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In general higher packaging densities, thinner substrates and lighter products can be achieved using flexible printed circuit boards (FPCB) compared to traditional thicker fiber-reinforced printed circuit boards. Polyimide (PI) is popular substrate material in FPCBs due to its excellent properties. Liquid crystal polymers (LCP), on the other hand, are more rarely used in FPCBs. This is mostly due to their high cost and poor availability. However, they have an excellent combination of thermal, mechanical, chemical and electrical properties, which have caused increased interest to use them in several applications. In addition to thermoplastic PI and LCP materials, thin FR-4 laminates can be used as FPCB material. They have lower flexibility than PI and LCP, but also lower price and excellent availability. In this study the effect of substrate material to the reliability of flip chip interconnections assembled with anisotropically conductive adhesives (ACA) was investigated. Three different FPCBs made from PI, FR-4, and LCP were used. Several different tests were conducted. Results clearly showed that the reliability of ACA joints on LCP substrates was better compared to PI and FR-4 substrates under humid and corrosive environments.