Board-Level Assembly Challenges and Tim Selections for the Large-Size Fcbga Packages

In recent years, high-power integrated chips have advanced rapidly toward higher density, greater integration, and multifunctionality. However, these advancements have introduced significant challenges in thermal engineering, particularly in the selection of thermal interface materials (TIM) and the reliability design of board-level components (PCB, chips, and heat sink modules). Vertically aligned graphene, renowned for its ultrahigh in-plane thermal conductivity and interfacial heat transfer efficiency, represents an ideal material for fabricating high-performance TIMs. Nonetheless, the mismatch in coefficients of thermal expansion (CTE) exacerbates warpage in large flip-chip ball grid array (FCBGA) chips. This study introduces a large-scale lidless thermal test vehicle (TTV) with a maximum heat flux of $2 ~\mathrm{W} / \text{mm}^{2}$ and a temperature measurement accuracy of $0.1^{\circ} \mathrm{C}$. Employing this system, a controlled air-cooled environment was established to evaluate the thermal dissipation and resilience performance of various TIMs. At 860 W power, TIM-B and TIM-C sustained chip temperatures of $85.7^{\circ} \mathrm{C}$ and $84.4^{\circ} \mathrm{C}$, respectively. While TIM-C demonstrated superior thermal conductivity, its suboptimal resilience resulted in uneven temperature distribution. This finding underscores the necessity of balancing thermal conductivity and resilience for optimal TIM performance. Measurements of chip warpage before and after PCB assembly revealed that the bond line thickness (BLT) of large FCBGA chips could reach up to $150 \mu ~\mathrm{m}$. Therefore, the use of traditional carbon-based thermal conductive materials such as silicone grease, phase change materials, has been unable to meet the system cooling requirements under the BLT parameters. Furthermore, Drop reliability tests identified potential damage risks at chip edges, highlighting the importance of buffering protection adjacent to the chip. Compared with silicone grease materials, carbon-based materials have a certain thickness and elasticity, which can effectively reduce the possibility of damage to bare die chips in extreme scenarios such as falling.