Moisture diffusion study in electronic packaging using molecular dynamic simulation

Moisture induced reliability concerns have been extensively studied in package design. Popcorning in plastic-encapsulated IC packages is a defect frequently occurring in solder reflow due to moisture penetration into the packages. Moisture diffusion has a detrimental effect on the epoxy/copper interfacial adhesion and drastically reduces the reliability of the encapsulated package. The present study is focused on the moisture diffusion in both the epoxy molding compound (EMC) and the EMC/Cu interface. In order to evaluate which is the dominating moisture transport mechanism at the interface, molecular dynamic (MD) models were built using the ´Materials studio´ software. Based on the conditions of 85degC/85%RH in qualifying tests in humidity chamber, the amount of water molecules are assigned to the packing cells in the MD models. All the MD simulations were performed at a temperature of 85degC using the constant-pressure and temperature ensemble (NPT). Non-bond interactions cut-off distance of 1.5 nm with a smooth switching function was used in all simulations. The simulation in each case study was performed with an interval of 1 femto second (fs) in each MD simulation step. The mean squared displacements of all water molecules were evaluated in each time-step to track the motion of the molecules. Constants of moisture diffusion in both bulk EMC material and EMC/Cu interface can be derived from the mean squared displacements. The MD results show that the value of the moisture diffusion coefficient at the EMC/Cu interface is higher than that in the bulk EMC material. It revealed that moisture can easily penetrate along the EMC/Cu interface. The MD simulation study has demonstrated that the seepage along the interface is the dominant mechanism for moisture diffusion into the EMC/Cu interface in plastic packages. This widely studied mechanism of moisture diffusion via the bulk EMC is an apparent a secondary moisture penetration path to the interface