Flex circuit solderability and life expectancy are influenced by handling and finishes.
What is the shelf life of a flex circuit if it stays in its original sealed packaging? And what is the life expectancy of a flexible circuit?
I assume the first question refers to long-term solderability and the second to the overall functionality of the flex circuit once installed in the final application.
As far as long-term functionality goes, flexible circuits in a static application really don’t have a post-installation “best if used by” date. There are flexible circuits over 25 years old still functioning without issue in military and avionics systems. Flexible circuits have also been used for decades in satellite applications due to their very low mass and high connection density. Considering the cost of a service call to a satellite, only the most reliable interconnects are used in these applications. That flexible circuits are still in use after all these years is a testament to their long-term reliability. The only exceptions to overall life expectancy are flexible circuits operating in very harsh environments, such as continuous temperatures above 275°F, strong acid or caustic exposure, abrasion, etc.