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 11.5 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.
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