Shear stress modeling of ACA joints in thermal tests

Anisotropic conductive adhesive (ACA) joints are used in various applications. Consequently, they may be exposed to various environments during their life-time, and reliability problems may occur, especially in demanding conditions. Accelerated life tests are typically used when reliability is studied, but they are fairly time-consuming. Utilisation of modelling would decrease the need for testing, and cost and time savings could be achieved. In this study the feasibility of using modelling in the reliability prediction of ACA flip chip joints was investigated. The aim was to examine if a relatively simple shear stress model together with reliability data of a test structure could be used to predict the reliability of slightly changed test structure or the effect of varied test method. First test samples with ACA flip chip joints were assembled and tested in different thermal environments. Next either the structure of the test samples was slightly varied by changing materials or dimensions, or different thermal tests were used. Failure times, modes and places between the test series were compared. In addition, shear stress modelling was used to estimate differences between the test series. Finally, the feasibility of using modelling in the reliability studies was evaluated by comparing the results of modelling and experimental testing. It was observed that it is important to take failure mechanisms into account in the modelling. When the failure mechanism of the ACA joints was related to the shear stresses, it was possible to utilise the shear stress modelling successfully in different thermal environments. On the other hand, if the failure mechanism of the polymeric interconnection was related to thermal expansion of the adhesive matrix, strain modelling which calculates the changes in thickness of an adhesive layer between contacts seemed feasible in reliability studies.