SUBJECT: Ph.D. Proposal Presentation
BY: Scott McCann
TIME: Wednesday, September 7, 2016, 12:00 p.m.
PLACE: MARC Building, 201
TITLE: Design for Mechanical Reliability of Redistribution Layers for Ultra-thin 2.5D Glass Packages
COMMITTEE: Dr. Suresh K. Sitaraman, Chair (ME)
Dr. I. Charles Ume (ME)
Dr. Shuman Xia (ME)
Dr. Rao R. Tummala (MSE)
Dr. Venkatesh Sundaram (ECE)


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 several reasons, however, glass packages do have challenges, such as debonding of copper redistribution layers (RDL) from the smooth glass surface, exacerbated warpage at ultra-thin and large substrate dimensions, and glass cracking due to dicing-induced defects and RDL stresses. To address these challenges, there is a need to understand plasticity effects on thin film adhesion, multiscale effects on mechanics of deformation, 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 the proposed research are to understand the fundamental factors that contribute toward the cracking of glass, debonding of RDL, and warpage of ultra-thin glass substrates, to design and demonstrate thermo-mechanically reliable 2.5D glass packages, and to develop design and process guidelines for such reliable glass packages. In particular, the proposed work studies delamination and warpage, dicing induced defects from which cracks can originate, and RDL stresses that can cause cracks to propagate. 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. Warpage is predicted using sequential finite-element modeling that mimics the fabrication process, and the warpage predictions are validated through shadow moiré measurements. Various dicing methods and the associated dicing defects are comprehensively quantified, and are used to reduce the chance for glass cracking. Stresses in glass caused by RDL are measured through birefringence and are correlated to modeling. Based on the findings of this work, test vehicles are designed and their reliability is demonstrated through 1000 thermal cycles. 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.