Thermal modeling of grease-type interface material in PPGA application

Heat sinks are widely used for cooling electronic packages. Typically, a highly conductive thermal interface material is used to enhance heat transfer from the package to the sink. One such material is thermal grease, which has the advantage of conforming to the heat sink and package surfaces. This paper focuses on the application of thermal grease to Plastic Pin Grid Array (PPGA) packages and thermal modeling of the resulting interface between the PPGA package and the heat sink. An experiment was performed to obtain the "bulk" and "apparent" thermal conductivity of a thermal grease. It was observed that the apparent thermal conductivity is lower than the bulk thermal conductivity, particularly for a thin grease thickness. The thermal interface resistance can be modeled by using bulk thermal conductivity in series with the contact resistance. Alternatively, an apparent thermal conductivity which incorporates contact resistance can be used. A second experiment was performed to understand the impact of grease voiding on thermal performance. A corresponding finite element model was created using the apparent thermal conductivity calculated from the first experiment. Both model and experiment showed that thermal performance is more sensitive to voiding at the package center than voiding at the package periphery.