
11-18
M68040 USER’S MANUAL
MOTOROLA
11.9.2 Forced Air
In this study, a small sample of MC68040 packages was tested in forced-air cooling in a
wind tunnel with no heat sink. The test was performed with approximately 6 W of power
being dissipated from within the package. As previously mentioned, since the variance in
θJA within the possible power range is negligible, it can be assumed for calculation
purposes that
θJA is constant at all power levels. Using the previous equations, Table 11-3
lists the results of the maximum power dissipation at maximum
θJC with airflow and no
heat sink for the MC68040, and Table 11-4 lists the results for the MC68LC040 and
MC68EC040. Refer to Table 11-1 for calculating other power dissipation values.
Table 11-3. Thermal Parameters with Forced Airflow
and No Heat Sink for the MC68040
Thermal Mgmt.
Technique
Defined Parameters
Measured
Calculated
MHz
Airflow Velocity
PD
TJ
θ
JC
θ
JA
θ
CA
TC
TA
25
33
100 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110
°C3 °C/W
12.7
°C/W
9.7
°C/W
91.1
°C
90.2
°C
84.9
°C
86.9
°C
86.0
°C
80.0
°C
29.90
°C
26.18
°C
00.76
°C
12.21
°C
08.40
°C
00.00
°C
25
33
250 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110
°C3 °C/W
11.0
°C/W
8.0
°C/W
91.1
°C
90.2
°C
84.2
°C
86.9
°C
86.0
°C
80.0
°C
40.70
°C
37.40
°C
15.40
°C
25.30
°C
22.00
°C
00.00
°C
25
33
500 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110
°C3 °C/W
9.9
°C/W
6.9
°C/W
91.1
°C
90.2
°C
84.2
°C
86.9
°C
86.0
°C
80.0
°C
47.63
°C
44.66
°C
24.86
°C
33.77
°C
30.80
°C
11.00
°C
25
33
750 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110
°C3 °C/W
9.5
°C/W
6.5
°C/W
91.1
°C
90.2
°C
84.2
°C
86.9
°C
86.0
°C
80.0
°C
50.15
°C
47.30
°C
28.30
°C
36.85
°C
34.00
°C
15.00
°C
25
33
1000 LFM
6.3 W
6.6 W
8.6 W
7.7 W
8.0 W
10.0 W
110
°C3 °C/W
9.3
°C/W
6.3
°C/W
91.1
°C
90.2
°C
84.2
°C
86.9
°C
80.0
°C
81.8
°C
51.41
°C
48.62
°C
30.02
°C
38.39
°C
17.00
°C
22.58
°C
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