On the design of a new test method to characterize the delamination of thermosetting polymer under thermal mixed mode loading

Overview & Synopsis

Crack propagation in integrated circuits is a major failure cause of electronic components. In the automotive branch, a highly competitive market, failure is not allowed and reliability can make the difference to gain market shares. For this reason, many efforts and researches were conducted in this field during the last decades. In automotive applications, electronic parts have to challenge heavy life conditions, such as vibrations, wide temperature variations, humidity, oil aggressions, etc. In particular, cyclic thermal variations bring incompatible thermal strains along materials interfaces, and leads to interfacial crack propagation (delamination) between copper leadframes used as conductor and polymeric molding compound used to encapsulate devices. Delamination causes then the cracking of the encapsulant and finally the failure of the electronic component. Predicting the interfacial delamination in electronic packages is key to enhance reliability. As detailed in this work, delamination strength of interfaces involving molding compounds depends on many influences. Its characterization required numerous kinds of test, involving heavy costs and long testing periods. In this work, a low cost test setup is proposed to characterize the molding compound/copper interface regarding the different influences already mentioned. The delamination toughness of the copper/polymeric molding compound is characterized with a classic molding compound of the market with respect to the mode mixity (ratio of loading mode II/mode I), temperature, and load rate. Moisture effect is also investigated. The critical energy release rate for the investigated interface is estimated using the classical numerical-experimental correlation used in fracture mechanics. For the investigated material pair, the interfacial toughness exhibits viscoelastic properties and an own time-temperature superposition principle. Obtained interfacial parameters are defined in Abaqus. The latter is widely benchmarked for problems involving delamination and viscoelastic materials. In the case of cracking between two dissimilar materials, the analytical solution of the stress and strain fields at delamination front depends on a characterisctic reference length. The choice of this refence length, conditioning the the mode mixity estimation, is also addressed. The reference length is experimentally characterized for the different investigated temperature.