Interfacial adhesion between the Epoxy Molding Compound (EMC) and the copper-based leadframe is one of the major concerns in the qualification of plastic packages. Since the conventional shear testing methods used in industry do not consider the residual stresses in the shear samples, they are only used as a qualitative testing method for the EMC qualifications. However, since these tests are based on the maximum force leading to interface delamination, they may cause erroneous results because of neglecting process-induced stresses, which may alter the required force needed to break the samples at the interface. Even classical fracture mechanics, based on mechanical load leading to crack propagation, may not fully characterize the interfacial fracture toughness, because the residual stresses available in the sample impede or facilitate the crack progress, depending on the state of the stresses at the crack tip. The aim of this work is to propose an effective selection criterion for finding the most suitable epoxy molding compound in terms of the intrinsic interfacial adhesion. The effect of residual stresses on the interfacial fracture toughness was investigated by performing an empirical approach to calculate the amount of the cure shrinkage by warpage measurement of the bi-material beams. The effective cure was implemented in the Finite Element Analysis (FEA) of the experimental fracture test to estimate the real interface adhesion. It was observed, that the proper molding compound candidate to fulfill the adhesion requirements was not the one which showed the maximum fracture force and material selection may be done wrongly, if the process-induced stresses are not considered in the FEA of the fracture tests
This work presents some recent progresses in reliability assessment of electronic assemblies in automotive industry and shows how coupled numerical-experimental techniques can help us save time and reduce the cost of IC package qualification. In order to fulfill the continuous trends in miniaturization of the electronic devices together with the demands to shorten the time-to-market, it is essential to use virtual qualification methods with the simulation tools. One of the main concerns in electronic packages is their structural integrity during the fabrication, surface mount process, and service life. A prominent example of failure in electronic assemblies is the interface delamination between two dissimilar materials. This failure mode is accelerated when the polymeric materials absorb moisture from humid environments. Moisture results in degradation of the physical properties of polymers, induces additional deformation due to hygroscopic swelling, and more importantl y, degrades the adhesion strength of the polymer to metal joints. This work provides conceptual understandings of the problem of moisture-driven interface delamination in plastic encapsulated microcircuits. In addition, it is shown how the developed method can enhance the material selection in order to improve the delamination resistance in the package and preserve the structural integrity
This study presents a multi-disciplinary approach in order to investigate the influence of hygroscopic stresses and stress relaxation in Epoxy Molding Compounds (EMC) on the apparent interfacial fracture toughness of Cu/EMC interfaces. Bi-material beams were designed and produced via transfer molding similarly to the process used in state-of-the-art semiconductor packaging. An empirical method was used to investigate the so-called "stress-free" temperature of the package using warpage measurement of the beams at elevated temperatures. Cure shrinkage was measured by introducing a fitting parameter to the Finite Element (FE) analysis to match the simulation results to the experimental warpage. The bimaterial beams were exposed to a humid environment and their warpage was observed to change during the moisture absorption. Warpage measurements together with a viscoelastic FE analysis allow for the calculation of two important material properties, namely, cure shrinkage and the Coefficient of Hygroscopic Swelling (CHS). Fracture toughness of the EMC/Cu interface as a function of exposure time to moisture was measured. End-Notched Flexure (ENF) tests at various intervals after moisture loading were performed. The Virtual Crack Closure Technique (VCCT) was applied to measure the adhesion strength in terms of interfacial fracture toughness by considering the sample manufacturing and sorption history
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