How much can a small SMD component really dissipate power

[LM]

I ask because there are more and more full boards here. Heat is supposed
to spread somewhere if I beliewe manufacturers, but in a full board
there is very little space to spread. Expesially if other side is full
too.

If I think about a simple case, there is a very small space around a
resistor to heat air above it. A small space because everywhere around
and under the part there are similar parts. What then is thermal
resistance of say square millimeter of FR4, or square inch if you want.
Does this make sense? A small component in the middle of similar parts,
all getting warm. The real life must be a mess.

Are there programs to simulate temperature of a full two sided board and
clever enough to be future proof.

IR cameras are one thing I can think to confirm how hot a part is.

Any other comments?

Regards

LM

 

[Tim Williams]

Guess that plain old copper clad will dissipate maybe 1W/in^2. (What was
the empirical figure, something like 150 K*in^2/W in free air?)

Adding bumps to the board will increase its power dissipation slightly,
and cutting up the copper foil reduces heat spreading ability (leading to
hotspots).

So if you have 100 little SMTs in a square inch, dissipating a total of
1W, you'll be pretty much at ratings. That's an average of 10mW per, but
the statistical distribution is probably a "power" law (of a different
kind of power), where 10% of the transistors dissipate 90% of the power,
or whatever. Keep this in mind when sizing thermal pads.

Tim

 

[Nemo]

If you look at the manufacturers' details for surface mount power
resistors, in the small print they say e.g. "for 1 watt dissipation, the
pads must be connected to 1 square cm of copper".

I believe you can buy surface mount heat sinks these days (ie, ones
which go on the board, to increase its effective surface area, not on
components).

how do you check how hot it is inside a
box. Hire some miniature chinese with thermometers?

I put an LM20 (temperature sensor from NatSemi) on the board. I only fit
it on the prototype.

Hah, we going to put the cards in vacuum.

This is an interesting point, even though you are joking. Does anyone
know what factor to increase power rating by for resistors in vacuum?
I've had to do this a couple of times, and used a factor of 3 or 4, and
they survived - but I think they were only dissipating for a few seconds
every minute.

It looks like there is not a good thermal simulator software.

I once worked at a company where we wanted to know what the average
temperature inside a box would be (for MTBF calculations). Someone
pulled a diagram out of a folder - a photocopy of a photocopy of an old
book - and said "well... we're dissipating an average of 100 watts in
that box. Its surface area is 1.3 square metres. So it'll be... um... 20
degrees hotter than ambient in there." This took all of 15 minutes to
gather the data and look at the graph. This was Too Easy, so the
managers bought a thermal CAD package and detailed an engineer to learn
how to use it, model the cards in the 3D space, and give them an answer
they could believe in. 1 week later he reports, "22 degrees". This is
suspiciously close to the non-credible first result, so they order a
full scale mock up (we don't have the cards yet, we had to make dummy
ones with resistor loads). This gives an answer of 20 degrees.

If anyone has a bit of web server room where we can upload that graph, I
have a JPG image of it, contact me here. It's always been spot on for me.

 

[Kim Enkovaara]

Very interesting. I have learned new things and perhaps got some
things confirmed. It looks like there is not a good thermal simulator
software.

There are, but they are not cheap, for example
http://www.mentor.com/products/mechanical/products/flotherm

--Kim

 

[Kim Enkovaara]

Not cheap, and they tend to have a huge learning curve. And they are
only as accurate as their input, which can be bad. Unless you do
thermal design full-time for IBM or something, it's usually faster and
cheaper, and certainly more accurate, to hack a physical model of your
situation (cardboard, duct tape, fans, heat sinks, copperclad) and
test it.

The tools are not that difficult but need experience in creating the
models etc. and also in understanding the results. There are also
consultants doing these things, if there is no inhouse experience
(or licenses).

The problem with hw models is that what-if analysis is hard to do. For
very power dense designs tens of rounds of different placement scenarios
are needed before things look better. It's hard to simulate in head
how different heatsinks and other mechanical structures affect to the
airflow of other components, how much the pcb conducts heat, would
heatpipes help etc. Also testing what happens if different fans fail is
interesting simulation in telecom designs that need high availability.

In very dense designs also hw models are needed, but they can be built
from the few good results from the simulations.

What does Flotherm cost? Is the air flow modeling really any good?

The modeling works, but of course experience is needed to get accurate
enough results. The price you can get from Mentor

--Kim