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"
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.