Fabrication of high aspect ratio micro-rod using a novel electrochemical micro-machining method

Abstract

In this paper, a novel electrochemical micro-machining method is proposed to fabricate a tungsten micro-rod with a high aspect ratio. In this method, the periphery of the iron needle is surrounded by an insulator so that only its end face with the diameter of 50 μm is exposed on the electrolyte as the cathode of the tool electrode. A tungsten rod with a diameter of 200 μm is taken as the workpiece and the anode. These two electrodes are immersed to a depth of 3 mm in the electrolyte of 2 wt% sodium hydroxide. This small end face of the tool electrode is against the workpiece in vertically reciprocating motion with a certain stroke to conduct the electrochemical micro-machining under DC electric field. During the machining, since the diameter of the workpiece is gradually reduced, the tool electrode is adjusted so that the gap between its end face and the anode remains 30 μm. The effects of supply voltage or current, rotational speed and stroke of workpiece, and relative position between the end faces of electrodes on the geometry of one end of the workpiece are investigated. This relative position can be used to control the workpiece geometry at one end and its amount of the length reduction. When this relative position is adjusted to a preset position at each stroke, the ratio of the length to the diameter can be increased, but the average decreasing rate in diameter remains invariable. The novel method developed in this paper can fabricate an extremely thin straight rod in diameter of 2 μm with an aspect ratio of 120 from a tungsten rod with a diameter of 200 μm using a tool electrode with an end diameter of 50 μm under two-stage procedure.

Introduction

Today, in many micro precise process or measuring system, it is necessary to use micro electrode as the tool electrode [1], [2] or as the probe [3], [4], [5]. To meet these demands, various methods to fabricate these micro electrodes have been proposed, such as a micro mechanical machining [6], [7], [8], a wire electro-discharge grinding (WEDG) [9], [10], a reverse electro-discharge machining (EDM) [10], [11], a scanning EDM [12], and a micro anode guided electroplating (MAGE) [13], [14], [15], etc. In recent years, an electrochemical micro-machining has received much attention in the fabrication of micro probe, since it offers several advantages including lower cost, no tool wear, higher machining rate, better precision and control, bright surface finish and absence of stress/burr in materials regardless of their hardness.

Electrochemical micro-machining is an anodic dissolution process in which a current is passed between a tool electrode (cathode) and a workpiece (anode) through an electrolyte so that material from the workpiece is dissolved to form the desired shape and the size in the workpiece. Lim et al. [16], [17] investigated the effects of current and voltage on the appearance of tungsten pins and a mathematical model was derived for controlling the diameter of the pin. Based on this model to control the current and voltage, a straight micro-pin with a diameter of 50 μm and a length of 4 mm was fabricated. Choi et al. [18] fabricated a straight micro-pin with a diameter of 5 μm and a length of 3 mm by controlling the applied voltage and the concentration of the electrolyte. Fan and Hourng [19] thought that the micro-electrode with diameter less than 0.1 mm could be fabricated under low applied voltage, high concentration electrolyte and appropriate rotational speed of the electrode. A higher rotational speed would result in an electrode of conical shape. Fan et al. [20] discussed the effects of applied voltage, pulsed period, duty factor and temperature on the fabrication of microelectrodes under a pulsed voltage. The micro-pin with a diameter of 0.1 mm and various lengths could be fabricated by a linear decay of applied voltage or duty factor.

In the past, when the micro-part with the high aspect ratio was fabricated by the electrochemical micro-machining, the tool electrode of the cathode mostly paralleled the tungsten rod of the anode. Since the thickness of the diffusion layer [16] and the oxygen [19] formed around the tungsten rod are not uniform, the geometry machined is easily influenced by the operating conditions so that it is hard to fabricate a truly cylindrical micro-rod.

On the other hand, using flat-end cathode against an anode workpiece to conduct the electrochemical drilling, the erosion gets deeper and the gap becomes larger, which will slow down the machining process by decreasing the electric field intensity [21]. Further, when the current density in the inter-electrode gap is higher than the one of the surrounding gaps, the workpiece is machined firstly at the inter-electrode gap [22]. Park et al. [23] investigated the effect of the tool electrode size on the machining rate. Hence, an insulation coating method was developed to reduce the tool electrode area. Their results showed that side insulation of the tool electrode is effective in minimizing size effects when the machining depth is high.

As mentioned above, it is easy to form the higher electric field intensity or current density in the gap between the electrodes, so that the machining rate is faster on a narrow area in the region with the smallest gap. Based on this idea, only the end face of the tool electrode with the diameter of 50 μm exposed on the electrolyte is proposed. The novel method of the electrochemical micro-machining is used to fabricate a tungsten micro-rod with a high aspect ratio which moves in vertically reciprocating motion with a certain stroke.