Two-laser deflashing of heat sinks in integrated circuit packages

During the plastic packaging process, mold flash is usually formed on a heat sink. In order to keep efficient heat transfer from the integrated circuit device to the exterior environment, the mold flash must be removed from the heat sink surface. In this paper, we demonstrate that a Nd:YAG laser deflashing is only available to remove thin flash of a few micrometers from heat sinks and cannot remove thick flash without damage to the heat sink, both from experimental and numerical analysis. Based on optical and thermal properties of heat sink and mold flash materials, we develop a two-laser deflashing technique for thick flash removal. A CO2 laser is used to remove the top layer of thick flash followed by Nd:YAG laser at low laser fluence and pulse number, which is used to remove the remaining thin flash without damaging the heat sink.