The demand for flexible printed circuit boards is continuously increasing as mobile phones, cameras, laptops and other portable devices become more and more widespread. The ROHS/WEEE regulations had to be applied for these types of materials as well. Some of the flexible materials can not withstand the soldering process temperatures even with tin-lead alloys, these are obviously not capable for lead-free soldering. However the influence of higher temperatures on the reliability of polyimide (PI) materials is of great interest. The other, lower temperature materials (PET, PVC, PEN), can be processed alternatively either by laser soldering or with conductive adhesives. This paper gives an overview of flexible base and joining materials, focusing on their applications and environmental specifications such as applicable temperature and humidity ranges. Environmental (moisture absorption, dimensional stability) tests have been done on the most widely used materials (PI, PEN and PET) to determine the influence of processing temperatures on material properties. Based on the results the processing temperature regimes, thus the joining materials (tin-silver-copper or low temperature solder alloy, conductive adhesive) for each base material have been chosen. Special test structures for assembly evaluation and reliability tests were built. The ageing of the samples has been accomplished traditionally by 85°C/85%RH and thermal shock tests (TS). The combination of thermal shock and HAST showed similar failure mechanism with the test duration reduced by a factor of 6 to 10. The component joints have been evaluated electronically and mechanically. The test results not only provide information on the expectable life time period of these circuits, but also help in optimizing the material choice for different application purposes.