Study on the curing process of no-flow and wafer level underfill for flip-chip applications

Flip-chip is a first-level interconnect technology to connect the IC chip to the substrate. Underfill is an adhesion layer in between the IC chip and the substrate to enhance the reliability of the flip-chip package. The inventions of no-flow underfill and wafer level underfill increase the productivity and decrease the cost of flip-chip manufacturing. In order to fundamentally understand the curing process of the underfill in these two flip-chip processes, the study on the curing process of no-flow and wafer level underfill is proposed. Curing kinetic models are developed to describe the curing behavior of the underfills of different resin chemistry and reaction mechanisms. Based on the kinetic modeling, the degree of cure (DOC) of the underfill during the solder reflow process can be predicted. A 3-D transient thermal analysis model is built in ANSYS to understand the effect of underfill curing on the temperature distribution of a flip-chip package during the solder reflow process. The gelation behavior of different underfill materials is studied through experiments as well as Monte Carlo Simulations. The gelation of the underfill during the solder reflow process is predicted and confirmed by experiments. The B-stage properties of the wafer level underfill are studied and optimized. The assembly of the flip-chip with the wafer level underfill is demonstrated. The wafer warpage introduced by the underfill curing is modeled using finite element analysis and measured experimentally. The post cure behavior of the no-flow and wafer level underfill is investigated. The material properties of the underfill under different post cure conditions are monitored. In order to meet the reliability requirement for flip-chip package, inorganic fillers are indispensable to achieve low coefficient of thermal expansion and high modulus. Material and process innovations are explored to incorporate silica fillers into no-flow and wafer level underfill.