Thermal
External Resistance
Internal Resistance
Radiation
Convection
Heat Sink
Thermal Interface Material
Die/Package
Die Junction
Package/Leads
Printed-Circuit Board
Radiation
Convection
External Resistance
(Note the internal versus external package resistance)
Figure 46. Package with Heat Sink Mounted to a Printed-Circuit Board
The heat sink removes most of the heat from the device. Heat generated on the active side of the chip is
conducted through the silicon and through the lid, then through the heat sink attach material (or thermal
interface material), and finally to the heat sink. The junction-to-case thermal resistance is low enough that
the heat sink attach material and heat sink thermal resistance are the dominant terms.
16.2.3 Thermal Interface Materials
A thermal interface material is required at the package-to-heat sink interface to minimize the thermal
contact resistance. For those applications where the heat sink is attached by spring clip mechanism,
Figure 47 shows the thermal performance of three thin-sheet thermal-interface 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. The bare joint results in a
thermal resistance approximately six times greater than the thermal grease joint.
Heat sinks are attached to the package by means of a spring clip to holes in the printed-circuit board (see
Figure 44). Therefore, the synthetic grease offers the best thermal performance, especially at the low
interface pressure.
When removing the heat sink for re-work, it is preferable to slide the heat sink off slowly until the thermal
interface material loses its grip. If the support fixture around the package prevents sliding off the heat sink,
the heat sink should be slowly removed. Heating the heat sink to 40-50•C with an air gun can soften the
interface material and make the removal easier. The use of an adhesive for heat sink attach is not
recommended.
MPC8540 Integrated Processor Hardware Specifications, Rev. 4
Freescale Semiconductor
83
FREESCALE [ Freescale ]
