The Glass Core with an Anti-crack Propagation Structure

Abstract

The trend of the physical properties of core layer in the PWB (Printed wiring board) is the low CTE (Coefficient of Thermal Expansion) and high modulus reducing the CTE mismatching between semiconductor chip and PWB to reduce the warpage of thin package. To jump up the physical property of the general organic material, thin glass less than 100 micron thickness is evaluated. In order to get the good physical properties, thin glass with the low CTE (3.2 ppm/°C) and high modulus (75GPa) is used for core material. And it is also advantageous in fine via pitch than with a high modulus metal core. But thin glass itself requires special handling to avoid cracks in the process. In order to handling without glass crack, the crack prevention structure was applied and the special insulation material was also laminated on the glass. The insulation layer having the glass fabric is laminated on the both side of thin glass to prevent crack propagation. And this material had no issue in the general PWB production line without any special treatment or Jig for the thin glass. Also the physical properties like CTE and modulus is outstanding. In the comparison of the low CTE organic core and glass core, physical properties of glass core was excellent than that of the organic core. In the final product, unit edge part exposing the thin glass is the weak point. Glass chipping or crack may occur during unit sawing process. But the crack on the sawing cut edge of glass can cause the horizontal crack issue of the glass. After reflow or thermal stress, the glass edge horizontal crack had propagated to the inside of the unit. So some special glass cutting line was added to block the crack propagation in the glass around the unit edge. The glass cutting line has been effective to block horizontal crack propagation into the inside of unit. With this structure and the insulation material, most of reliability test items that are applied in common circuit board were passed through.