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Polymers are frequently used as the insulating material in printed circuit boards (PCBs). However, many polymers tend to absorb moisture and this may impair their electrical and mechanical properties. In this study three flexible PCB materials; polyimide (PI), fluorinated ethylene-propylene (FEP) and polyethylene terephthalate (PET) were aged in high humidity test in which a temperature of 85°C and relative humidity of 85 percent were used. The aim was to study whether the high humidity test has an effect on the water absorption and mechanical strength of the parameters examined. Water absorption was measured using two different methods; precision scale and moisture analyzer. The modulus of elasticity was determined with a thermomechanical analyzer (TMA) in order to gain information about changes in mechanical strength. High humidity testing seemed to decrease the water absorption of PI but it had no effect on the modulus of elasticity. On the other hand, water absorption of FEP remained very low after high humidity testing but the modulus of elasticity decreased. For PET the high humidity test was very harsh and it became fragile during aging. Testing did not affect water absorption of PET before it became brittle but after that the water absorption increased markedly. The modulus of elasticity of PET seemed to decrease during high humidity testing but as the standard deviation was very high, it is not certain whether or not the mechanical strength was already impaired before the embrittlement.