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Non-conductive adhesives (NCA) are used to attach flip chip components onto substrates. They have many advantages compared to solders. However, in a humid environment the use of NCA may cause some reliability problems. Long-term tests are essential when the reliability of NCA joints is determined. These tests, however, are often very time-consuming. Nevertheless, without the tests it is difficult to predict the failure times and places. One of the long-term failure mechanisms is the deterioration of the adhesion. In order to study the effects of moisture on the adhesion of NCA, strength tests were conducted after a humidity test, and the adhesion was found to decrease as a function of time. Additionally, finite element models of moisture and thermal expansion were formed, and on the basis of the models the most likely places for failures were defined to be the corner joints. Finally, the results of the models were compared to the results of adhesion measurements and the expected failure time was estimated to be 13 500 hours in test conditions. No failures were seen during 4000 hours of testing. Thus, this structure was found to be extremely reliable in a humid environment.