Molecular Dynamics study of the traction-displacement relations of epoxy-copper interfaces C. K. Y. Wong 1 , S. Y. Y. Leung 1 , R.H. Poelma 1 , K.M.B. Jansen 1 , C. C. A. Yuan 1 , W. D. van Driel 1, 2 and G. Q. Zhang 1, 2 1 Faculty 3mE, Department PME Delft University of Technology Mekelweg 2, 2628 CD DELFT, The Netherlands 2 Philips Lighting Mathidelaan 1, 5611 BD Eindhoven, The Netherlands k.y.leung-wong@tudelft.nl Phone: +31 (0)15 27 85738 Abstract The traction-displacement relations of the epoxy- copper interfaces are studied using an atomistic model. The reaction force of the epoxy layer in response to displacement of the interface is calculated during molecular dynamics simulation. A parametric study in terms of displacement rate and the step size of displacement increment has been performed. The traction-displacement relations are found sensitive to the step size of the displacement increment. The traction- displacement relations are better described with a small displacement increment in the initial region where the epoxy-copper interface is in close contact. The interfacial energy as calculated by the traction- displacement model is -0.28 Jm -2 which is comparable to the value obtained from a static model. This calculated value is also close to the thermodynamic work of adhesion (-0.26 Jm -2 ) of an epoxy-copper system as reported in the literature [1]. 1. Introduction Cohesive Zone Models (CZMs) are widely adopted to study fracture and delamination processes in metal, polymer and their interfaces [2-6]. Rather than assuming an infinitely sharp crack as in fracture mechanics, CZMs propose the presence of a fracture process zone, where the external work is transferred to both the forward and the wake regions of the propagating crack [5]. The traction-displacement relations which are used in CZMs, describe the major parameters such as peak traction, separation in crack propagation. While the relations are difficult to be determined experimentally, molecular models (molecular dynamics or coarse grain molecular dynamics) are usually adopted to study the relations. Here we use a Molecular Dynamics model to simulation the traction-displacement relations of an epoxy-copper interface. Reaction forces acting on the epoxy as the copper substrate moves away from the epoxy are calculated by a force summation method. A parametric study of the displacement rate and the step size of the displacement increment have been performed. The aim of this work is to propose a MD approach for determination of the traction-displacement relations of the cohesive zone model. 2. Molecular dynamics simulation An atomistic model of the epoxy-copper interface is presented. Molecular dynamics is applied to calculate the reaction force of atoms in the epoxy layer in response to displacement of the copper layer. All models are created with the commercial computational materials program Material Studio 5.0 and the module Forcite from Accelrys [7]. All the simulation was conducted using the COMPASS forcefield. The scripts are developed with the programming language Perl. 2.1 Boundary condition Fig. 1a and b, illustrate the snapshots of the separating interface in different displacement distances (0, 2.5 and 5 Å in –z direction) for 2D- and non-periodic boundary conditions, respectively. The gap between the layers behaves differently with the 2D and the non-periodic setting. Upon the copper layer displacement, the gap between the two layers in the non-periodic model changes in response to the motion of the copper layer. Atoms in the epoxy layer were separated from the copper layer. On the contrary, the gap between the two layers remains unchanged with the 2D periodic boundary conditions. Atoms in the epoxy layer were not separated from the copper layer. Due to the 2D periodic boundary conditions setting, some atoms in the epoxy layer are connected to other atoms in the neighbor cell. This constrains the motion of the connected atoms and also fails the separation of the layers. With this observation, non-periodic boundary condition is adopted in all the models in this study. 978-1-4577-0106-1/11/$26.00 ©2011 IEEE 2011 12th. Int. Conf. on Thermal, Mechanical and Multiphysics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2011 — 1 / 5 —