Multi-scale X-ray Tomography of Solder Interconnects in Microelectronics

Advanced packaging, including 3D IC integration, is one of the main drivers in packaging and system integration to meet the requirements for miniaturized smart systems with high functionality and high performance. For 3D stacking of wafers or dies, interconnections like micro solder bumps and Cu pillars are used. Figure 1 (left) shows a stack with a TSV interposer structure [1]. 3D-stacked products and advanced packaging challenge materials and process characterization. The control of the micro-bump quality is a particular issue. Special tasks are the characterization of the geometry of the solder bumps to estimate the stress enhancement risks, the nondestructive imaging of micron-size pores and of intermetallic phases as well as the visualization of cracks. Several NDE techniques for metrology and failure analysis are currently under discussion. In this paper, the potential and the limits of micro XCT and nano XCT for NDE of solder interconnects are described. Strategies for nondestructive evaluation of geometry, materials and defects are discussed. It is shown that multi-scale imaging with several resolution ranges is one potential approach. Micro XCT (resolution about 1 m) and nano XCT (resolution about 50 nm) are very useful lab-based techniques with a promising prospect for the future. We demonstrate the capabilities for nondestructive imaging of multi-die stacks with TSVs and micro solder bumps. Figure 1 (middle and right) right demonstrates a micro XCT overview and a nano XCT ROI study of such a multi-die stack with solder interconnects. An analysis of individual solder bumps reveals mismatches in relative positioning, variability in the shape, micron-size pores, and the distribution of intermetallic phases. This information is important to evaluate the respective process steps (process control) and the product reliability (quality control). Since deviations from the targeted geometry and defects are difficult to locate precisely from a two-dimensional image, X-ray computed tomography has to be applied