Graduation Semester and Year

Spring 2024



Document Type


Degree Name

Doctor of Philosophy in Mechanical Engineering


Mechanical and Aerospace Engineering

First Advisor

Dr. Dereje Agonafer


Bump-less direct Cu-Cu hybrid bonding interconnection technology helps achieve high-density and fine-pitch applications such as high-performance computing. It provides much lower electrical resistivity and lower electromigration compared to C4 (controlled collapse chip connection) solder bumped flip assembly and C2 (chip connection, micro-bump, or Cu-pillar with solder cap) bumped flip chip assembly. In this study, multi-level sub-modeling approach is utilized to study the bump-less Cu-Cu bonded interconnect reliability of the 3D TSV package during Cu-Cu thermal compression bonding (TCB) process. Most direct Cu-Cu TCBs requires diffusion of Cu atoms across the interface to form monolithic copper at high temperature (350 – 400°C). Fracture mechanics parameters was calculated at Cu-Cu bonded region and Si/TSV region under TCB thermal loads. Further, multivariable design optimization is carried out to improve the reliability of advanced interconnects.

Detailed study of material compatibility of the various electronics packaging materials in immersion cooling environment is essential to understand their failure modes and reliability. The modulus and thermal expansion are critical material properties for electronics mechanical design. PCBs and substrates are a critical component of electronic package and heavily influences failure mechanism and reliability of electronics both at the package and board levels. Three different types of PCBs and four different types of the substrates with different glass transition temperature (Tg) was immersed in single-phase dielectric fluids. The first part of the study focuses on signal substrate material. Impact of thermal aging in air and di-electric fluid on the thermo-mechanical properties is investigated. The test samples are immersed in ElectroCool EC100 dielectric fluid, and air for 720 hours at three different temperatures: 25°C, 50°C, and 75°C. Dynamic Mechanical Analyzer (DMA) was used to measure storage and loss modulus to calculate complex modulus for both the immersed and non-immersed samples and compared to assess the impact of immersion and thermal aging. The second part of the study focuses on the effect of the material’s glass transition temperature (Tg) on the immersed samples and its impact on the thermo-mechanical properties. Three different types of PCB samples were immersed in mineral oil for 720 hours and thermally aged at different temperatures. Similarly, three different types of substrates were immersed in two different types of di-electric fluid for 720 hours and thermally aged at 85°C. The complex modulus is characterized for all the PCBs and substrates samples and compared with the non-immersed and non-thermally aged samples.

3D numerical study aimed to optimize the geometric configuration of the manifold microchannel heatsink and compare the heat transfer enhancement with linear microchannel for fixed volume ranging from 0.7 mm3 to 0.9 mm3. Three-dimensional numerical study of steady, laminar, incompressible flow for forced convection was used for the computational fluid dynamics with a goal driven optimization, adhering to global constraints. Results of the numerical investigation shows that for all fixed volume, manifold microchannel heatsink gave the best results for minimization of peak temperature.


Cu-Cu Direct hybird bonding, TSV, immersion cooling, Thermo-mechanical properties, Manifold microchannel heatsink


Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

Available for download on Thursday, May 28, 2026