Datasheet

MPC8241 Integrated Processor Hardware Specifications, Rev. 10

Freescale Semiconductor 49

System Design Information

7.7.1 Internal Package Conduction Resistance

For the PBGA, die-up, packaging technology, shown in Figure 28, the intrinsic conduction thermal

resistance paths are as follows:

• The die junction-to-case thermal resistance

• The die junction-to-ball thermal resistance

Figure 30 depicts the primary heat transfer path for a package with an attached heat sink mounted to a

printed-circuit board.

Figure 30. PBGA Package with Heat Sink Mounted to a Printed-Circuit Board

For this die-up, wire-bond PBGA package, heat generated on the active side of the chip is conducted

mainly through the mold cap, the heat sink attach material (or thermal interface material), and finally

through the heat sink where forced-air convection removes it.

7.7.2 Adhesives and Thermal Interface Materials

A thermal interface material should be used between the top of the mold cap and the bottom of the heat

sink minimizes thermal contact resistance. For applications that attach the heat sink by a spring clip

mechanism, Figure 31 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. Thermal grease significantly reduces the interface thermal resistance. That is, the bare joint offers

a thermal resistance approximately seven times greater than the thermal grease joint.

A spring clip attaches heat sinks to holes in the printed-circuit board (see Figure 28). Therefore, the

synthetic grease offers the best thermal performance, considering the low interface pressure. The selection

of any thermal interface material depends on factors such as thermal performance requirements,

manufacturability, service temperature, dielectric properties, and cost.

External Resistance

Internal Resistance

Radiation Convection

Heat Sink

Printed-Circuit Board

Thermal Interface Material

Package/Leads

Die Junction

Die/Package

(Note the internal versus external package resistance)