# Comparison between Laser and Stamping without Die (SWD) for Micro Tapered Hole Forming
## 1 Introduction  
The back covers of consumer electronics products, such as mobile phone, tablet Person computer (PC) and so on, are required of microholes. The microholes are tiny, but light could pass through them. Without damaging the aesthetic sense of mobile device design by using microperforation illumination technology, the micro holes provide indication functions or replace the input function of keyboard or mouse. The tapered microhole offers higher light extraction efficiency and better illumination uniformity. The current microhole processing methods include laser processing, electro-discharge machining, micro stamping, and wet etching. Among these processing methods, the electro-discharge machining has higher machining accuracy and lower surface roughness, but the processing rate is low. Thus, it is not suitable for mass production. The wet etching process could remove the surface material selectively or globally at the highest etching rate with the best cost effectiveness, but is limited to the specific materials. Meanwhile, the following pollution is always an issue. In copper and aluminum alloy microhole processing, the laser processing and micro stamping are better choices in terms of machining efficiency, and both processing methods have their own merits and demerits. The laser drilling results in high depth-diameter ratio and machining efficiency. Itcan implement high speed drilling, especially applicable to large quantity and high density drilling without tool wear. Due to the Gaussian beam and the effect of defocusing, the microholes by laser drilling are mostly tapered holes [1]. The laser drilling utilizes the high power density of laser to vaporize the workpiece rapidly, and then the melted material is extruded by the surplus pressure of vapor to form holes. As it is a thermal ablation process, the pulsed laser processing is likely to result in char black, molten slag and material deterioration [2; 3]. This study uses Nd:YAG laser for drill experiment. In the application of micro stamping, [PERSON] _et al._ developed the wire electro-discharge grinding (WEDG) mechanism in 1985, which was used in micro-discharge machining, and the microelectrode was created successfully [4]. This technology was also used to make micro punches for micro stamping [5; 6]. Grinding technology [7; 8] or reverse-electrical discharge machining (EDM) [9] can be used for making a micro punch as well. In this study, we uses reverse-EDM for creating a micro punch. The major problem in making a microhole by micro stamping is the precision alignment of the punch and lower die, which creates bottleneck in current micro punching. While this issue can be overcome by guide pin [10] or image alignment [11], it remains complicated for mold design. Therefore, this study proposes a stamping without die (SWD), where a higher moment of inertia is produced by a thicker sheet, thus, overcoming the bending moment generated by the stamping process. Micro stamping is a micro forming techniques. This plastic forming considers complex structural mechanics and material problems. In recent years, regarding plastic forming analysis, many studies have used finite element analysis software DEFORM-3D to analyze mold wearness, and the flow of material in molds. According to the simulation results, the simulation and experiment are matched well [12; 13; 14; 15]. In order to study the feasibility of the proposed process, and find the optimal experimental parameters quickly, the DEFORM-3D is used to predict forming and loading in our micro stamping process. The simulation result can used in the experiments. The experimental setup of micro stamping is as shown in Figure 1a. The whole setup is on an engraving EDM machine (Sodick AP1L, Baginton Coventry, UK). The punch head is affixed to the \(Z\)-axis. Figure 1b shows the schematic diagram of the holder fixing workpiece. The design concept is that the workpiece is fixed by screws and the central part of workpiece is suspended. Figure 1c shows the tapered punch for stamping, which is made by reverse-EDM.