Effect of Interfacial Adhesion of Copper/Epoxy under Different Moisture Level

Understanding interfacial adhesion subjected to different levels of moisture content and temperature elevation is of significant interest to the electronic packaging industry. The conventional study has focused primary on moisture diffusion into the encapsulated molding compound (EMC) of plastic packages. This paper looks at the alternative path of interfacial seepage into the EMC and copper interface of the IC package. In this study, the effect of moisture on interfacial bonding energy was calculated by running a molecular dynamics simulation. A series of MD models consisting of a network of epoxy macromolecules and copper atoms with different amount of water molecules at its interface were built with the Discover module. The mass ratio of water molecules to epoxy varied from 1% to 6% while the surrounding temperature kept at 85degC in order to simulate a similar environment condition in MSL-1 qualification test. Calculations were carried out at different humidity level with a prescribed moisture concentration value, using the NVT ensembles. From the simulation results, it is observed that the interfacial bonding energy decreases with the increase of mass ratio of water molecule to the epoxy due to locking of water molecules at the nanopores at the epoxy/Cu interface. Interfacial bonding energy between the epoxy and copper substrate weakens when water molecules increasingly accumulate at the interface. To verify the simulation results, epoxy was molded on pre-cleaned copper substrate and undergone MSL-1 test. Adhesion strength of the sample set was evaluated by button shear test at different times. They show a strong qualitative correlation between the MSL-1 test data and the MD simulation results. It is concluded that the interfacial moisture diffusion is also an important factor contributing to delamination in plastic packages