Characterization of EMC/LF interfacial fracture toughness using the advanced mixed-mode bending method

Tests on epoxy molding compound (EMC) and copper lead frame (LF) interfaces in form of bimaterial beams have been carried out with the Advanced Mixed-mode Bending (AMB) test method. The AMB test is a fracture mechanical test which enables the loading of the interface by a superposition of the opening mode (mode I) and the in-plane shear mode (mode II). In this work the test specimens are extracted from mold maps consisting of two different EMC systems on Cu lead frames. Additionally, in order to study the effect of the assembly process, beams cut from an unlabeled package without a die were experimentally tested. The experimental fracture tests, performed with the AMB, are numerical simulated by means of finite element method (FEM). For specific fractured states of the beam, the crack length is determined using digital image correlation (DIC) algorithm. For the evaluation of the energy release rate G versus mode-mixity ψ FEM analysis were carried out accounting for actual test data, crack length, geometry and linear elastic material data. The extraction of G and ψ was implemented by the crack surface displacement extrapolation method (CSDEM). The presented results summarize normalized G(ψ) curves of two molding compounds used in the two different sample configurations, which are the mold map and the package specimen. The data consists of energy release rate values in a mode-mixity range of ψ = -25° to 90°. Each curve is resulting from tests of 3-11 samples proofing the capability of the AMB to produce reliable measurement data with lowest number of specimens. A qualitative comparison of the achieved G(ψ) values is discussed with respect to existing curve fitting models and examined effect of the lead frame surface finish.