Quality Estimating Model Based on Temperature of Hole Bottom in Laser Drilling of Blind Holes

Abstract

The printed wiring board (PWB) has becomes relatively smaller due to the downsizing of electric devices. Higher densification has been advanced by the circuit formation of multi-layer PWBs in the current manufacturing of these boards. In current manufacturing of multi-layer printed wiring boards, a method frequently used is to laminate the core with insulating resin as build-up layers. Microvia drilling using laser technology has become the prevailing method of machining smaller blind via holes. Aramid fiber reinforced plastic (AFRP) is considered suitable material for the build-up layers, because it is efficient in laser drilling. However, heat damage in the hole has been a problem because the laser drilling cause a heat damage to the PWB materials. The poor hole quality, such as the carbonization of the resin and the peel around circuit copper foil/core material, causes decrease in the reliability of circuit connections. In the present report, first, we took multi layer PWB-reinforced by aramid unwoven cloth, and measured the temperature distribution of the circuit copper foil during laser drilling using a thermocouple. Second, we proposed a heat input prediction model using a finite element method (FEM), considering the change of laser absorption of the circuit copper foil surface. Finally, we carried out a thermal stress analysis based on the temperature distribution, and confirmed the efficiency of this analysis. As a result, the calculated temperatures by this model, considering the variation of the absorption of foil surface during laser drilling, are in good agreement with the experimental temperatures. It is confirmed that this model is effective in estimating the temperatures and thermal stresses in the bottom copper foil during laser drilling of the build-up layers.