The fundamental aspects of pulse plating are reviewed mostly on the basis of theoretical and experimental results. The main features of pulse plating are due to the effect of pulse current on the mass transport process. A pulsed current produces a pulsating diffusion layer in which the concentration of the ionic species fluctuates periodically with time near the electrode surface. Mathematical analysis of mass transport revealed that there are mutual restrictions among the pulse parameters-pulse current density, pulse width and off-time-, although pulse current can be increased up to some ten times the d.c. limiting current. Results obtained in pulse plating of tin and zinc are in good agreement with this theoretical prediction. While the morphorogy of the deposits, such as unevenness and porosity, is mainly influenced by mass transport, their crystal structure -grain size, crystallinity and preferred orientation -is strongly affected by activation overpotential. The corrosion resistance, magnetic and electrical properties, and the residual stress of the deposits are sometimes improved by pulse plating. It is concluded that pulse plating has more potential than d.c. plating in obtaining electrodeposits having specific physicochemical properties.
The application of pulse electrolysis to single metal deposition is reviewed. In comparison with deposits obtained from DC electrolysis, those obtained by pulse electrolysis with pulsed current (PC), periodic reverse current (PR) and asymmetric alternating current (AAC) show a far wider range of change in such properties as hardness, porosity and ductility. Each of the plating parameters-cathodic peak current density ip, on-time Ton, off-time Toff, anodic peak current density iap- strongly influences the properties of the deposits. A high ip, which results in smaller grain size and greater brightness has the effect of increasing hardness relating to structual modification. This is due to the high overpotential that developes along with high ip. A short Ton results in the formation of uniform thin diffusion layers that produce smooth deposits. A long Toff controls the morphology of the deposits because it is during Toff that the recovery of ion concentration, the dissolution and recrystallization of deposits, and the adsorption of anions and surfactants occur. While ductile but soft deposits have been obtained by pulse plating with an iap in additive-free baths, it seems fairly well established that the use of pulse plating parameters that result in grain refinement will also lead to a reduction in porosity of deposits as well as increase in ductility. Pulse electrolysis appears to have great potential for obtaining electrodeposits with a wide range of properties.
Pulse electrolysis of alloy baths brings about two principal effects on the alloy deposits. One is that the change in the composition of alloy deposits with plating conditions is less pronounced in pulse electrolysis, since it is possible to maintain the solution composition in the vicinity of the electrode surface. The second advantage is the smaller grain size resulting from higher cathodic overvoltage during the pulse on-time. The effects of pulse electrolysis are briefly discussed with respect to the mass transport process and crystallization behavior in alloy electrodeposition. The features and the available application fields of alloy deposits prepared by the pulse plating method are also described.
Amorphous alloys can be electrodeposited by direct or pulsed current. According to electrocrystallization theory, a high crystallization overpotential produces microcrystalline structure in the deposits. Thus if the amorphous phase consists of microcrystals, pulsed current is preferable for preparing amorphous alloys. However, another newly proposed theory holds that the amorphous phase structure is microcrystalline but icosahedral. This would mean that future study is needed effect of pulsed current.
At present, the term of pulse plating denotes not only square-pulsed current plating but also various other types of modulated current plating. These various types of pulse plating are categorized electrically, and the gap between pulse plating theory and it's practical application are presented. And it is suggested that the research papers on pulse plating must be studied enough in proposing to bring pulse plating to the practical level. The method to choose more suitable pulse plater to it's purpose is presented, and it is mentioned that the improvement of the equipment for pulse plating including DC wiring increases the effect of pulse plating.
Problems encountered in pulsed electrodeposition were reviewed. It has been said that pulsed current can cause a profound change in the morphorogy of electrodeposits making them harder, more uniform and more compact, with better adherence to the substrate. The effect of the pulsed current, however, is not necessarily the same for all metals because the rate determining step of the deposition reaction affected by the pulsed current in each case is the rate-determining step. The use of pulsed current improves microcurrent distribution provided concentration polarization develops at the crests and recesses in the electrode surfaces.
Platinum was electrodeposited from diamminedinitroplatinum (II) complex solution by pulsedcurrent electrolysis, and the pulse method was used to investigate the possibility of improveing current efficiency and the properties of the electrodeposit. In the electrolysis of platinum, increasing lp and decreasing Ton and Toff reduced the amount of co-deposited hydrogen, and improved current efficiency. The increases in Ip shifted the cathode potential to the negative, increased overvoltage, and resulted in minute crystals that produced a smooth surface on the electrodeposit. The surface of the electrodeposited platinum had a relatively random crystal structure with preferential orientation on the (111) plane. SEM photographs confirmed the smooth surface resulting from the increase in Ip and the decreases in Toff and Ton, indicating that the effect of pulse electrolysis using the experimental solution is promising.
