Is there anything wrong with running a trace between the pads of another component?

[John Larkin]

Your 70K/w for a 20 mil dia hole is the same as my 50C for a 28 mil hole.
500uR per 1 oz square cu works for me too.
Given a fixed heat transfer area and no solder fill, a lot of little vias
are better than one big air hole.
Let's check your 150K k/w with a 20" x 0.010" 1 ohm cu trace:
Rt= 0.1*20/(.010*.0014) = 143K.. not too bad!
Do you use/agree with any of the trace width calculators or even IPC(2222)
that are based on convection cooling? They work for me when the trace is
1.50" long, shorter traces can be thinner, longer traces must be fatter.
Most PCB have internal planes that spread heat and thermal conduction in
traces greatly out performs thermal convection. I strive for <12mV drop in
all traces. This will keep temp rise in all traces to <5C due to conductive
cooling at the trace end points.

Right: usually I care about voltage drop before I care about trace
heating, and if you keep the voltage drop down, the heating is no big
deal. Usually.

John

 

[John Larkin]

If you pack N vias of d diameter in a line such that they touch each other
the N vias are equivalent to a big D=N.d via
So the thermal resistance decrease as 1/N for a compact set of vias inside a
square (N.d)^2.
Of course the gain will not be this big, but the advantage is still way in
favor to N small vias.



Now that seems a pretty nice and useless figure to throw up in air during
some meeting.
Just look at those jumping on their pencil to take note and you'll know who
to not work with.

I've designed a few oven-type things where I wanted to get current
into/out of a box with minimum heat loss. It turns out that brass is
better than anything else I could find, more like 300,000 (k/w)/ohm.

This is a serious issue in cryogenics, getting signals up/down a
temperature gradient with minimal heat transfer [1], but the situation
is more complex, since both r and theta per unit length change a lot
as a function of temperature.

John

[1] Liquid nitrogen costs about as much as beer, and liquid helium
costs about as much as whiskey.

 

[Fred Bartoli]

On Wed, 10 May 2006 21:00:45 +0200, "Fred Bartoli"

Now that seems a pretty nice and useless figure to throw up in air during
some meeting.
Just look at those jumping on their pencil to take note and you'll know who
to not work with.

I've designed a few oven-type things where I wanted to get current
into/out of a box with minimum heat loss. It turns out that brass is
better than anything else I could find, more like 300,000 (k/w)/ohm.

This is a serious issue in cryogenics, getting signals up/down a
temperature gradient with minimal heat transfer [1], but the situation
is more complex, since both r and theta per unit length change a lot
as a function of temperature.

Oh yes. But this isn't the typical design review
I thought of this, because at a design review I once saw half the audiance
jumping on their notebook when I "calculated" the resitance of a track and
had to explain how to do this.
I'm always amazed at how little it takes to be a guru.

John

[1] Liquid nitrogen costs about as much as beer, and liquid helium
costs about as much as whiskey.

And which one tastes best? I mean LN or LH.

 

[John Larkin]

Oh yes. But this isn't the typical design review
I thought of this, because at a design review I once saw half the audiance
jumping on their notebook when I "calculated" the resitance of a track and
had to explain how to do this.
I'm always amazed at how little it takes to be a guru.

We could start a group project, to create a file (eventually a book?)
of assorted lore, equations, rules of thumb, handy approximations,
tricky algorithms.

John

[1] Liquid nitrogen costs about as much as beer, and liquid helium
costs about as much as whiskey.

And which one tastes best? I mean LN or LH.

Kinda changes the meaning of "a cold one."

John

 

[joseph2k]

0.06pF oops


indeed.

In this case it was a very short length of inner trace, 8mil from the
-ve input trace. Of course it was a poor layout that allowed that to
happen in the first place, but I was surprised at how small the
calculated capacitance was when I checked the PCB layout. So I measured
the spike carefully, along with dV/dt on the offending signal, and sure
enough

I = Ccoupling*dV/dt

injected into the high impedance summing node caused the observed output
spikes. I tested the theory in spice, and then managed to isolate the
offending trace by drilling out a couple of vias, and manually
re-routing the trace. Voila, no spikes. routing the wire past the opamp
input made the spikes come back.

it was a pretty good introduction to how circuit layout affects performance.

Cheers
Terry

I think that i would call that more on the order of senior project in a
truly well built curriculum.