Camber evolution and stress development during cofiring of dielectric and ferrite bilayer laminates

Camber evolution and stress development of low-temperature cofirable dielectric and ferrite laminates were investigated as a function of the heating rate and the thickness ratio of two layers. Isotropic uniaxial viscosities of individual layers were determined through a vertical sintering technique by in-situ measuring shrinkage curves via an optical dilatometer. Sintering mismatch stresses as a result of the densification rate difference were calculated by using the as-measured curvature of bilayers. A good agreement was achieved in the evolution laws of experimentally measured and theoretically predicted cambers and stresses. However, quantitatively analyzed data indicated obvious differences between experimental results and theoretical predictions, especially where high heating rates and small thickness ratios of dielectric to ferrite layers were adopted. This discrepancy was ascribed to the porosity gradients and preferred pore orientations in the cofired layers as a result of the existing mismatched stresses.