Definition

Polyimide (PI) is a high-performance polymer with excellent thermal stability (continuous use to 260°C), chemical resistance, and mechanical flexibility. In PCBs, it's used as the base film for flexible circuits (Kapton is DuPont's brand), high-temperature rigid laminates, and coverlay material. Polyimide has higher moisture absorption than FR-4, which can affect electrical properties and requires bake-out before soldering. Its excellent dimensional stability and high Tg make it preferred for aerospace, military, and implantable medical devices.