Foldable smartphones require Flexible Printed Circuit Assemblies (FPCAs) with substrates that endure 100,000+ folding cycles, extreme flexibility, and stable electrical performance across -40°C to 85°C. The core FPCA substrate materials are Polyimide (PI), Liquid Crystal Polymer (LCP), and Polyethylene Terephthalate (PET), each tailored to balance flexibility, heat resistance, and cost. PI dominates mainstream foldable devices due to its optimal combination of performance and processability, while LCP serves high-frequency applications and PET targets low-cost, non-critical components.

Polyimide (PI) films (e.g., DuPont Kapton®, Toray Upilex®) are the primary FPCA substrates for foldable hinges and high-flex zones. With a Tg of 250–260°C, continuous operating temperature up to 260°C, and exceptional foldability (100,000+ cycles at 1.5mm radius), PI maintains mechanical integrity and insulation during repeated bending. It bonds well with rolled copper foil (preferred for flexibility over electrolytic copper) and resists solvents, radiation, and thermal aging. PI’s low CTE (12–15 ppm/°C) minimizes stress between the substrate and copper traces, critical for preventing crack formation in foldable regions.

Liquid Crystal Polymer (LCP) substrates are used in high-frequency FPCA sections (e.g., 5G antennas) due to their ultra-low Dk (0–5) and Df (0.001–0.003), ensuring signal integrity at millimeter-wave frequencies. LCP offers better high-frequency stability than PI but is more expensive and harder to process, limiting its use to performance-critical areas. PET substrates (Tg ≈80°C) are low-cost but have poor heat resistance and foldability, suitable only for non-flex, low-temperature FPCA components in foldable devices. Advanced foldable FPCAs often use multi-layer PI with LCP core layers to combine flexibility and high-frequency performance.