Interface crack propagation between epoxy moulding compound and copper

For the MMB delamination experiments on CU-EMC interfaces as previously discussed in [1-6], the interface fracture shows the characteristics of a "brittle interface". This is because of the fact that the actual fracture appears between the brittle EMC and the brittle CU-oxides (being present on the CU lead-frame). Low cycle fatigue fracture or sub-critical fracture under cyclic loading conditions is generally considered not to be occurring for "brittle interfaces". Therefore, it was somewhat surprising that in [7] fatigue fracture was reported to occur for CU-EMC interfaces. The reason for this surprising behaviour could possibly be found in the overloading of the CU-substrate above its yield limit or just due to local plastic phenomena in the CU near the crack tip. In such a case the J-Integral value at the crack tip could possibly rise even under non-progressive cyclic loading. In order to research this phenomena for the MMB test of [1-6], a number of cyclic deformation simulations is applied to explore the J-Integral value for the case that the CU-lead-frame is being bend above its yield limit. In this paper following steps will be discussed: As during non-progressive cyclic loading extremely small changes of the J-Integral value are expected to occur and these should be well registered, first a separate comparison of the J-Integral evaluation of the MMB test for 3 different FEM packages is discussed (ANSYS, ABAQUS, MARC). The simulation results for the J-Integral values during non-progressive cyclic loading at room temperature (=Glassy state of the EMC), are evaluated. It occurs that Shake down to Elastic action occurs after the first cycles. As a result, based on the simulation results at room temperature, sub-critical fracture is not likely to occur. It is expected that at high temperatures (=Rubbery state of the EMC) again Shake down to Elastic action will occur after the first few cycles. Consequently, plastic behaviour in the CU is not considered as the root cause of sub-critical fracture. On the other hand, in packages under cyclic loading including high temperature, the exposure at high temperature will be accompanied by continuing aging of the EMC. In parallel research [15] it was found that due to aging of the EMC the deformation and stress state of a package is dramatically changing with time. This dramatically changing state could well be the origin of previously observed sub-critical fracture. Here further research will be required.