V. N. N. Trilochan Rambhatla's research while affiliated with Georgia Institute of Technology and other places

Microelectronic packages continue to become smaller and more complex. Interfacial delamination is a common failure mechanism present in microelectronic packages due to the mismatch in the coefficient of thermal expansion (CTE) between different materials. Epoxy Molding Compound (EMC)/copper is a common interface found in microelectronic packages and is susceptible to delamination due to CTE mismatch. This work analyzes interfacial delamination of an EMC/copper interface and the impact of temperature and humidity conditioning on interfacial fracture energy using a double cantilever beam test. The critical interfacial fracture energy is obtained for as-received, thermally-aged, and humidity-conditioned samples. These experimental results can be used in models to predict delamination in an EMC/copper interface and how these interfaces are affected by factors such as time, temperature, and humidity conditioning.

Experimentally characterized critical interfacial fracture energy is often written as an explicit trigonometric function of mode-mixity and is used to determine whether an interfacial crack will propagate or not under given loading conditions for an application. A different approach to assess whether an interfacial crack will propagate is to employ a failure locus consisting of the critical fracture energies corresponding to different fracture modes, represented by an implicit formulation. Such a failure locus can be linear, elliptical, among other shapes. As it is nearly impossible to obtain isolated GIc or GIIc values through experimentation, extrapolations are used to determine these two extreme values based on intermediate experimental data. However, the magnitude of these extreme values as well as the shape of the two forms of failure curves are at risk of being inconsistent should proper care not be taken. An example of such an inconsistency would be to use a trigonometric formulation to obtain the extreme values through extrapolation and then employ those values in simulation through an elliptical failure. In this work, we have employed a series of commonly used interfacial fracture energy measurement techniques over a range of mode-mixities for a metal/polymer interface to demonstrate the potential discrepancy in the two approaches and to underscore the need for a consistent approach in evaluating interfacial crack propagation.

Microelectronic packages contain numerous bimaterial interfaces which influence both device design and reliability. The failure of these bimaterial interfaces have been observed to be a function of both the relative shear and tensile loads, otherwise referred to as “mode-mixity.” While the failure of such bimaterial interfaces has been the focus of much study, their performance under fatigue, in particular with respect to mode-mixity, is underexplored. Double cantilever beam tests for a copper /epoxy molding compound interface have been performed for several different mode-mixity conditions, both monotonically and cyclically. The resulting Paris’ laws are reported. In particular, the impact of mode-mixity on fatigue crack propagation is explored. The Paris’ law coefficients and exponents have been seen to be dependent on mode-mixity. The dependency of fatigue behavior on mode-mixity means that some of the properties of fatigue crack propagation can be determined from a bimaterial interfaces monotonic fracture behavior. Finally both numerical analysis in ANSYS and SEM surface characterization are performed to further the understanding of the mechanisms behind the observed trends in fatigue interfacial delamination propagation behavior as a function of mode-mixity.

Fracture mechanics is an essential field of study towards the improvement and development of electronic packages. In combination with modern simulation method such as finite element analysis (FEA), fracture mechanics is widely used and appreciated in the industry. Many different approaches have been developed to calculate the fracture parameters for interfaces or bulk material under given loads in order to compare them against previously measured failure criteria. While many publications are available that have described the different simulation approaches in detail or compare the different fracture test methods, there have been few comparisons of these simulation approaches with respect to their use in research and development of electronic packages. The objective of this work is to compare different delamination modeling methodologies and their applications for electronic packaging. The work highlights the differences in theory behind each approach as well as the differences in their practical use to predict delamination or asses a fracture risk in electronic packages. The intention was to use commercially available FE-codes in conjunction with a well-defined set of adhesion strength tests. During this work, energy based fracture criteria were applied by means of the virtual crack closure technique (VCCT), the J-integral and the cohesive zone material model (CZM) methods. These methodologies are most commonly used but can differ significantly from each other as will be shown in this comparison. To demonstrate the use of these techniques, copper lead frame to epoxy mold compound (EMC) delamination was assessed, representing a very common packaging failure mode. Critical energy release rates were measured on multiple Copper-EMC test specimens under varying load phase angles. ANSYS was used to build mechanical simulation models of a selected device. Existing post processing procedures were applied to assess delamination risk based on above mentioned techniques. The simulation study considers realistic monotonic loading conditions and results will also be compared to existing failure analysis images for demonstration and validation purpose. As an outcome, the paper will include a ranking of the approaches as well as a summary of advantages and disadvantages, based method and accuracy. An outlook on future developments such as fatigue or aging phenomena will finish the work.

Fatigue crack propagation for copper/epoxy molding compound interfaces is modeled in this work by conducting cyclic loading on double cantilever beam test specimens. The continued increase in mechanical compliance of test specimens as the crack propagates through hundreds of cycles is used to determine the crack length and thus, the crack growth rate per cycle which is used to determine the Paris’ law constants as a function of strain energy release rate range. When monotonic debonding testing is conducted, it is seen that the critical strain energy release rate initially increases with the crack length and then stabilizes demonstrating the increasing resistance for the epoxy/copper interface. When such an increasing R-curve is used to normalize the strain energy release rate range, it is observed that the Paris’ law constants can be determined with good consistency for a wide range of specimens over different crack lengths.