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Cyrix 6x86 Thermal Design Considerations Revision 1.1, 12/17/96 Introduction This Application Report serves as a guide in the thermal design of a lpersonal computer using the Cyrix® 6x86TM Microprocessor. A simplified thermal model is presented that utilizes thermal resistances to describe the heat flow from the CPU. Two case studies are included to show how to measure the thermal performance of the microprocessor in a typical computer enclosure. Additional examples illustrate the calculation of expected maximum case and ambient temperatures. The D.C. Specifications and thermal data in the 6x86 Microprocessor Data Book are expanded and updated by the Appendix in this Application Report. Heat Flow The 6x86 CPU dissipates as much as 25 watts of power depending on the CPU clock frequency. The CPU is mounted up-side-down in a PGA package (Figure 1). Most of the heat is concentrated at the surface of the semiconductor chip and is passed to the package through three main paths: (1) through the bulk of the silicon chip to where the chip is mounted to the package, (2) through the bond wires to the package, (3) through radiation across the void between the chip and the bottom of the package. COPPER HEAT SPREADER SILICON CHIP CERAMIC (NOT TO SCALE) BOND WIRES Figure 1. 6x86 PGA Package Cross-Sectional View Cyrix Application Report 1 The package is cooled by radiation, convection and conduction. Some heat is conducted through the pins and the socket, but most of the heat passes from the package into the flowing air stream that carries the heat out of the equipment enclosure. The transfer of heat from the package to the ambient air can be greatly enhanced through the use of a heatsink. Our thermal model will concentrate on the heat flow from the case and heatsink to the surrounding air. Thermal Resistance Model As heat flows from a heat source to a cooler object, there is a temperature drop (T0 - T1) which is similar to the voltage drop (E) across an electrical resistor. Electrical power dissipated in the chip (P) generates heat. The heat flows away from the source analogous to electrical current (I). By dividing the temperature drop (T0 - T1) by the power producing the heat (P), we obtain thermal resistance () expressed in Celsius degrees (°C) per watt (W). (T0 - T1) = ----------P °C/W E ( similar to: R = ---- ) I 2 Cyrix Application Report Thermal Resistances Three thermal resistances (Figure 2) can be used to idealize the heat flow from the case of the 6x86 CPU to ambient: CS = thermal resistance from case to heatsink in °C/W, SA = thermal resistance from heatsink to ambient in °C/W, CA = CS + SA, thermal resistance from case to ambient in °C/W. Additional symbols are used for the temperatures of the, case, heatsink and ambient air: TC = case temperature (top dead center) in °C, TS = heatsink in °C, TA = ambient (free air) temperature in °C. The power applied to the semiconductor is P = power applied, VCC * ICC in watts (W). TC Case CS T