Electroless Nickel Plating Process Model for Plated-Through-Hole Board Manufacturing R. TENNO, 1 K. KANTOLA, 1,3 and A. TENNO 2 1.—Control Engineering Laboratory, Helsinki University of Technology, FIN-02015 TKK, Finland. 2.—Department of Computer Control, Tallinn University of Technology, 19086 Tallinn, Estonia. 3.—E-mail: kalle.kantola@hut.fi In this paper, the electrochemical reaction mechanism is used to develop a mathematical model for an electroless nickel plating process of plated- through-hole board. The model is calibrated against experimental data and the result, which is in good agreement with measured data, applied for state estimation of the unobservable processes. The electrical, chemical, and board parameters are estimated from measurements that are standard in the nickel plating industry. Key words: Electroless plating, nickel, modeling, state estimation INTRODUCTION Electroless nickel plating is a widely used process in many industries including microelectronics where plat- ing through hole (PTH) technology is applied. For this technology, precise control is crucial, and to improve the control, a mathematical model of the process is required. In this paper, a new mathematical model for the process is presented. The model was partially developed as a general model of electrochemical plat- ing, either in the framework of the chemical reaction theory or electrochemical reaction theory. 1–6 The mixed potential model 4–6 developed in the latter theory is similar to the more advanced porous electrode model 7–9 originally developed for a battery. These mod- els, along with the neutralization models, are applied in this paper as a complex mathematical model for electroless nickel PTH boards. The proposed model is calibrated against data measured in industrial experi- ments and applied in state estimation. By using the model, the unobservable electrical, chemical, and board parameters, including the current densities, potentials, deposition speeds, and reaction rates, are estimated from standard PTH industry measurements. In a multiple process system, it is relatively simple to verify a model of some partial processes if the other processes are assumed known or constant. However, model verification of all of the processes is more com- plicated, especially if the processes are multiply related and most of them unobservable. This problem is rarely analyzed in the plating literature. In the study of Ramasubramanian et al., 10 electroless cop- per plating is analyzed without experimental data (simulation). In the study of Kim and Sohn, 6 electro- less nickel plating is analyzed on a rotating disk elec- trode. In both papers, the plating process is assumed to be limited by mass transfer caused by diffusion, which is not the case in PTH boards (as discussed in the ‘‘Types of Model’’ section). The purpose of this paper is to propose a complete model for monitoring the unobservable processes of PTH board manufacturing. This model occupies the middle ground between black box treatment and first principle analysis. The model is highly nonlinear; its state and parameters should be estimated as an infinite dimensional estimator, 11,12 which is imprac- tical. A much simpler approach is applied in this paper. The parameters of the theoretical model are estimated numerically using a nonlinear least- squares method. The unobservable state is estimated directly from the model using the measurements that are readily available in the PTH industry. MODEL Several reaction mechanisms have been proposed to describe the plating chemistry, 1–6 including the electrochemical reaction mechanism, 2,4,6 which was found to match with the measured data also in this paper. This mechanism, described through the ano- dic and cathodic reactions, can be represented as the following system. Anodic reaction-hypophosphite oxidation: H 2 PO  3 1 2H 1 1 2e  )H 2 PO  2 1 H 2 O; U1 ¼0:504 V ð1Þ Cathodic reactions-phosphorous deposition, hydro- gen gas evolution, and nickel deposition: (Received January 2, 2006; accepted May 8, 2006) Journal of ELECTRONIC MATERIALS, Vol. 35, No. 10, 2006 Regular Issue Paper 1825