This is a note I wrote some years ago during the
AO-40 project. If people think it's useful, perhaps I could post it to
Wikipedia.
Alan
N1AL
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Our
local PC board technical expert here at HP has done an extensive study on power
dissipation of surface-mount parts. While his report is based on non-space
applications, there are a number of conclusions that are very
interesting.
I won't include the entire report, partly because it is much
too long, and partly because it is HP proprietary. But here are a few
highlights.
He found surprisingly little discussion of these issues in
the literature. So he performed a series of experiments using an
Inframetrics 760 infared camera system to measure component temperatures.
All measurements were taken on similar-sized PC boards mounted horizontally with
no significant airflow.
One main conclusion: "Part ratings by the
vendors have little meaning by themselves since trace width and part density
have a huge impact on heat dissipation."
For example, what is the
allowable power dissipation of an 0603 resistor? The answer is somewhere
between 12 mw and 526 mw, depending on layout!
This is based on a
60 degC temperature rise. (Most resistors have a 125 degC max case
temperature spec.) Temperature rise is directly proportional to power
dissipation.
The 12 mw number was measured on 112 parts packed into a
0.75 square inch area and all dissipating the same power. The 526 mw
measurement was on a single part soldered between two half-board-sized ground
planes and tied through many vias to another ground plane on the back side of
the board.
Manufacturers rate 1206-case resistors at 125 mw, versus 62.5
mw for 0603-case resistors, a 2:1 ratio. Actually, for most reasonable
trace widths, an isolated 0603 resistor can dissipate roughly 80% as much as a
1206. When parts are packed densely together, power dissipation is limited
by the maximum watts per square inch. Note that I used a 30 degC
temperature rise for the following
table:
0603 1206
Isolated
Resistor:
Large ground plane: 263
mw 403 mw
0.060 inch
traces: 170 mw 199
mw
0.040 inch traces: 148
mw 177 mw
0.012 inch
traces: 106 mw 128
mw
0.005 inch traces: 77
mw 100 mw
0.0025 bond
wires: 55 mw 79
mw
High-density part layout, 0.75 square
inch (4.84 cm^2) area:
Number of
parts
112
32
Power per
part 6 mw 20
mw
Total
power 672 mw 640
mw
Number of parts
56
16
Power per
part 1.5 mw 43
mw
Total
power 644 mw 688
mw
Number of
parts
20
8
Power per
part 31 mw 80
mw
Total
power 620 mw 640
mw
Since the power dissipation depends so strongly on trace width, then
clearly most of the heat must be conducted, not radiated, on SMT resistors (and
other parts). Even in the thin-trace case, much of the heat is conducted
to, and radiated from, the PC board. You can see that on the infared
photos: the PC board surrounding the part is quite hot.
Thermal
resistance of isolated SOT-23 transistors was very similar to 0603 resistors up
to 0.040" line widths. SOIC-8 voltage regulators had about 1/2 the thermal
resistance of SOT-23.
The above numbers are probably conservative, since
they are based on still air, even though most earth-based applications have
forced-air cooling or at least natural convection (vertical PC board). The
rule of thumb I generally use is 1 watt per square inch (155 mw/cm^2), which
gives an average temperature rise of around 35 degC in still air.
I have
been told that for space applications, 15 mw/cm^2 is a more appropriate
limit. Assuming radiation cooling is only 1/10 as efficient as convection
cooling, then that seems a reasonable spec.
Conclusion:
I have
heard several rules of thumb on what percentage you should derate component
power specifications for space applications. For through-hole devices,
such a rule of thumb probably makes sense. THD devices dissipate most of
their heat from the component body, and relatively little heat is conducted out
the leads. On earth, most of that heat is conducted to the air; only a
little is radiated. In space, radiation is the only heat-dissipating
mechanism, so the power must be derated by a large factor.
But SMT
devices are cooled mainly by conduction out the leads to the copper traces on
the PC board, so they are not directly affected by the lack of air. It
seems to me that as long as I keep within the 15 mw/cm^2 limit on the PC board,
that allowable power dissipation of individual components should be governed by
trace width, per the above table. If I have a hot component, I'll connect
it to lots of copper and make sure no other hot components are
nearby.
<END>
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