mpc8640 Freescale Semiconductor, Inc, mpc8640 Datasheet - Page 111

• Manufacturer Part Number: mpc8640
• Description: Integrated Host Processor
• Manufacturer: Freescale Semiconductor, Inc

Heat generated on the active side of the chip is conducted through the silicon, then the heat sink attach
material (or thermal interface material), and finally to the heat sink where it is removed by forced-air
convection.
Because the silicon thermal resistance is quite small, the temperature drop in the silicon may be neglected
for a first-order analysis. Thus the thermal interface material and the heat sink conduction/convective
thermal resistances are the dominant terms.
19.2.2
A thermal interface material is recommended at the package-to-heat sink interface to minimize the thermal
contact resistance.
materials (silicone, graphite/oil, floroether oil), a bare joint, and a joint with thermal grease as a function
of contact pressure. As shown, the performance of these thermal interface materials improves with
increasing contact pressure. The use of thermal grease significantly reduces the interface thermal
resistance. That is, the bare joint results in a thermal resistance approximately seven times greater than the
thermal grease joint.
Often, heat sinks are attached to the package by means of a spring clip to holes in the printed-circuit board
(see
interface pressure, and is recommended due to the high power dissipation of the MPC8640. Of course, the
selection of any thermal interface material depends on many factors—thermal performance requirements,
manufacturability, service temperature, dielectric properties, cost, and so on.
Freescale Semiconductor
Figure
59). Therefore, synthetic grease offers the best thermal performance, considering the low
Thermal Interface Materials
MPC8640 and MPC8640D Integrated Host Processor Hardware Specifications, Rev. 3
1.5
0.5
2
1
0
Figure 61. Thermal Performance of Select Thermal Interface Material
Figure 61
0
10
shows the thermal performance of three thin-sheet thermal-interface
20
Contact Pressure (psi)
30
40
50
Silicone Sheet (0.006 in.)
Bare Joint
Fluoroether Oil Sheet (0.007 in.)
Graphite/Oil Sheet (0.005 in.)
Synthetic Grease
60
70
80
Thermal
111