Towards a predictive behavior of non-conductive adhesive interconnects in moisture environment

Flip chip on flex interconnections with non-conductive adhesive paste are taken as test vehicle to study the behavior of adhesive interconnects in moisture environment. A dedicated test structure is used to enable a proper four-wire resistance measurement of single interconnects. A new and very powerful approach is used, focusing on the response of the contact resistance of the interconnect to humidity. This response of the contact resistance to increasing or decreasing humidity levels reveals much more information about degradation kinetics and physical mechanisms behind it, than can be obtained from any steady-state accelerated humidity test, used today. Two types of accelerated tests have been performed with on-line measurement of the contact resistance with high measuring resolution. The first test aims at the durability of the interconnects and is done at 85°C with humidity cycling between 30%RH and 85%RH. Two adhesives were tested for over 2800 hours. Time to failure has been determined applying different failure criteria. Experimental results show that the response to the moisture during the first cycles is already a good indicator for the eventual time to failure. A simple power law equation applies to relate the response to time to failure. The second test is step stress-type test and consists of increasing and decreasing the relative humidity in steps of 10% to 15%RH at the fixed temperature of 85°C. This test shows that the relative response of the contact resistance to humidity is much larger at high relative humidity levels, compared to the response at low relative humidity levels. Combining the results of both types of experiments allows defining a simple transform for non-conductive adhesive interconnects in a changing humidity environment. Failure analysis is performed to understand the physical behavior of the interconnects during the accelerated tests.