| dc.description.abstract | Advanced electronic assemblies are expected to perform with greater reliability in mission-critical applications under harsh environmental conditions, including high G-shock loads (up to 25,000 g), thermo-mechanical stresses, and long-term exposure to elevated temperatures (up to 150°C) and humidity in various defense, aerospace, and automotive underhood applications. The interfaces at critical junctions Potting/PCB, Resin/Copper, Chip/Underfill, and EMC/Substrate) degrade much faster than the bulk materials. It is necessary to understand these critical interfaces to predict the failure mechanisms, develop mitigation strategies, and optimize material selection for long-term reliability. Through this understanding, this dissertation embarked on a holistic investigation into the degradation of interfacial properties at board-level and component-level assemblies, with particular emphasis on potting material, PCB resin, underfill, and EMC interfaces. In this regard, the cohesive zone parameters used in the finite element simulations are determined by experimental test techniques and predictive modeling to investigate the interfacial fracture behavior. These efforts have been made to assess interface reliability, besides making predictions of failure modes due to harsh mechanical and thermal conditions, thereby helping design reliable electronics. The investigation starts with high-temperature-aged potting/PCB interfaces at 100°C and 150°C from 30 to 360 days and is tested under dynamic four-point bend loading. Stress intensity factors (KI & KII) and energy release rates in a steady state were measured at different aging intervals. The results reveal the degradation in interfacial properties with time due to thermal exposure and an increased tendency for delamination at extended durations of harsh environment exposure. The extracted fracture toughness parameters were used to calculate cohesive zone parameters and validation through finite element modeling. Additionally, shock load orientations at 0°, 30°, and 60° were also investigated, bringing out the sensitivity of the drop orientation to the reliability of potted electronic assemblies. The developed predictive finite element model was validated using experimental data for which high-speed imaging and 3D-Digital Image Correlation (DIC) techniques were employed. This predictive model successfully captured the behavior of potting/PCB interfaces, emphasizing the importance of cohesive zone parameters in understanding damage propagation in potted assemblies. Pad cratering failures at the resin and copper layer of the PCBs due to multiple reflow conditions have been discussed at board–level interfaces. The research characterizes the evolution of bulk resin properties and resin-copper interfacial strength through multiple reflow cycles and their interaction in determining the susceptibility of pad cratering. The interfacial fracture toughness was measured using four-point bend tests for various resin-copper-glass fiber combinations. The results show that multiple reflows degrade bulk resin properties, accelerating the loss of resin-copper interface strength and increasing pad cratering. A predictive regression model to identify material combinations offering superior pad cratering resistance was developed to improve board-level reliability. In flip-chip ball grid array (FCBGA) packages used in automotive applications, the primary focus was the delamination in chip/UF and EMC/substrate interfaces. The thermo-mechanical loads in automotive underhood environments, such as engine control units, advanced driver assistance systems, and safety and critical systems, could degrade the FCBGA interfaces, which have not been studied widely yet. The research reported here has investigated the monotonic and fatigue behavior of chip/UF interfaces after aging at 100°C and 150°C using bi-material specimens aged up to 360 days. The interfacial fracture toughness and Paris law constants are determined to characterize the interfacial strength. Results show that the properties of the cohesive zone evolve with aging, which plays a vital role in crack growth rate and long-term reliability. The presented research follows up with a predictive finite element modeling of the FCBGA packages, the cohesive zone parameters developed from the experiments. To model the interfacial fracture behavior of the FCBGA under thermal cyclic loading (-40°C to 125°C), cohesive elements were used in critical interfaces such as chip/UF, substrate/underfill, and TIM/Cu. The SDEG parameters show that the TIM/Cu interfaces have the highest degradation among the other FCBGA interfaces. These simulations offer predictive insights into possible failure mechanisms occurring during long-term operations. This dissertation embodies significant contributions toward understanding the mechanisms of interfacial degradation and failure in complex electronic assemblies under harsh environments. These findings give critical insights into the evolution of interfacial properties under mechanical and thermal loads, bringing out the importance of cohesive zone modeling for accurately predicting failure behavior. The predictive models developed in this work would go a long way toward being useful to designers and manufacturers for optimizing material selection to ensure electronic component reliability in such a harsh environment. The practical impact of this research is extensive in defense, aerospace, and automotive applications, where long-term electronics reliability is crucial. Herein, a state-of-the-art advance is made in investigating the interfacial fracture toughness and its predictive modeling to enable the development of next-generation electronics for extreme environmental conditions. | en_US |
