0................................................................ Electrolytic Metal Deposition

Electrolytic Metal Deposition

There are three types of electrolytic metal deposition processes: direct current electrodeposition, pulse plating, and laser-induced metal deposition.

Direct Current Electrodeposition

The parts to be plated are placed on racks (for still plating) or barrels (for barrel plating of small components such as fasteners) and the racks or barrels immersed in the electrolyte. The parts act as a negatively charged cathode. The positively charged anode(s) completes the electric circuit. A power source in the form of a battery or rectifier provides the necessary current. Electrodes, immersed in the electroplating bath (electrolyte), are connected to the output of a DC current source. A DC (direct current) electric current is passed between the items (negative terminal), the electrolyte and the anode. The composition of the electrolyte, the temperature and current are all controlled within close limits to give consistent results. In most cases, the electrolyte is filtered continuously to remove solids, which might otherwise stick to the part being plated and so produce rough deposits.

The geometric shape and contour of parts to be plated affect the thickness of the deposited layer. In general, parts with sharp corners and features will tend to have thicker deposits on the outside corners and thinner ones in the recessed areas. The cause of this difference in the resulting layer thickness is that the DC current flows more densely to sharp edges than to the less accessible recessed areas. In other words, the current distribution is not uniform. Therefore the process must be controlled sharply.

Pulse Plating

Pulse plating gives an "electrical tool" which is used to give a further optimization of the plating system. With periodic short anodic pulses applied to the cathode the direction of the current flow is altered. During the anodic pulses the current density can be for example 2.5 - 3 times higher than during DC -deposition. Electrodeposition using pulsed currents is known as pulse plating. The pulsed currents can be unipolar (on–off) or bipolar (current reversal). Pulses can be used along or be superimposed on a DC feed. By using the bipolar pulse, metal deposition occurs in the cathodic pulse period, with a limited amount of metal being redissolved in the anodic period. This repeated deposition and partial re-dissolution could improve the morphology and the physical properties of the deposit.

Laser-Induced Metal Deposition

In laser-induced metal deposition, a focused laser beam is used to accelerate the metal deposition. Experiments have shown that the deposition rate can be increased by 1000 times. The plating equipment mainly consists of a laser head with focusing optics and the electrolytic cell. In the laser-assisted metal deposition techniques, lasers act as directed photon –as well as energy sources providing a micro –reaction volume by the illuminated zone. Chemical reaction on the substrate(surrounding media interface occur because the absorbed laser photons cause a temperature increment near the interface excited electronic states or even free electrons, so that irradiated areas can operate as catalytic active sites in the metallization.