Ph.D. Proposal Defense by Sakethraman Mahalingam
Monday, July 26, 2004

(Dr. Suresh K. Sitaraman, Chair)

"Reliability of Flip Chip Packages Using Nano-Filled Underfill Materials"

Abstract

Solder joint reliability in flip chip on organic substrates (FCOB) is enhanced through underfill application. Traditionally, underfill materials are resins filled with ?m sized filler particles. The filler content in the underfill material is often tailored to achieve suitable mechanical properties such as Young’s modulus and Coefficient of Thermal Expansion (CTE). Traditional underfilling process involves dispensing the underfill, letting the underfill flow through the gap between the die and the substrate through time-consuming capilliary flow, and curing the underfill. No-flow underfill, a recent development in the electronic packaging industry, is able to cure during solder reflow process and thus eliminates the separate, time-consuming, and costly process of post-reflow underfill dispensing and curing. However, no-flow underfills cannot be filled with ?m sized particles as the filler particles interfere with the soldering process. A viable alternative to this problem is the use of nano-sized filler particles that do not interfere with the soldering process and yet can be used to give a wide range of mechanical properties. Nano-filled underfills (NFU) are optically transparent and therefore are excellent candidates for the wafer level underfilling process as well.

The objective of this work is to study the thermo-mechanical reliability of NFU in microelectronic packages through experimental reliability testing and theoretical modeling. As part of the theoretical modeling, the delamination of the underfill from the die surface under monotonic and fatigue loading will be investigated. Two approaches will be employed to study such an interfacial fracture problem: 1) Conventional fracture mechanics and energy release rate and 2) Cohesive zone modeling. In parallel to the predictive models, experiments will be conducted to characterize the NFU-die interfacial fracture toughness over a wide range of mode mixity. Both montonic and fatigue-based interfacial delamination propagation will be investigated. The theoretical models and experimental characterization data will be applied to flip-chip on organic board test vehicle assemblies underfilled with NFU. The test vehicles will be thermal cycled between -55 C to 125 C, and the reliability of the test vehicles will be assessed. Such experimental data will be used to validate the predictions from the numerical models that take into consideration temperature- and direction-dependent material properties. Based on the models and experiments, design guidelines for reliable FCOB assemblies with NFU will be developed.