
FAULT-FINDING 229
Photo 22 Example of an
incorrectly made joint; large
solder blobs, excessive flux due
to dirty component leads, and
solder bridges between adjacent
tracks
Where T
}
is the maximum junction temperature ( C) specified by the
manufacturer (usually before derating is applied), T
d
is the ambient
temperature (°C), and $
jc
,0
cs
and ö
sa
are the thermal resistance of junction
to case (specified by the manufacturer), case to heatsink, and heatsink to
air respectively (measured in °C/W).
The previous formula may be conveniently rearranged so that the value
of #
sa
may be found:
Τ
(»je +
OJ C/W
As an example, consider the case of a semiconducto ...