Power and Thermal Characteristics
77
Figure 11 Package with Heat Sink Mounted to the Printed Circuit Board
Radiation
External Resistance
Convection
Heat Sink
Thermal Interface Material
Die/Package
Die Junction
Package/Leads
Internal Resistance
Printed Circuit Board (PCB)
Radiation
Convection
External Resistance
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 convection.
Because the silicon thermal resistance is quite small, for a first-order analysis, the
temperature drop in the silicon may be neglected. Thus, the thermal interface material
and the heat sink conduction/convective thermal resistances are the dominant terms.
Heat Sink Selection Example
For preliminary heat sink sizing, the die-junction temperature can be expressed as follows:
T j = T a + T r + (θjc + θint + θsa ) x P d
where:
T j is the die-junction temperature
T a is the inlet cabinet ambient temperature
T r is the air temperature rise within the computer cabinet
03
FREESCALE [ Freescale ]
