Hydrophobic self-assembly monolayer structure for reduction of interfacial moisture diffusion

Interfacial delamination is one of the primary concerns in electronic package design. Pop-corning during the solder reflow of plastic-encapsulated IC packages is a frequently occurred defect due to moisture penetration into the packages. Moisture absorption has a detrimental effect on the EMC/Cu interfacial adhesion and drastically reduces the reliability of plastic packages. To improve package reliability and to prevent interfacial delamination, it is important to design the EMC/Cu interface for high hydrophobicity and good adhesion. The object of this paper is an investigation of both adhesion and moisture absorption at the EMC/Cu interface using MD simulations. Three kinds of models containing SAM1, SAM2 and a mixture of SAM1 and SAM2, have been used to evaluate the bonding energy and moisture absorption between EMC and SAM coated Cu substrate in this study. In each model, SAM1 or SAM2 or mixture of SAM1 and SAM2 chains were aligned on the copper substrate. MD simulations were performed at a given temperature using the constant-volume and temperature ensemble (NVT). Non-bond interactions cut-off distance of 1.25 nm with a smooth switching function was used in all simulations. The simulations were performed with an interval of 1 femto second (fs) in each MD simulation step. Moisture distribution and binding energy were calculated from simulation for each model. MD simulation results showed that the SAM1 has the higher bonding energy, while SAM2 has the higher hydrophobicity. It was also found that a mixture of SAM1 and SAM2 has both a higher bonding energy and a higher hydrophobicity which can be used as an interface promoter for adhesion and moisture inhibitor in electronic packages. This study shows that MD simulation can be an efficient tool for optimization of SAM to create a hydrophobic interface, which can provide useful pointers of the selection of the SAM structure.