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 non-destructive and non-contact 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.