PC8245 Просмотр технического описания (PDF) - Atmel Corporation

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PC8245

Integrated Processor Family

Atmel Corporation 

Internal Package Conduction

Resistance

For the TBGA, cavity down, packaging technology, shown in Figure 6 , the intrinsic con-

duction thermal resistance paths are as follows:

• the die junction-to-case thermal resistance,

• the die junction-to-ball thermal resistance.

Figure 6 depicts the primary heat transfer path for a package with an attached heat sink

mounted to a printed-circuit board.

Figure 6. TBGA Package with Heat Sink Mounted to a Printed-Circuit Board

External Re sistance

Radiation Convection

Heat Sink

Internal Resistance

Printed-Circuit Board

Thermal Interface Material

Mold Cap

Die Junction

Die/Substrate/C5 Solder Balls

Adhesives and Thermal

Interface Materials

External Resistance

Radiation Convection

(Note the internal versus external package resistance)

For this die-up, wire-bond TBGA 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 it is removed by forced-air

convection.

A thermal interface material is recommended between the top of the mold cap and the

bottom of the heat sink to minimize the thermal contact resistance. For those applica-

tions where the heat sink is attached by spring clip mechanism, Figure 7 shows the

thermal performance of three thin-sheet thermal-interface materials (silicone, graph-

ite/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 inter-

face 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 7). Therefore, the synthetic grease offers the best thermal per-

formance, 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, etc.

18 PC8245

2171D–HIREL–06/04

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