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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