Development of Thermal Solution for High Power Flip Chip CPU Package

摘要

As the clock speed of central processing unit (CPU) becomes faster, the heat generated from CPU device is increased significantly. Therefore, the thermal solution of the flip chip (FC) CPU package is one of the important technical issues. The FC CPU package mounted on the board in a system requires an additional external heat sink for better thermal dissipation. Therefore, most of the heat generated from the processor is dissipated through the topside of the FC CPU package. This paper focuses on the two thermal interfaces between chip backside and heat spreader (Lid) of ceramic PGA FC package, and between heat spreader and external heat sink. For better thermal dissipation, it is required that the thermal interface material (TIM) should have the high thermal conductivity, small voids in the TIM and at the interface, and good adhesion with the chip backside, heat spreader, and external heat sink. Other impacts on the thermal performance are the material type of the heat spreader and the design of the external heat sink. In addition, the thermal performance of the package should not be degraded after reliability tests such as temperature cycle, pressure cooker test, and high temperature storage. In general, the thermal conductivity of thermal grease is around 1 W/mK. However, such thermal grease can't give a thermal solution for P_d max. of 80W, Θjc (junction to case) of 0.27℃/W. Therefore, the advanced thermal grease of which thermal conductivity is more than 3.0 W/mK was used for developing of a thermal solution for Pd max. of 80W. Moreover, thermal epoxy having thermal conductivity of 18W/mK was also evaluated to guarantee P_d max. of 100W and Θjc of 0.20℃/W. Consequently, this paper illustrated that the thermal performance of FC CPU package with a heat spreader is strongly influenced by the defect of the TIM such as voids, poor adhesion, and what the thermal solution for P_d max. of 80W is. This paper also included the experiments and simulations on the different TIM, heat spreader, and external heat sink design.