Datasheet
DD2.X
PowerPC 750CL Microprocessor
Preliminary
5.8.2 Internal Package Conduction Resistance
For the exposed-die packaging technology shown in Table 3-3, Package Thermal Characteristics, on
page 22, the thermal paths illustrated in Figure 5-8 are as follows:
• Die junction-to-case thermal resistance (primary thermal path), defined as the thermal resistance from
the die junctions to the top surface of the package.
• Die junction-to-lead thermal resistance (not normally a significant thermal path), defined as the thermal
resistance from the die junctions to the circuit board interface.
• Die junction-to-ambient thermal resistance (largely dependent on customer-supplied heat sink), defined
as the sum total of all the thermally conductive components that comprise the end user's application.
Ambient is further defined as the air temperature in the immediate vicinity of the thermally conductive
components, including the contributions of surrounding heat sources.
Figure 5-8 is a thermal model, in schematic form, of the primary heat transfer path for a package with an
attached heat sink mounted to a printed-circuit board.
Figure 5-8. C4 Package with Heat Sink Mounted to a Printed-Circuit Board
External Resistance Components
(Defined by Customer Application)
Radiation
Convection
Custom/Application Specific
Heat Sink
Thermal Interface Material
Die Junction to Case
Chip Junction
RJC
RJB
Internal Resistance Components
(Package and Die)
Die Junction to Lead
Resistance
(Package and Leads)
Printed-Circuit Board
Radiation
Convection
External Resistance Components
(Defined by Board Design and Layout)
(Note the internal versus external package resistance.)
Heat generated in the chip is conducted through the silicon, then through the package to the top of the
package, then through the heatsink attach material (or thermal interface material), and finally into the heat
sink and the ambient air. 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.
System Design Information
Page 58 of 70
Version 2.5
December 2, 2008
IBM [ IBM ]
