Cohesive Zone Parameters for a Cyclically Loaded Copper-Epoxy Interface

Samet, D., Kwatra, A., and Sitaraman, S. K., “Cohesive Zone Parameters for a Cyclically Loaded Copper-Epoxy Interface,” 66th Electronic Components and Technology Conference, IEEE-CPMT and EIA, Las Vegas, NV, May 2016.
Bimaterial interfaces, like the one consisting of epoxy mold compound (EMC) cured over copper leadframe, are commonly present in microelectronic packages. Failure in such bimaterial interfaces can be simulated through the use of cohesive zone modeling (CZM). To date, nearly all CZM modeling of bimaterial interfaces has been performed for monotonic loading conditions. However, most of the interfacial failures in microelectronic packages occur during operating conditions where repetitive or fatigue loading conditions are present. The CZM work for fatigue seen in literature utilizes methods which while demonstrably accurate, are data correlation schemes rather than derivations formulated from the underlying micromechanical behavior. Most proposed fatigue CZM models incorporate fatigue damage by decreasing the critical strain energy release rate (SERR), Gc, though a modified CZM damage parameter. This work presents a new characterization approach which continuously modifies CZM parameters as fatigue loading occurs. This new characterization method offers the potential of higher predictive value as it requires no assumptions beyond that of energy conservation, an assumption already inherent to the formulation of cohesive zone modeling. Through the addition of fatigue effects to CZM behavior, this method is capable of predicting and modeling crack propagation for loads below the monotonic critical level.
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