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 (back side) of your conductor will be considerably higher than that on the bare-copper side (front side, or pcb-facing side).
You might be tempted to think that this will be OK because even with if the back side of the trace is messed up because of the nickel plating you've still got a good copper surface on the front (pcb-facing side) of the trace. Working by analogy to Figure 1, if the front-side surface acts in parallel with the nickel-plated side, even if the nickel-plated resistance becomes infinite the overall resistance of the configuration can never exceed the resistance of the front side alone.
Unfortunately the parallel-resistances idea is a very bad analogy. At high frequencies, current distributes itself around the periphery of a trace in a pattern that minimizes the total inductance of the trace configuration without regard to the surface resistivity of the trace. The name of the particular effect that controls the distribution of current around the periphery of a conductor is the "Proximity Effect". In a typical pcb microstrip trace configuration the Proximity Effect forces about 2/3 of the signal current to flow on the surface of the trace facing the reference plane, and only 1/3 on the opposite surface.
If you change the surface resistivity of one side of the trace (by plating it with Nickel), you hardly change the distribution of current around the periphery of trace. The Proximity Effect still holds the current in the approximate 2/3 and 1/3 ratio. A better circuit model for current flowing on the front and back surfaces of a microstrip traces involves two independant current sources, pre-set in a proportion of current mandated by the Proximity Effect (Figure 2).
The high-frequency behavior stands in marked contrast to the behavior of current at low frequencies. By the term low frequency I mean the audio band, up to 100KHz or perhaps 1MHz. At low frequencies, current distributes itself to minimize the total power dissipated in a conductor—the so-called, least resistive distribution. At high frequencies, current distributes itself to minimize inductive effects.
Let's do a concrete example. Suppose you have two resistors, R1 and R2 , connected in parallel, as in Figure 1. These resistances represent the surface resistances of the front and back sides of a microstrip trace, respectively. Give the two resistors each a value of two ohms. The overall (parallel) resistance of the circuit formed by their parallel combination then equals 1 ohm.
Now, double the value of resistor R2 (changing its value to 4 ohms). The DC resistance of the parallel combination becomes (2*4)/(2+4) = 4/3, not that much greater than where you started. If you measure the currents, you will see more current flowing in R1, and less in R2, than before. By sharing the current, allowing it to flow in the most efficient manner, the least-resistive principle minimizes the total power dissipation. No matter how high you make the value of R2, the overall DC resistance of the parallel combination can never exceed R1.
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 front and back sides 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 R1 and R2, suppose that resistor R1 represents the front surface of your microstrip trace and resistor R2 the back 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 R2 simply doubles its dissipation. The total power dissipated in that case would now be three units (one for R1, plus two for R2), making the overall resistance appear 50 percent greater than its original value. If you multiply resistance R2 by 10, the overall power dissipation goes up by a factor of 10 in R2, making a total of eleven units of power, or 5.5 times the original dissipation. Continued increases in the surface resistivity of the back side generate unlimited increases in the overall effective resistance.
Let's calculate how bad the effect can get for a nickel-plated trace.
- The resistivity of nickel exceeds that of copper by a factor of k=4.5
- The relative magnetic permeability, μR, for nickel at 1 GHz lies in the range of 5 to 20 (take 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 (kμR), which works out to about 6.7
If the current density on the back 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.
Dr. Howard Johnson