High-Density Interconnect Rdl-Fpc Hybrid Substrate for Compact Sip Packaging

This paper presents a novel RDL-FPC hybrid substrate structure developed to meet the growing demand for compact, high-performance electronic packaging. By integrating fine-pitch redistribution layers (RDL) into flexible printed circuit (FPC) fabrication, the proposed architecture achieves significant thickness reduction while supporting highdensity interconnects. A low-loss dielectric material with a dielectric constant of 2.8 and a dissipation factor of 0.0060 at 20 GHz was employed to ensure signal integrity in high-frequency applications. The process was implemented on a temporary glass carrier and includes multi-layer RDL build-up, photoimageable dielectric (PID) and photoimageable coverlay (PIC) lamination, laser debonding, and surface finishing. Reliability was evaluated through thermal cycling, heat soak, thermal shock, and mechanical bending tests, all of which showed no failures. Warpage and delamination were mitigated via structural optimization, including symmetric layer design and selective PIC patterning. The final structure, approximately <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$150 \mu ~\mathrm{m}$</tex> thick, exhibits excellent electrical, mechanical, and thermal performance, making it suitable for advanced system-in-package (SiP) applications.