Stress Characterization of Ceramic Substrates by Laser Speckle Photometry

Direct copper bonding (DCB) substrate consisting of an Al2O3-ceramic layer in between two thick copper layers is one of the most powerful ceramic substrates for electronic applications. During the manufacturing process or under the service loading, high thermal or mechanical stress would be occurred. Extreme stress concentration might increase and in consequence copper structure rip off the ceramic substrates by cracking and conchoidal fractures. To avoid the crack initiation and guarantee a good quality state of ceramic substrates, the residual stress condition should be monitored. In this paper, an optical non-destructive testing method - Laser Speckle Photometry (LSP) will be demonstrated to determine mechanical stresses in ceramic substrates. The static speckle pattern is generated on the sample surface by illumination of a laser source, and sample is loaded by a 3-point-bending device to introduce surface strain, which activates a time resolved speckle signal. By recording and analyzing the quasi static speckle pattern, the speckle signal can be related to applied external mechanical signal. The resulting measurement signal was calibrated by stresses, calculated via a finite-element-model FEM. The potential of LSP for non-destructive characterization and monitoring of stress condition will be shown in details.