AMD AMD-K6-2/400 User Guide - Page 338

Preliminary Information AMD-K6™-2E+ Embedded Processor Data Sheet 23542A/0-September 2000 The thermal resistance of a heatsink is determined by the heat dissipation surface area, the material and shape of the heatsink, and the airflow volume across the heatsink. In general, the larger the surface area the lower the thermal resistance. The required thermal resistance of a heatsink (qSA) can be calculated using the following example: If: TC = 70°C TA = 45°C PMAX = 19.50W Then: qCA ˆ Ë Í T---P-C---M-----A--T--X--A--ÝÛ = ----2---5---•---C----19.50W = 1.28•C ‰ W Heat Dissipation Path Thermal grease is recommended as interface material because it provides the lowest thermal resistance (@ 0.20°C/W). The required thermal resistance (qSA) of the heatsink in this example is calculated as follows: qSA = qCA - qIF = 1.28 - 0.20 = 1.08°C/W Figure 115 illustrates the heat dissipation path of the processor. Due to the lower thermal resistance between the processor die junction and case, most of the heat generated by the processor is transferred from the top surface of the case. The small amount of heat generated from the bottom side of the processor where the processor socket blocks the convection can be safely ignored. Ambient Temperature Case temperature Thin Lid Figure 115. Processor Heat Dissipation Path 316 Thermal Design Chapter 17