Reflection by Embedded Copper Plane and Volumetric Heating in Laser Microvia Drilling

Abstract

A finite difference thermal model is developed to analyze volumetric heating during laser drilling. The substrates of interest are multilayered polymeric sheets containing an embedded copper plane, which are generally used for high-density microelectronics packaging, and the laser of interest is a CO2 laser of wavelength 9.3 μm. Both the incident laser beam propagating toward the embedded copper plane and the beam reflected upward by the copper plane contribute to the volumetric heating of the substrate, which is the main mechanism for material removal. When the polymer layer above the copper sheet thins down as drilling progresses, the exit of the laser beam from the polymer layer after reflection by the copper sheet and the heat conduction within copper increase the energy loss significantly, making the process energetically inefficient. Consequently, the material-removal capability of the process diminishes, and a very thin polymer layer is left as residue on top of the copper sheet. The thickness of the residue depends mainly on the laser pulse width as predicted by the model. A nanosecond pulsed laser is found to effectively reduce the residue thickness to about 0.1 μm. The reflected laser energy, however, is found to contribute to the drilling process when the thickness of the polymer is higher than the absorption depth.