System Design Information
temperature—airflow, board population (local heat flux of adjacent components), heat sink efficiency, heat sink
attach, heat sink placement, next-level interconnect technology, system air temperature rise, altitude, etc.
Due to the complexity and the many variations of system-level boundary conditions for today's microelectronic
equipment, the combined effects of the heat transfer mechanisms (radiation, convection, and conduction) may vary
widely. For these reasons, we recommend using conjugate heat transfer models for the board, as well as system-level
designs.
For system thermal modeling, the MPC7445 and MPC7455 thermal model is shown in Figure 30. Four volumes will
be used to represent this device. Two of the volumes, solder ball, and air and substrate, are modeled using the
package outline size of the package. The other two, die, and bump and underfill, have the same size as the die.
Dimensions for these volumes for the MPC7445 and MPC7455 are given in Figure 20 and Figure 22, respectively.
The silicon die should be modeled 9.10 × 12.25 × 0.74 mm with the heat source applied as a uniform source at the
bottom of the volume. The bump and underfill layer is modeled as 9.10 × 12.25 × 0.069 mm (or as a collapsed
volume) with orthotropic material properties: 0.6 W/(m • K) in the xy-plane and 2 W/(m • K) in the direction of the
z-axis. The substrate volume is 25 × 25 × 1.2 mm (MPC7445) or 29 × 29 × 1.2 mm (MPC7455), and this volume
has 18 W/(m • K) isotropic conductivity. The solder ball and air layer is modeled with the same horizontal
dimensions as the substrate and is 0.9 mm thick. It can also be modeled as a collapsed volume using orthotropic
material properties: 0.034 W/(m • K) in the xy-plane direction and 3.8 W/(m • K) in the direction of the z-axis.
Die
z
Bump and Underfill
Substrate
Conductivity
Value
Unit
Solder and Air
Bump and Underfill
Side View of Model (Not to Scale)
x
k
k
k
0.6
0.6
2
W/(m • K)
x
y
z
Substrate
18
Substrate
Die
k
Solder Ball and Air
k
k
k
0.034
0.034
3.8
x
y
z
y
Top View of Model (Not to Scale)
Figure 30. Recommended Thermal Model of MPC7445 and MPC7455
MPC7455 RISC Microprocessor Hardware Specifications, Rev. 4.1
58
Freescale Semiconductor
FREESCALE [ Freescale ]
