Author: Sukrut Phansalkar
Title: Mold process induced residual stress prediction using cure extent dependent viscoelastic behavior.
Date/time: July 11th at 10:00am EST
Location: 2162 DeWALT Conference Room, Glenn L. Martin Hall.
Committee members:
Professor Bongtae Han, Advisor & Chair
Professor Peter Sandborn
Professor Avik Dutt
Professor Michael Azarian
Professor Sung W. Lee, Dean’s Representative
Abstract:
Molding processes produce residual stresses in encapsulated components. They are combined with the stresses caused by the coefficient of thermal expansion (CTE) mismatch to dictate the final warpage at room and reflow temperatures, which must be controlled for fabrication of redistribution layer (RDL) as well as yield during assembly.
During molding process, EMC is continuously curing and the mechanical properties continue to evolve; more specifically, the equilibrium modulus and the relaxation modulus. The former defines behavior after complete relaxation while the latter describes the transient behavior. It is thus inevitable to measure cure-dependent viscoelastic properties of EMC to be able to determine mold induced residual stresses accurately. This is the motivation for this thesis.
In this thesis, a set of novel methodologies are developed and implemented to quantify a complete set of cure-dependent viscoelastic properties, including the fully cured properties. Firstly, an advanced numerical scheme has been developed to quantify cure kinetics of thermosets with both single and dual-reaction systems. Secondly, a unique methodology is proposed to measure the viscoelastic bulk modulus – of EMC using hydrostatic testing. The methodology is implemented with a unique test setup based on inert gas. The results of viscoelastic testing along with the master curves and temperature-dependent shift factors of fully-cured EMC are presented.
Thirdly, a novel test methodology utilizing monotonic testing has been developed to measure two sets of equilibrium moduli – and of EMC during curing. The main challenge for the measurements is that the equilibrium moduli could only be measured accurately only when the EMC has fully relaxed. The temperatures for complete relaxation typically occur above the glass transition temperature, , where the curing rate is also high. A special measurement procedure is developed, through which the EMC moduli above can be determined quickly at a near isocure state. Viscoelastic testing of partially-cured EMC is followed to determine the cure-dependent shift factors of EMC. The test durations have to be very long (several hours) and it is conducted below of the EMC where the reaction is slow (under diffusion-control)
Finally, a numerical scheme that can utilize the measured cure-dependent viscoelastic properties is developed. It is implemented to predict the residual stress evolution of molding packages during and after molding processes.