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Tue, 17 Mar 1998 12:45:15 -0500 |
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Welcome Valerie,
I'll try to put my two cents worth.
1. Current Distribution:
Assume the Cu trace and the Au plating as two
separate conductors.
Now the conductivities are
Conductivity of Cu = 5.76 x 10^7 S/m (siemans per meter)
Conductivity of Au = 4.10 x 10^7 S/m (siemans per meter)
Copper is more conductive than gold.
The formula for the resistivity for a slab of metal is
R = rho*Length/Cross-sectional Area
rho is equal to 1/conductivity.
So, for equal slabs of Au and Cu ...
R(Cu) = (1/5.76x10^7)*L/A
R(Au) = (1/4.10x10^7)*L/A
If we put the Au in reference to Cu, we find that
Conductivity of Au approx = 0.70*Conductivity of Cu
Also the Au plating cross-sectional area is
1/10 the Cu trace cross-sectional area.
So ...
R(Au) = (1/0.70*5.76x10^7)*L/(A/10) this reduces to
R(Au) = 14*(1/5.76x10^7)*L/A or 14 times the Cu.
What the heck does this gobbledygook mean?
This means:
a. The Au plating is 14 times more resistant than the Cu trace.
b. The Au plating will carry 1/14 the current of the Cu trace.
c. Valid ONLY for Au and Cu materials with corresponding 1:10
ratios of area AND assuming the currents through
both materials travel parallel.
2. Frequency Effects:
Yes. Burn these two concepts into your brain
Skin Effect and Skin Depth.
Skin Effect - The tendency for the resistance of
a material to increase at higher frequencies.
The current travels closer to the surface of a
conductor at higher frequencies.
The *mechanism* by which this happens is called
Skin Depth - One skin depth is where approx 63% of the
current resides. Varies with frequency.
Sorry for making this long.
Regards, Doug
----------
> From: Valerie Webber <[log in to unmask]>
> To: [log in to unmask]
> Subject: [TN] Plating Effects on Current Distribution
> Date: Friday, March 13, 1998 10:29 AM
>
> Valerie Webber
> 03/13/98 10:29 AM
> Ok, bear with me as I am new to this e-mail forum thing. This is my
first
> attempt at posting.
>
> I would like information on the effects plating has on the current
> distribution throughout my conductor. For example, say I have two 40 um
> traces with 40 um space between edges sitting on a dielectric. I'm using
a
> copper alloy of only 40% IACS. Now I throw 3 um of gold plating (78%
IACS)
> on the 3 exposed sides of my traces.
>
> What is the distribution of current through this multi-layer conductor?
> How does this distribution change with increasing frequency?
> Is there a non-conductive barrier between the copper and the plating?
> Trace pitch has stayed the same, but edge-to-edge spacing and trace
> cross-section have changed. Do I use the plated configuration in my RLC
> calculations?
> Is there modeling software suited for this? I have a 2-d FEA package
> (Ansoft), but it does not seem to handle 2 conductors sharing a boundary.
> Maybe it does, but I'm doing something wrong?
>
> I would appreciate being pointed in the direction of any literature,
> experts, etc. that have investigated this issue.
>
> Valerie Webber
>
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