Intel X5365 Design Guide - Page 38

Thermal/Mechanical Reference Design 2.4.2 the heat source to the fins directly impact the thermal performance of the heatsink. In particular, the quality of the contact between the package IHS and the heatsink base has a higher impact on the overall thermal solution performance as processor cooling requirements become strict. Thermal interface material (TIM) is used to fill in the gap between the IHS and the bottom surface of the heatsink, and thereby improves the overall performance of the thermal stackup (IHS-TIM-Heatsink). With extremely poor heatsink interface flatness or roughness, TIM may not adequately fill the gap. The TIM thermal performance depends on its thermal conductivity as well as the pressure load applied to it. Refer to Section 2.4.2 for further information on the TIM between the IHS and the heatsink base. • The heat transfer conditions on the surface on which heat transfer takes place - Convective heat transfer occurs between the airflow and the surface exposed to the flow. It is characterized by the local ambient temperature of the air, TLA, and the local air velocity over the surface. The higher the air velocity over the surface, the resulting cooling is more efficient. The nature of the airflow can also enhance heat transfer via convection. Turbulent flow can provide improvement over laminar flow. In the case of a heatsink, the surface exposed to the flow includes the fin faces and the heatsink base. An active heatsink typically incorporates a fan that helps manage the airflow through the heatsink. Passive heatsink solutions require in-depth knowledge of the airflow in the chassis. Typically, passive heatsinks see slower air speed. Therefore, these heatsinks are typically larger (and heavier) than active heatsinks due to the increase in fin surface required to meet a required performance. As the heatsink fin density (the number of fins in a given cross-section) increases, the resistance to the airflow increases: it is more likely that the air will travel around the heatsink instead of through it, unless air bypass is carefully managed. Using air-ducting techniques to manage bypass area is an effective method for maximizing airflow through the heatsink fins. Thermal Interface Material TIM application between the processor IHS and the heatsink base is generally required to improve thermal conduction from the IHS to the heatsink. Many thermal interface materials can be pre-applied to the heatsink base prior to shipment from the heatsink supplier and allow direct heatsink attach, without the need for a separate TIM dispense or attach process in the final assembly factory. All thermal interface materials should be sized and positioned on the heatsink base in a way that ensures the entire processor IHS area is covered. It is important to compensate for heatsink-to-processor attach positional alignment when selecting the proper TIM size. When pre-applied material is used, it is recommended to have a protective application tape over it. This tape must be removed prior to heatsink installation. The TIM performance is susceptible to degradation (i.e. grease breakdown) during the useful life of the processor due to the temperature cycling phenomena. For this reason, the measured TCASE value of a given processor can decrease over time depending on the type of TIM material. Refer to Section 2.4.7.2 for information on the TIM used in the Intel reference heatsink solution. 38 Quad-Core Intel® Xeon® Processor 5300 Series Thermal/Mechanical Design Guidelines (TMDG)