
MOTOROLA
M68040 USER’S MANUAL
11-19
Table 11-4. Thermal Parameters with Forced Airflow and No Heat Sink
for the MC68LC040 and MC68EC040
Thermal Mgmt.
Technique
Defined Parameters
Measured
Calculated
MHz
Airflow Velocity
P
D
4 W
5 W
6.3W
T
J
θ
JC
θ
JA
θ
CA
T
C
T
A
20
25
33
100 LFM
110
°
C
3
°
C/W
12.7
°
C/W
9.7
°
C/W
98
°
C
95
°
C
91.1
°
C
98
°
C
95
°
C
91.1
°
C
98
°
C
95
°
C
91.1
°
C
98
°
C
95
°
C
91.1
°
C
98
°
C
95
°
C
91.1
°
C
59.2
°
C
46.5
°
C
29.9
°
C
66
°
C
55
°
C
40.70
°
C
70.4
°
C
60.5
°
C
47.63
°
C
72
°
C
62.5
°
C
50.15
°
C
72.8
°
C
63.5
°
C
51.41
°
C
20
25
33
250 LFM
4 W
5 W
6.3W
110
°
C
3
°
C/W
11
°
C/W
8
°
C/W
20
25
33
500 LFM
4 W
5 W
6.3W
110
°
C
3
°
C/W
9.9
°
C/W
6.9
°
C/W
20
25
33
750 LFM
4 W
5 W
6.3W
110
°
C
3
°
C/W
9.5
°
C/W
6.5
°
C/W
20
25
33
1000 LFM
4 W
5 W
6.3W
110
°
C
3
°
C/W
9.3
°
C/W
6.3
°
C/W
Reviewing the maximum ambient operating temperatures illustrates that using an all small
buffer configuration of the MC68040 with a relatively small amount of airflow (100 LFM)
achieves a 0–70
°
C ambient operating temperature. However, depending on the output
buffer configuration and available forced-air cooling, additional thermal management
techniques may be required.
11.9.3 With Heat Sink
The designer must consider many factors in choosing a heat sink: heat-sink size and
composition, method of attachment, and choice of a dry or wet (i.e., thermal grease)
connection. The following paragraphs discuss the relationship of these decisions to the
thermal performance of the design noticed during experimentation.
The heat-sink size is one of the most significant parameters to consider in the selection of
a heat sink. Obviously a larger heat sink provides better cooling. Under forced-air
conditions as low as 100 LFM, the difference between the
θ
CA
is very small (0.4
°
C/W or
less). This difference continues to decrease as the forced airflow increases.
The area of this example heat-sink base perimeter is 1.8"
×
1.8", with a height of 0.65".
The heat-sink used a pin-fin (i.e., bed-of-nails) design composed of aluminum alloy. Figure
11-32 illustrates the heat sink, which can be obtained through Thermalloy, Inc.