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The drive toward new first level interconnection technologies is running in parallel with the need to study their reliability as such, as well as in further processes such as second level reflow soldering. Both material properties and process settings have a significant effect on the reliability of adhesive interconnections of flip chips on flexible foil substrates. Integrated circuits (ICs) with pitches of 200 and 300 μm bonded on two different foil types were subjected to various moisture preconditioning treatments, and subsequently reflow soldered. Measurements of the daisy chain resistance are used to monitor the yield before and after reflow testing, and to qualify the endurance behavior in the 85°C/85% RH stress test. We address here the possible failure mechanisms.