Application of Laser Speckle Photometry for Strain Characterization in Ceramics

Direct bonded copper (DBC) substrate is the base material and the key component for power electronic modules because of its excellent thermal performance and high current loading ability. However, in the manufacturing process, thermal stress occurs at the interface between copper and ceramic layers due to their different thermal expansion coefficients. Local stress concentration will lead to crack initiations in the ceramic, which will finally result in the functional failure of the substrate. To guarantee a good quality of DBC substrate, a sensor concept based on laser speckle photometry (LSP) is developed, showing its potential to determine local strain and stress in ceramics. LSP is a nondestructive and noncontact technique to characterize surface conditions by detecting and analyzing speckle patterns. The presented work aims at establishing correlation models between speckle parameters and mechanical strains in ceramics (Al2O3, LTCC). For this purpose, strains were introduced into ceramic samples by a three-point bending process, and the corresponding speckle patterns were recorded and analyzed simultaneously using LSP technique. Both simulation and experimental results show that surface strains in ceramics can be characterized by using static and quasi-static LSP parameters.