Interfacial Adhesion Study for SAM Induced Covalent Bonded Copper-EMC Interface by Molecular Dynamics Simulation

Abstract

Copper-epoxy molding compound (Cu-EMC) interface is known to be the weakest joint in the electronic packages, which causes delamination during a reliability test. A prime reason is the lack of adhesion between Cu and the epoxy compound. To solve the problem, a self-assembly monolayer (SAM) is introduced to improve adhesion of copper-epoxy system. Thiols/disulfides, which can effectively deposited on Cu surface, were selected as the SAM to act as surface modifier to Cu substrate. The selection of thiols/disulfides candidate with an appropriate tail group is essential for the adhesion enhancement. To promote adhesion, the SAM structures should be able to form high density covalent bonds with the EMC. This paper focuses on the use of molecular dynamics (MD) simulation to study the covalent bonds effect on the adhesion in Cu-SAM-EMC system. The data is used as a means to select SAM candidates with good interfacial adhesion strength. In this study, MD simulation models of the Cu-SAM-EMC system with covalent bonding and optimized SAM density were built to evaluate the interfacial bonding energy between the EMC and SAM coated substrate. The results show that the interfacial bonding energy changed with different SAM coatings on the Cu substrate. The modelling results were compared with the experimental button shear data. The shear test shows that when Cu substrates have been coated with SAM with amine/amide groups, adhesion increased significantly. A consistent qualitative trend is observed in the results calculated by the MD simulations. The sheared samples, which were analysed by the time-of-flight secondary ion mass spectrometer techniques, further confirmed the existence of covalent bonds at the interface. This proves that the covalent bonding at the interface is a key mechanism in enhancing the interfacial adhesion. This work illustrates that MD can help in understanding the behavior of bonding of SAM to polymer at the molecular scale. The MD can be a useful tool to select SAM structure for adhesion promotion in a Cu-SAM-EMC system.