A laser speckle photometry based non-destructive method for measuring stress conditions in direct-copper-bonded ceramics for power electronic application
1. BACKGROUND Power electronic application come along with very high requirements in terms of reliability. Thermal loadings stress whole electronic packages while powering the semiconductor and by environmental temperature changes. Direct copper bonding (DCB) substrate are widely used in such electronic application due to its high thermal conductivity performance. They consists of copper layers sintered onto alumina ceramic sheet. Because of the very high process temperature thermal inducted stresses arise along the substrate while cooling to ambient temperatures. The DCB substrates carry residual stresses usually. Along the edges of copper structures the residual stresses are concentrated. Under service condition or even during the electronic manufacturing process the stress concentration might increase and in consequence copper structure rip off the substrate by cracking and conchoidal fractures in the ceramic sheet. To avoid the critical cracking situation the knowledge about the residual stress condition is required. The target will be a stress mapping across the DCB substrate. Within the paper on potential approach to measure the stress condition will be presented. / 2. METHODOLOGY The presented work demonstrate a laser-speckle-photometry (LSP) based method to determine mechanical stresses in a non-destructive testing approach. For this purpose, Al2O3-ceramic samples were systematically loaded by a 3-point-bending experiment in a dynamic mechanical analyser to induce certain stress condition. The mechanical test procedure was simultaneously observed by the LSP measurements setup. During the experiment a defined surface on the alumina ceramic region is illuminated by laser light. By reason of the optical rough surface of the sample a so called speckle pattern reflects, which is recorded by a CCD-Camera system. The speckle pattern depends on the sample surface condition and the mechanical strain condition. In the next step the recorded images of the pattern are evaluated by using different mathematical algorithms. Correlations between stress condition and measurement signal was observed and evaluated. This resulting measurement signal was then calibrated bystresses calculated via a finite-element-model of the 3-point-bended Al2O3-ceramic. / 3. RESULTS The authors observe an excellent accordance between the LSP measurement signal and the stresses introduced into the surface of the sample. Furthermore a study of influence was realised, to demonstrate the independence of the measurement signal from external effects for example ambient light, exposure time etc. These results enable to enhance the LSP-method for mapping stresses across processed assemblies in a production line in future.
Sudip, Shohag Roy