Nickel-Plated Traces
High-Speed Digital Design Online Newsletter: Vol. 5 Issue 06
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 in. of Nickel followed by .000005 in. of Gold over the copper pads. I am investigating the effect of two alternate procedures:
- Coating all the traces/pads [with nickel] and then applying solder mask, versus
- 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 !!
Thanks for your interest in High-Speed Digital Design. Since writing Steel-plated power planes, I've received a number of letters pointing out that nickel is magnetic, too, and that 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 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 in this case. 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 surface resistivity of the trace. That is, if you change the skin- effect resistance on one side (by plating), it turns out that at high frequencies you hardly change the distribution of current around the periphery of trace at all.
That acts in contrast to the behavior of current at low frequencies. For objects like pcbs, the term low frequency means the audio band, up to 100KHz or perhaps 1MHz. At low frequencies, 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 becomes (4*2)/(4+2) = 4/3. At low frequencies 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 (above 1MHz on a pcb) 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 effect and do not respond to (moderate) changes in the surface resistivity of the two surfaces.
Going back to the example of resistances A and B, suppose that resistor A represents the top surface of your microstrip trace and resistor B the bottom surface. Suppose that you begin with both resistors dissipating one unit of power. In an inductively dominated circuit, where the current distribution in the resistors does not change with their values, doubling the resistance of A doubles its dissipation. The total power dissipated in that case would now be three units (two for A, one for B), making the overall resistance appear 50 percent greater than its original value. If you multiply the resistance of A by 10 the overall power dissipation goes up by a factor of 10 in A, unity in B, making eleven units, or 5.5 times the original dissipation. 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 lies 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. My conclusion? In a skin-effect limited system, nickel-plating cuts 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 in., much thinner than your nickel plating, so my calculations should be about right. If you could make the nickel plating as thin as the gold then the current would submarine below the nickel into the copper, and you would not suffer any increase in resistance. I bet that doesn't work, though, because the nickel will not act as a good oxidation barrier layer if it is made that thin.
In a time-domain reflectometer waveform (TDR), any series resistance present in the conductor under test causes an upward tilt to observed waveform. You could say that the trace shows a slightly lower impedance at first (high frequencies), then gradually transitions to a larger value as time goes by (lower frequencies). The amount of upward tilt relates to the amount of series resistance. I predict that 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 a long line (perhaps 10 inches) in your TDR trace, you should see on a long trace 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
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