glyoxylic acid in the solution of electroless copper plating were introduced. We got the consumption laws of composition in the solution, determined the adding specification and tested on different materials. The content of copper sulfate was determined by chemical titration, the content of glyoxylic acid was accurate determined by ultraviolet spectrophotometry. Experimental results show that, the error is less than0.5%when we determine the content of copper sulfate, the error is less than0.5%when we determine the content of glyoxylic acid, the linear correlation coeflficient r=0.9982(n=5), accord with the requirements of constant analysis completely. The best adding specification obtained as follows:according to the surplus content of copper sulfate,1.3times of glyoxylic acid is supplemented referenced to the theory consumption of glyoxylic acid. Additives are supplemented referenced to20%of the initial potassium sodium tartrate dosage,10%of the initial2,2’-bipyridine dosage. Complexing agents are supplemented referenced to1%of the initial content. Electroless copper plating solution can be used more than20times supplemented by this method. The coating surface resistance is less than100mΩ/sq in the aspect of surface metalization of nonmetals such as fabric, glass, ceramic and polyimide. The bonding strength and the appearance of the coating is good.The effects of complexing agents, additives and technological conditions to glyoxylic acid on cathode and anode process of electroless copper plating were studied by the electrochemical workstation linear scanning curve determination. Micromorphology, structure and composition of the coating were characterized by scanning electron microscopy and x ray diffraction. The experimental results show that the additive significant effect on oxidation of glyoxylic acid and reduction of cupric ions, the coating structure and performance. Trace2,2’-bipyridine and potassium ferrocyanide increase the polarization degree of anodic polarization and cathode polarization, inhibit the electrode process of Cu(I), reduce the copper layer deposition rate, make the grain finer, and the crystal face (220) is preferred orientation. Trace polyethylene glycol has depolarization on anodic oxidation of glyoxylic acid, less affects to the cathodic reduction of copper sulphate and speeds up deposition rate by increasing the oxidation reaction speed of glyoxylic acid. The crystal structure is basically unchanged. With the effect of additives, the coating is density, the porosity is decreased and the adhesion and conductivity are increased. It is depolarized on anodic oxidation of glyoxylic acid and cathodic reduction of copper sulphate that increasing reaction temperature. Dual promoting effect accelerates the reduction of cupric ions and copper deposition, but the crystal structure is basically unchanged. The pH controls the reaction process of electroless copper plating by controlling the oxidation of glyoxylic acid and less affects to the cathodic reduction of copper sulphate. When the pH is increased, copper deposition speed is accelerated, grains are larger, crystal structure is unchanged, crystal face (220) is preferred orientation. The magnetic field can make the coating uniformity, the crystal face (111) is preferred orientation, but have no affection on the crystal structure of the coating.