MPC7410 RISC Microprocessor Hardware Specifications, Rev. 6.1
44
Freescale Semiconductor
System Design Information
8.8.1 Internal Package Conduction Resistance
For the exposed-die packaging technology, shown in Table 3, the intrinsic conduction thermal resistance paths are
as follows:
The die junction-to-case (or top-of-die for exposed silicon) thermal resistance
The die junction-to-ball thermal resistance
Figure 28 depicts the primary heat transfer path for a package with an attached heat sink mounted to a printed-circuit
board.
Heat generated on the active side of the chip is conducted through the silicon, then through the heat sink attach
material (or thermal interface material), and finally to the heat sink where it is removed by forced-air convection.
Since the silicon thermal resistance is quite small, for a first-order analysis, the temperature drop in the silicon may
be neglected. Thus, the heat sink attach material and the heat sink conduction/convective thermal resistances are the
dominant terms.
Figure 28. C4 Package with Heat Sink Mounted to a Printed-Circuit Board
8.8.2 Adhesives and Thermal Interface Materials
A thermal interface material is recommended at the package lid-to-heat sink interface to minimize the thermal
contact resistance. For those applications where the heat sink is attached by spring clip mechanism,
Figure 29 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. That is, the bare joint results in a thermal resistance approximately seven 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 26).
This spring force should not exceed 5.5 pounds of force. Therefore, the synthetic grease offers the best thermal
performance, considering the low interface pressure. 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.
External Resistance
Internal Resistance
Note the internal versus external package resistance.
Radiation
Convection
Radiation
Convection
Heat Sink
Printed-Circuit Board
Thermal Interface Material
Package/Leads
Die Junction
Die/Package