MODELLING AND DESIGN OF A CONTROLLER FOR pH AND TEMPERATURE CONTROL DURING ELECTROLESS PLATING

Abstract

To seek out the desired performance, a process is restrained to operate in a predetermined manner i.e. it is controlled. Restraint is usually achieved by controlling a selected physical variable vital to the performance of the system. In a highly sturdy non-linear process such as control of the coating weight and its uniform composition in electroless NiXP coating, the application of a controller is quite limited and still remains out to be a challenging province of study. There are two basic parameters, which directly affect the weight of the coating being deposited. The first is the variation of temperature at which the coating is being deposited because of a linear relationship existing between the temperature and rate of deposition. The second one is, the changes in the operating pH that has a negative relation with the rate of deposition. PH is also important in deciding the composition of the deposit. Affecting the rate of deposition and composition, these variables thus directly influence the homogeneity of the electroless deposit being obtained. The aim of this work is to develop a control strategy to maintain an optimum pH and temperature in the electroless bath, based on mathematical modeling. For variations in the load as well as of pH and temperature from the set point, the manipulated variable `V0' (flow rate) is to be regulated frequently. The fluctuations in `pH' depends on the inflow rate `Vo' and feed concentration of NH4OH, used as a pH-controlling reagent in the electroless process. The variation in the `V0' and temperature of the incoming bath (Ti) will further induce variations in the temperature of the process from the desired set point. Thus directing a control on feed-concentration and feed rate will enable us to maintain a constant rate of disposition of electroless NiXP coating globules. In this context, the chemistry of the process is studied and amount of NH4OH to be added to maintain the desired `pH' is calculated. Further a mathematical model is constructed on the basis of rate law governing the process. The whole system is designed in a batch-type process and the conditions are being evaluated for definite time intervals. The concentrations of the ingredients consumed and produced, left in the bath after deposition, and to be fed regularly for continuous deposition, are thus determined. The results obtained from the calculations abiding by the model are then studied for developing a strategy for the control accordingly. Due to the continuous replenishment of the bath, with a controlled pH and temperature, the central aim of getting a uniform coating of consistent composition can be accomplished