Mass Transfer Within the Location Where Micro Electroplating Takes Place

Abstract

Electroplating and electroforming are the two electrochemical processes extensively used in metal fabrication. Electroplating provides a thin metal film to bestow the surface with desired property such as abrasion and wear resistance, corrosion protection, lubricity and aesthetic qualities; electroforming leads to a deposition of metal skin onto a mandrel which is then removed and then the metal deposit was thickened to obtain precise fabrication of molds. Both the electrochemical processes are carried out in the bath where sufficient concentration of metal salt is supplied in presence of an electric field. The electrochemical kinetics is determined not only by the strength of electric field but also by the mass transport phenomenon of the electrochemical active ions. The electric field employed in the electroplating is relatively lower and the field distribution is homogeneous. In contrast, the electrical field exerted in electroforming seems to be much stronger and the field distribution becomes less homogeneous. In 1995, a novel localized electrochemical deposition (LECD) process was pioneered by Hunter [1] to fabricate three-dimensional (3D) metal microstructures. The LECD brings the electrochemical process to a new era. However, in the LECD process, the electrical field exerted at the electroplating site is super high and the distribution of field strength is ultra heterogeneous. The phenomenon of mass transport in such a strong field distributed in extremely heterogeneous is the case which we have never encountered in doing usual electrodeposition. In the process of micro electroplating, the site where LECD taking place was experimentally controlled to along the track guided with a microanode. Accordingly, the micro metallic features could be fabricated electrochemically along the motional track guided by the microanode [2]. Due to this fact, LECD was also named as microanode guided electroplating (MAGE) process. The schematic diagram of MAGE is shown in Fig. 1. A platinum wire (diameter in the range from 25 to 125 μm) was fixed coaxially, and cold mounted with epoxy resin in polymethylmethacrylate (PMMA) tube (inner and outer diameters are 3 and 5 mm, respectively) to expose a disk (25 ~ 125 μm in diameter) acting as the microanode. The micranode was driven to move by a stepping motor in an electroplating bath thus guiding the micro electroplating way according to the program built in the micro-CPU. The micoanode assembly and microanode was driven to move by a