Flexible Two-layered Photo-Imageable Dielectric and its Application to Thin Form-Factor and High-Density FPC (Flexible Printed Circuit) using SAP (Semi-Additive Processes)

Abstract

We demonstrate the development of a flexible photo-imageable dielectric (FPID), as well as a thin and high-density flexible printed circuit (FPC) using a dielectric with semi-additive process (SAP). We developed a two-layered FPID in a dry film format. The top layer provides excellent thermal and mechanical properties and the bottom layer provides excellent chemical resistance and adhesion with core materials. We made a copper layer on FPID by spattering and electrolytic copper plating up to $\mathrm{20} \mu \mathrm{m}$ by SAP. The peel strength between copper layer and FPID was measured by 90 ° peel method. As a result, the FPID demonstrated excellent adhesion strength to copper (> 9.0 N/cm). Also, this FPID is enough to form reliable fine traces on the dielectric. We conducted between-the-traces reliability test ( $\mathrm{L}/\mathrm{S}=\mathrm{8} \mu \mathrm{m}/\mathrm{8} \mu \mathrm{m}$ ) and between-layers ( $\mathrm{20} \mu \mathrm{m}$ thickness) reliability test under biased highly accelerated stress test (BHAST) conditions. No failures by ion migration were observed for over 300 hours in between-the-traces test and for 200 hours in between-layers. Regarding the resolution of FPID, our method can obtain $\mathrm{35} \mu \mathrm{m}$ via holes when the dielectric thickness is $\mathrm{15} \mu \mathrm{m}$ . Then we fabricated a thin four-layered FPC using the dielectric as solder resist layers as well as build-up layers. The height of the four-layered FPC is thus reduced by > 50% compared to the conventional process. We successfully developed a flexible dielectric dry film material with reliable thin form-factor and high-density.