The codeposition behavior of thallium, which acts as a grain refiner in gold electrodeposition, was studied in both DC and pulsed current electrolysis. In jet plating, the maximum current density that produced shiny deposits was the same for both methods, but the codeposition behavior of thallium in gold was quite different. In DC plating, the amount of thallium codeposited in gold increased with greater current density and thallium concentration, but decreased with an increase in the concentration of conductivity salts. In pulse current plating, on the other hand, the amount of thallium codeposited was constant at about 300ppm and independent of these factors. Thus the effect of such pulsed current parameters as pulse duration and waveform were studied in relation to thallium codeposition behavior. It was found that at optimum pulsed current conditions the amount of thallium codeposited in gold was suppressed to about 80ppm. It was also found that periodically reversed current was effective in suppressing the amount codeposited to the same level. The differences in the codeposition behavior of thallium in DC and pulsed current conditions were explained satisfactorily using a diffusion layer model of gold cyanide electrolysis and the grain refining mechanism of thallium proposed by McIntyre and Peck.
Pb-Sn-Cu alloy plating from fluoborate baths has been carried out by the constant current method (CCM) and the pulse current method (PCM). It has been found from polarization measurements that the existence of Pb2+ in the plating bath decreases the limiting current density of copper deposition. The copper content in plated layers made by CCM qualitatively corresponded with the limiting current density of copper deposition. In the case of PCM, the copper content in the plated layer reached a maximum at a frequency of about 10Hz. During the PCM off-time, a layer of high copper content was formed at the electrode surface by a Pb+Cu2+→Pb2++Cu replacement reaction. This is the chief distinction of PCM. The tin content of the plated layer did not change with current density in CCM and increased with the addition of hydroquinone to the plating bath. The preferred orientation of lead in plated layers obtained by CCM using a bath containing gelatin and hydroquinone was (220) plane, but this was not recognized for PCM except in the case of low duty ratios. Plating layers having a compact, smooth sureface morphology were prepared by PCM at a low frequency and a duty ratio of about 0.5.
Pulse plating of Cu-Zn alloy from cyanide brass plating baths was investigated with respect to the effect of pulse electrolysis with galvanostatic square pulses on the potential-current curves, current efficiency and composition of brass deposit. It was found that pulse plating yielded a brass deposit of uniform composition with a brighter appearance and finer crystal grain than that obtained under direct current electrolysis, and that the effect of pulse plating at a high pulsed-current density was mainly influenced by the increased deposition overpotential and the off period in each cycle where there is zero current in the external circuit. It was also noted that the electrodeposition mechanism changed from the regular type to the irregular type with increasing pH in the vicinity of electrode surface, since high pulsed current electrolysis resulted in a greatly increased hydrogen evolution reaction.
In high-purity gold-plating baths for electronic components, a trace amount of thallium is added as a grain refiner. At high current densities, however the thallium content of the gold deposits increases and wire bondability deteriorates. In this study, pulse plating was investigated as a method of decreasing thallium content. It was found that there was little thallium in deposits obtained using pulsed waves of 125Hz frequency and a 75% duty cycle. Micrographs and X-ray diffraction patterns showed that the crystals were large and grew in a relatively random under pulse plating.
Pulse-plating of tin was studied in N, N-dimethylformamide baths without the presence of additives with regards to the current efficiency, the morphology of the deposits and X-ray diffraction. The tin deposits plated under the duty-cycle of 1-5% at the average current density of 10mA/cm2 exhibited a bright, smooth, fine-grained surface.
The grain size of the CdTe deposits obtained by potentiostatic pulse method increased with increases in pulse frequency, and high frequency pulse plating resulted in films having improved photo-electrochemical characteristics.
The influence of pulsed current on the morphology of gold electrodeposited from phosphate solutions was studied. Deposits having a bright, laminar structure were formed in the small duty cycle range of the pulsed current even without codeposition of less noble metals.
The effect of high pulsed crrent on the properties of electrodeposited gold film was investigated. The deposits were analyzed by means of an electron microscope and X-ray diffraction, and it was found that the corrosion resistance of gold films was far better when formed by high pulsed current than when formed by direct current.