Modeling, design, fabrication and reliability characterization of ultra-thin glass BGA package-to-board interconnections
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Recent trends to miniaturized systems such as smartphones and wearables, as well as the rise of autonomous vehicles relying on all-electric and smart in-car systems, have brought unprecedented needs for superior performance, functionality, and cost requirements. Transistor scaling alone cannot meet these metrics unless the remaining system components such as substrates and interconnections are scaled down to bridge the gap between transistor and system scaling. In this regard, 3D glass system packages have emerged as a promising alternative due to their ultra-short system interconnection lengths, higher component densities and system reliability enabled by the tailorable coefficient of thermal expansion (CTE), high dimensional stability and surface smoothness, outstanding electrical properties and low-cost panel-level processability of glass. The research objectives are to demonstrate board-level reliability of large, thin, glass packages directly mounted on PCB with conventional BGAs at pitches of 400µm SMT and smaller. Two key innovations are introduced to accomplish the objectives: a.) Reworkable circumferential polymer collars providing strain-relief at critical high stress concentration areas in the solder joints, b.) novel Mn-doped SACMTM solder to provide superior drop test performance without degrading thermomechanical reliability. Modeling, package and board design, fabrication and reliability characterization were carried out to demonstrate reliable board-level interconnections of large, ultra-thin glass packages. Finite-element modeling (FEM) was used to investigate the effectiveness of circumferential polymer collars as a strain-relief solution on fatigue performance. Experimental results with polymer collars indicated a 2X improvement in drop performance and 30% improvement in fatigue life. Failure analysis was performed using characterization techniques such as confocal surface acoustic microscopy (C-SAM), optical microscopy, X-ray imaging, and scanning electron microscopy/energy dispersive spectrometry (SEM/EDS). Model-to-experiment correlation was performed to validate the effectiveness of polymer collars as a strain-relief mechanism. Enhancement in board-level reliability performance with advances in solder materials based on Mn-doped SACMTM is demonstrated in the last part of the thesis.The studies, thus, demonstrate material, design and process innovations for package-to-board interconnection reliability with ultra-thin, large glass packages.