Effect of Cu conductor properties on crack generation around through holes of Cu/glass ceramic co-fired substrate

We investigated crack generation and methods for preventing it in glass ceramics when Cu-glass paste and glass ceramics were co-fired. Cu-glass paste was used to fill the through holes of a glass-ceramic green sheet consisting of borosilicate glass/Al2O3/cordierite by screen-printing. Cracks are generated in the glass-ceramic portion that surrounds the through holes parallel to the circumference. These cracks occur when the tensile stress generated at the interface between the conductor in the through hole and the glass ceramic as a result of a mismatch in the coefficients of thermal expansion (CTEs) between Cu and glass ceramic exceeds the mechanical strength of the glass ceramic. The magnitude of the tensile stress was calculated by performing stress analysis using a model based on a thick multilayer cylinder. These cracks can be prevented by either greatly reducing the CTE of Cu conductor by adding a large amount glass or by reducing the Young's modulus of the Cu conductor and without adding additives to the Cu paste for promoting sintering (e.g., glass), which increase the density of the Cu conductor.