Two-dimensional transient modeling of CO2 laser drilling of microvias in high density flip chip substrates

Thermal modeling is essential to understand the laser-materials interactions and to control laser drilling of blind micro holes through polymeric dielectrics in multilayer electronic substrates. In order to understand the profile of the drilling front irradiated with different laser beam profiles, a two-dimensional transient heat conduction model including vaporization parameters is constructed. The absorption length in the dielectric is also considered in this model. Therefore, the volumetric heating source criteria are applied in the model and the equations are solved analytically. The drilling speed, temperature distribution in the dielectric and the thickness of the residue, termed smear, along the microvia walls and at via bottom/copper pad interface are studied under different laser parameters. An overheated metastable state of material is found to exist in the workpiece. The overheating parameters are attained for various laser drilling parameters to control the thermal damage and to minimize smear residue on the micro hole wall/bottom diameter.