10-2
M68020 USERS MANUAL
MOTOROLA
Solving Equations (10-1) and (10-2) for K gives:
K = PD (TA + 273
°C) + qJAPD2
(10-3)
where K is a constant pertaining to the particular part. K can be determined from equation
(10-3) by measuring PD (at thermal equilibrium) for a known TA. Using this value of K, the
values of PD and TJ can be obtained by solving equations (10-1) and (10-2) iteratively for
any value of TA.
The total thermal resistance of a package (
qJA) can be separated into two components,
qJC and qCA. qJC represents the barrier to heat flow from the semiconductor junction to the
package (case) surface, and
qCA represents the barrier to heat flow from the case to the
ambient air. These terms are related by the equation:
qJA=qJC + qCA
(10-4)
qJC is device related and cannot be influenced by the user. However, qCA is user
dependent and can be minimized by such thermal management techniques as heat sinks,
forced air cooling, and use of thermal convection to increase air flow over the device.
Thus, good thermal design on the part of the user can significantly reduce
qCA so that qJA
approximately equals
qJC. Substitution of qJC for qJA in equation (10-1) results in a lower
semiconductor junction temperature.
10.2.1 MC68020 Thermal Characteristics and
DC Electrical Characteristics
MC68020 Thermal Resistance (
°C/W)
The following table provides thermal resistance characteristics for junction to ambient and
junction to case for the MC68020 packages with natural convection and no heatsink.
CharacteristicNatural Convection and No Heatsink
qJA
qJC
Thermal Resistance
PGA Package (RC Suffix)
PPGA Package (RP Suffix)
CQFP Package (FE Suffix)
PQFP Package (FC Suffix)
26
32
46
42
3
10
15
20
Resistance is to bottom center (pin side) of case for PGA and PPGA packages, top center of case
for CQFP and PQFP packages.
MC68020 CQFP Package
Table 10-1 provides typical and worst case thermal characteristics for the MC68020
CQFP package both with and without a heatsink. The heatsink used is black anodized
aluminum alloy, 0.72"x0.75"x0.6" high with an omnidirectional 5x6 array of fins.
Attachment was made using Epolite 6400 one part epoxy.
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
..
.