Fabrication of a micro-punching head using LIGA and stacking processes

Abstract

A simple stacking process to fabricate microstructures with a high aspect ratio of 30 is presented in this study. For the x-ray absorption of a substance, an analytical model of the absorption dosage (exposure dosage) along a depth of x is developed. It can accurately estimate the time and dosage required for exposure. Based on the calculated result, 1 mm thick PMMA (polymethylmethacrylate) photoresist with a separate gold (Au)/graphite mask is exposed using x-rays followed by a developing process. Then a stacking process is applied for these 1 mm thick PMMA microstructures to form a microstructure with a high aspect ratio. With this stacking method, a PMMA microstructure with a height of 3 mm has been fabricated. In addition, a nickel–cobalt (Ni–Co) alloy electroplating process is applied to fabricate a metallic mold with high hardness. The hardness of the mold by electroforming is the key factor to the quality of the micro-punch process. As an integrated micro-punch head for IC leadframes, the hardness should be higher than 500 Vickers hardness (Hv). In addition, it was found experimentally that changing the ratio of Ni sulfamate to Co sulfamate can affect the deposit hardness and internal stress. The deposit internal stress varies with Co content. Thus, it is necessary to add a stress reducer to improve the hardness and reduce internal stress. A hardness of the mold of 550–560 Hv with 26–28 wt.% of Co and zero internal stress have been made successfully after adding a stress reducer. With this electroplating technique, a micro-punching head with high hardness and high aspect ratio can be fabricated by LIGA technology.