Design for mechanical reliability of redistribution layers for ultra-thin 2.5D glass packages

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McCann, Scott
Sitaraman, Suresh K.
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Packaging for high-performance computing requires multiple logic and memory dies assembled on a single substrate. Such a 2.5D package demands a large (≥35x35mm) and ultra-thin (≤100μm) substrate with asymmetric build-up, high density wiring, and ultra-fine pitch interconnects (≤35μm). Glass is an ideal substrate material for such packages due to its excellent electrical properties, tailorable coefficient of thermal expansion (CTE), high mechanical rigidity, availability in large and thin panel form, and smooth surface for fine line fabrication. However, glass packages do have challenges, such as glass cracking due to dicing-induced defects and RDL stresses as well as debonding of copper redistribution layers (RDL) from the smooth glass surface. To address these challenges, there is a need to understand plasticity effects on thin film adhesion, the role of dicing defects on glass cracking, and process-induced stresses due to RDL. However, the existing literature does not adequately address several of these. The objectives of this research are to understand the fundamental factors that contribute toward the cracking of glass and debonding of RDL, to design and demonstrate thermo-mechanically reliable 2.5D glass packages, and to develop design and process guidelines for such reliable glass packages. This work studies how RDL stresses propagate dicing-induced defects into cohesive cracks as well as interfacial delamination, how geometry and process modifications could mitigate such failures, demonstrates prototypes that are reliable through processing and thermal cycling, and develops design guidelines for current and future glass packages. As part of experimental validation, stresses in glass caused by RDL are measured through birefringence and are correlated to modeling. Warpage is predicted using sequential finite-element modeling that mimics the fabrication process, and shadow moiré measurements are used to validate the package warpage and thus, the model predictions. Various dicing methods and the associated dicing defects are comprehensively quantified, and are used to reduce the chance for glass cracking. Based on the findings of this work, test vehicles are designed and their reliability is demonstrated through 1000 thermal cycles. To enable a wider design space, three alternative solutions to glass cracking, edge coating, two-step dicing, and laser dicing, are proposed, analyzed, and demonstrated. An innovative method to determine the critical energy release rate for peeling of a copper thin film from a glass substrate is developed, and the developed technique is employed to enhance adhesion of copper wiring. In addition, general design and process guidelines for mechanical reliability, which are applicable to other packaging applications, such as mobile substrates, filters for RF, and power, are developed.
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