TIM Coverage Evaluation for High Performance Computing Application

Abstract

High performance computing (HPC) system are improving computing performance to meet ever-increasing challenge continuously, such as high-end central processing unit (CPU) and server. Regarding this requirement, the high thermal conductivity (High-K) thermal interface material (TIM) which between die top surface and heat spreader in EHS-FCBGA is developed to solve the overheating problem. Although there have been many research on materials with high-K TIM in the recent years, the TIM coverage is also one of the most influential factors in high power requirement. At the beginning of the product life (T0), the silicone-based TIM coverage is usually maintained above 95%, but it always occurs the void or delamination issue after reliability test such as TCT and HTSL test which in turn affects the actual application. In summary, the optimization of TIM coverage evaluation after reliability test is inevitable.For system level cooling, the bulk of the heat is normally dissipated through the top of the package. However, thermal resistance junction-to-case (Theta JC) was usually used to judge the thermal performance of upward heat dissipation for package level, which is not enough to explain the overall thermal behavior from system level. In view of that, the maximum junction temperature (Tj) which contain overall thermal characterization is used to define the limitation of TIM coverage in this research.In this study, we present a method which can define the TIM coverage limitation with thermal design power (TDP) up to 100~500W for EHS-FCBGA in different package size and die/package ratio under air cooling system. The influence of the TIM coverage will be more sensitive since the die/package ratio is lower. In conclusion, Tj will be affected by the external cooling system extremely. Theta JC cannot indicate the overall thermal behavior of the entire system, nor can it be used to define the TIM coverage. While defining the TIM coverage, the quality of the external cooling system should be considered at the same time.