A Compliance-Based Approach to Study Fatigue Crack Propagation for a Copper-Epoxy Interface

Abstract

As the microelectronics industry continues to advance the boundaries of size and performance, focus on the impact of systems packaging has risen to the forefront of design and material considerations. As interfaces are often constructed of multiple heterogeneous layers, interfacial delamination is an important failure mechanism to consider in microelectronic packaging. This failure is due to, among other factors, the stresses arising from high mismatches in coefficient of thermal expansion (CTE). Most work to date has focused on interfacial crack propagation under monotonic loading that is incurred during fabrication steps such as deposition or curing which occur at elevated temperatures and subsequent cooling to room temperature. This is an important design consideration but it is not sufficient as the operational life of these devices involve high numbers of heating and cooling cycles which result in crack propagation under fatigue loading. As such the study of fatigue effects on these interfaces is paramount to improving the lifetime of microelectronic devices as the field pushes towards both thinner and wider packages. One such exploration, which is the subject of this work, is to determine the interface incremental crack growth rate as it relates to cyclic loading. In this work, double cantilever beam (DCB) tests are performed at various stress ratios on samples with epoxy mold compound (EMC) atop a copper leadframe. For these tests, force versus displacement curves will be obtained. Given the small dimensions of the interfaces in question, it is desirable to develop a test methodology that does not require in-situ measurement of crack length. Thus, in these tests the compliance of the samples is determined from the force versus displacement curves and used to infer the progress of the crack through an indirect approach. The advantage of this method is that it does not require the observational measurement of the crack length potentially allowing crack monitoring absent any optical or imaging methods. Using the determined crack propagation rate with fatigue cycle under various loading conditions, a generalized fatigue crack propagation model will be developed for mold compound and copper interface, and such a model can be employed to assess packaging reliability in operating conditions.