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Design Considerations
Thermal Design Considerations
A complicating factor is the existence of three common ways for determining the
junction-to-case thermal resistance in plastic packages.
•
To minimize temperature variation across the surface, the thermal resistance is
measured from the junction to the outside surface of the package (case) closest to the
chip mounting area when that surface has a proper heat sink.
•
To define a value approximately equal to a junction-to-board thermal resistance, the
thermal resistance is measured from the junction to where the leads are attached to the
case.
•
If the temperature of the package case (T
thermal resistance is computed using the value obtained by the equation
(T
– T
)/P
.
J
T
D
As noted above, the junction-to-case thermal resistances quoted in this data sheet are
determined using the first definition. From a practical standpoint, that value is also suitable for
determining the junction temperature from a case thermocouple reading in forced convection
environments. In natural convection, using the junction-to-case thermal resistance to estimate
junction temperature from a thermocouple reading on the case of the package will estimate a
junction temperature slightly hotter than actual temperature. Hence, the new thermal metric,
thermal characterization parameter or Ψ
gives a better estimate of the junction temperature in natural convection when using the
surface temperature of the package. Remember that surface temperature readings of packages
are subject to significant errors caused by inadequate attachment of the sensor to the surface
and to errors caused by heat loss to the sensor. The recommended technique is to attach a
40-gauge thermocouple wire and bead to the top center of the package with thermally
conductive epoxy.
4-2
) is determined by a thermocouple, the
T
, has been defined to be (T
JT
DSP56367
– T
)/P
. This value
J
T
D
MOTOROLA
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