Development of a non-destructive inspection system for micro bump joints

The performance of electronic devices has been improved by increasing packaging density using advanced packaging technologies. Typical highly densified packages are SiP (System in a Package) and MCM (Multi Chip Module). These package and module structures include multiple LSI chips in them. Though conventional LSI or memory chips have been planarly aligned on a substrate, 3D-stacked chip structures have started to be used in actual products to minimize signal delay and to maximize assembly density. However, the structural complexity and difficulty of assembly process of 3D-stacked structures give rise to cause the decrease of production yield. Thus, the production cost has to be minimized for realizing such 3D-stacked structures. Therefore, a nondestructive evaluation system for detecting delaminations between a chip and micro bumps in 3D-stacked structures is indispensable for highly reliable and low-cost manufacturing. In these stacked structures, it is hard to inspect the adhesion condition of metallic bumps that connect a lower chip with an upper chip because most of bump interconnections are invisible. We have, therefore, proposed a new nondestructive evaluation method for detecting delamination between a chip and metallic bumps by measuring the local surface deformation of chips. We have already validated that the local deformation of thinned chips significantly changes by defects of bump interconnections, such as lack and delamination of bumps. In this research, we fabricated flip chip samples with intentionally delaminated bumps, and measured the local deformation of the Si chips for validating the possibility of detection of thin delamination by our inspection method. Additionally, to improve the accuracy of this evaluation method, noise factors that affect local surface deformation of a chip was discussed and the inspection algorithm for eliminating these noise factors was developed. By applying this inspection algorithm that consists of the acquisition of regular distribution of the surface deformation, numerical processing for eliminating the effect of a 3D-stacked structure, the noise reduction processing of frequency-dependent and amplitude-dependent noises, and the comparison between the reference and measured deformation, it is possible to detect open failures of bump interconnections in 3D-stacked structures during assemble process precisely. Finally, in order to validate the effectivity of this inspection method, we manufactured area-arrayed 3D-stacked sample chips and measured the local deformation at surface of sample chips. A laser measurement technique was used for high-speed inspection. Based on the measured change of the amplitude and period of the local deformation at a position of intentionally introduced delaminated bumps, 3 mum-thick delamination was detected precisely. The inspection speed reached about 5 minutes per a chip with 400 bumps. Therefore, we have established a nondestructive inspection system for micro bump interconnections in 3D-stacked structures.