Sorry if this is a little long,
but allow me to put my 2 cents in -
And if anyone can find a flaw in this please correct.
I'm a little rusty with units.
Resistance of something is calculated using
R = rho * L / A
where R = resistance (in ohms)
rho = resistivity of the material (in ohms-cm)
L = Length (in cm, since resistivity is in ohms-cm)
A = Width * Thickness of slab (in cm^2)
+----------------------+
/ /|
/ / |
L / / |
/ / |
/ / |
/ / +
/ / /
+----------------------+ /
| W | /
| | /
| T | /
| Area = W*T | /
| | /
+----------------------+
Now, "ohms per square" assumes your working with a
"SQUARE" slab when viewed from above. Ohms per square
doesn't care whether you are using inches, feet, or
miles. The ohms-per-square of copper is going to be
the same no matter how large the square. Different
thicknesses of the slab will of course give you
different values. But, as long as the thickness
is constant and the sides are square, the
ohms-per-square will be the same for no matter
what size square slab you have.
Anyway, since we're talking about a square slab,
we have to assume that the Width is the same as
the Length, i.e. square looking down from above.
The equation given above reduces to:
Given: R = rho * L / A
= rho * L / (W*T), and since L = W,
R = rho / T
Now, I have rho for copper as
Cu = 1.724*10^-6 ohm-cm
I must convert the cm to whatever units I'm using
for thickness units. Let's say mils. So, it just
happens that
0.000001724 centimeters = 0.0006787 mils
Now we say
rho Cu (mils) = 6.787*10^-4 ohm-mils
So our ohms-per-square formula for the units of
thickness in mils is
Rsq Cu (mils) = 6.787*10^-4 ohm-mils / Thickness (mils)
Thickness in mils Ohms-per-Square
0.7 (1/2 oz Cu) 9.696*10^-4 ohms
1.4 (1 oz Cu) 4.848*10^-4 ohms
2.8 (2 oz Cu) 2.424*10^-4 ohms
Roughly, 1/2 oz Cu is 1 milli-ohm per square
1 oz Cu is 1/2 milli-ohm per square
2 oz Cu is 1/4 milli-ohm per square
I believe this is correct. But I would
really like to know if I'm wrong.
Regards, Doug
----------
> From: [log in to unmask]
> To: [log in to unmask]
> Subject: [DESIGNERCOUNCIL] Resistivity of copper
> Date: Thursday, September 25, 1997 3:08 PM
>
> Is there a chart or formula out there somewhere that will allow us
to
> figure the resistivity of copper / sq. I recall seeing one at one
> time, and am not sure if it was part of an IPC Standard.
>
> Any help is appreciated.
>
> John Glowacki
> Sr. PC Designer
> Vicor Corp.
> Andover, MA
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