Nickel-Plated Traces

HIGH-SPEED DIGITAL DESIGN - online newsletter -
Vol. 5 Issue 6

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*----------------------(QUESTION)---------------------*

Nickel-plated traces

Paul Greene writes,

I am an engineer with Rexcan Circuits , a PCB manufacturer in Ontario. We are evaluating various processes for the use of immersion gold. We are depositing approx .00012"" Nickel followed by .000005 Gold over the copper pads. I am investigating the effect of two alternate procedures:

(1) coating all the traces/pads [with nickel] and then applying solder mask, versus

(2) applying solder mask over the traces first and then coating only the pads [with nickel and gold].

We have been advised that due to the changes to the skin effect caused by the Ni/Au on the traces for high frequency RF designs we could be building in a problem. Can you find time to comment on this as I've run some characteristic impedance tests at around 65 Ohms using a TDR tester and not seen much difference. Is this a real potential problem ??

Hope you don't mind me contacting you , but your web page has lots of good stuff in this area !!

*--------------(REPLY FROM DR. JOHNSON)---------------*

Thanks for your interest in High-Speed Digital Design. I've received a number of inquiries since writing "Steel-plated power planes" pointing out that nickel is magnetic, too, and a well- established chemistry exists for plating it onto copper. That's an interesting point. Of course the magnetic permeability of nickel is nowhere near that of steel, so you won't get nearly as dramatic an effect, but it might be worth investigating.

Regarding your very interesting question, I've often wondered about the same issue. I presume you are familiar with the concept of the skin-effect and how current at high frequencies flows only on the outer surface (skin) of your conductor, not in the middle. Because of the high magnetic permeability of nickel plating, the skin-effect resistance on the nickel-plated side of your conductor will be considerably higher than that on the bare-copper side (core side).

You might be tempted to think that this will be OK because even with if one side of the trace is messed up because of the nickel plating you've still got a good copper surface on the bottom of the trace. The bottom-side surface acts in parallel with the nickel-plated side, so even if the nickel- plated resistance were infinite the overall resistance of the configuration you might think would be no more than twice as bad as an all-copper trace. Unfortunately that is a *very* bad analogy. At high frequencies the current distributes itself around the periphery of your trace in a pattern that minimizes the total inductance of the trace configuration WITHOUT REGARD TO THE SKIN RESISTANCE OF THE TRACE. That is, if you change the skin- effect resistance on one side (by plating), then in the limit at very high frequencies you hardly change the distribution of current around the periphery of trace at all.

At DC the current distributes itself to minimize the total dissipated power.

For example, suppose you have two resistors A and B in parallel, both with resistance 2. The overall (parallel) resistance is 1. If you double the value of A (changing its value to 4) the DC resistance of the parallel combination is (4*2)/(4+2) = 4/3. At DC you get less current in A, more in B, in a combination that minimizes the total dissipation. No matter how high you make the value of A, the parallel combination never gets bigger than 2.

At high frequencies the same effect does not prevail. At high frequencies the current distributes itself in a way that minimizes the overall inductance (this minimizes the energy stored in the magnetic field surrounding the circuit). In a pcb trace, this means the ratio of currents on the top and bottom of the trace are fixed by the inductance and not able to respond to (moderate) changes in the surface resistivity of the two surfaces.

Going back to the example of resistances A and B, suppose A represents the top surface of your microstrip trace and B the bottom surface. If you double the resistance of surface A the current doesn't change, so the power dissipated in A doubles. If A started out with half the power, the total dissipation goes to: (1/2)*2+(1/2) = 3/2. For a constant-current circuit where the power is proportional to the effective resistance, you can say that the effective resistance of the combination goes up by 50% (to 3/2 of the original value). If you multiply the resistivity of A by 10 the effective resistance goes up by (1/2)*10+(1/2)= 5.5 times. Continued increases in surface resistivity of the top side make unlimited increases in the overall effective resistance.

Let's calculate how bad the effect can get.

The resistivity of nickel exceeds that of copper by a factor of k=4.5

The relative magnetic permeability of nickel at 1 GHz is in the range of 5 to 20 (take u=10 as a nominal value)

The increase in surface resistance of nickel at 1 GHz (above and beyond that of copper at 1 GHz) equals the square root of k*u, which works out to about 6.7.

If the current density on the top side of a 50-ohm FR-4 pure-copper microstrip contributes about 1/3 the total dissipation, and if you increase that 1/3 of the dissipation by a factor of 6.7, then I would expect an overall increase in resistive trace loss by a factor of ((1/3)*6.7 + 2/3) = 2.9. That's roughly a tripling of the resistive trace loss. At frequencies on the order of 1 GHz the nickel- plating will cut in third the effective useful length of your traces.

I checked the skin depth of nickel at 1 GHz and found it's about 0.000055", much thinner than your nickel plating. If you could make the nickel plating as thin as the gold it won't have much of an effect, but I bet it doesn't work as a barrier layer if it's that thin.

In a TDR waveform, any series resistance present in a conductor causes an upward tilt to observed waveform. You can think of this as the trace showing a slightly lower impedance at first (high frequencies), then gradually transitioning to a larger value as time goes by (lower frequencies). The amount of tilt is related to the amount of series resistance. I predict your nickel-plated traces will show a greater upward tilt than your bare-copper traces. That's one way you can determine the extent of the effect (ultimately, this measures the magnetic permeability, and thus the purity, of your nickel).

If you look carefully at the step edge that returns from the far end of the line in your TDR trace, you should see on a long trace (perhaps 10 inches) a noticeable degradation in risetime. This degradation will be worse on the nickel-plated traces than on the bare copper traces.

This effect is real, and commonly understood in the microwave community (see next message).

Best Regards,
Dr. Howard Johnson

*-------------(ADDITIONAL CORRESPONDENCE)-------------*

Dear Howard,

I have an additional idea for your iron power ground plane: any magnetic material will do. Every now and then we encounter someone climbing the microwave learning curve using magnetic nickel somewhere in the RF ground path and asking, "Why are my losses so high?" I also understand that magnetic nickel is plated on plastic cases to reduce EMI. Point you might want to mention is that, in the skin effect region, the equivalent surface resistance increases with the sqrt(mu). Thus, the conducted EMI in the magnetic ground path gets significantly attenuated, while DC currents are unaffected by mu. However, the designer should be very careful not to introduce magnetic metals into the ground current return path for any high speed or high frequency signals, or they will find themselves climbing the same learning curve us microwave guys have.

Sincerely, Jim Rautio

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