Laser via formation in flexible substrates for high density electronic assembly

This paper deals with the technology of double sided, high density printed circuit boards, based on advanced flexible materials, created especially for the interposer (the substrate element) of chip scale packages. The new technology requires new methods for the realization of via holes. These via holes are realized by laser drilling. The required laser drilling process and technology is under development at the Budapest University of Technology and Economics (BUTE, Budapest, Hungary), and later at RUC/TFCG (Gent, Belgium), using frequency multiplied Nd:YAG laser at BUTE, and excimer/CO₂ lasers at RUG. For the experiments, double sided flex laminates (e.g. polyimide flex), covered on both sides with 9-12 μm of Cu layers are used. The final processing step of the substrate is the patterning and etching of the Cu layers on both sides of the laminates. The developed technology enables fabrication of a flex CSP substrate suitable for interconnecting a chip on one side (high-density side, e.g. flip chip technology) and solder balls on the other side (low density side)