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Trouble in Your Tank: At 40 GHz, Everything Matters, Part 1

The electronics industry supply chain is moving very quickly with the explosion of AI servers, high-performance computing, ADAS, quantum computing, and 5G/6G. Signal integrity is under scrutiny in an entirely new role. With many factors influencing signal integrity, one area that contributes to signal distortion at high frequencies is the PCB’s final finish layer deposits. Frequencies above 40 GHz are susceptible to such distortions, and data transfer rates of 224 GB/s are driving changes in PCBs and IC substrates.

The insertion loss of a high-frequency PCB can decrease the usable signal levels of a system, whether in a receiver or a transmitter. Additional details on insertion loss can be found in a previously published paper.¹ Certainly, the particular conductors’ loss and the conductors’ attributes (surface roughness, surface profile, final finish topography, and particular metal) influence attenuation loss at higher frequencies. Metal finishes have different conductivity values as well and contribute to signal loss.²

A second concern for solderable finish circuits is the skin depth of the conductor. As frequency increases, skin depth decreases. This pushes the transmission more toward the surface of the circuit trace. Thus, the rougher the deposit and the varying conductivity of the metal itself, the more loss can be expected.

With the exception of silver, all other finishes, when plated over a copper conductor, will increase the loss compared to a copper conductor without the finish. This makes selecting nickel in the metal stack somewhat problematic for higher-frequency applications.

Alternative Finishes
As signal integrity becomes more critical at high frequencies, engineers are seeking final finishes that satisfy both electrical and assembly requirements, and several alternative finishes are now commercially available. Key considerations include:

• Lead-free assembly solderability
• Wire-bonding capability
• Reduced signal loss for product boards and advanced substrates

These needs are especially relevant in advanced packaging, where chips may be wire-bonded on one side of the substrate while solder balls are attached to the bottom side.

Over the past five to seven years, researchers have increasingly focused on final-finish metal stacks that eliminate electroless nickel, since nickel is significantly more lossy than silver or copper.

Except for silver, all other finishes listed in Table 1 will increase loss when plated over a copper conductor compared to a copper conductor without the finish. This makes the selection of nickel in the metal stack somewhat problematic.

Nickel’s lower conductivity raises an important question for high-frequency design: Should solderable finishes such as Electroless Nickel-Immersion Gold (ENIG) or Electroless Nickel-Palladium-Immersion Gold (ENEPIG) still be used?

In practice, these finishes remain widely used because they offer proven benefits for demanding applications:

• They have a strong track record in advanced packaging
• They support the needs of high-reliability circuit designs
• They support wire bonding and solderability

Rather than eliminating nickel entirely from the solderable finish stack, a more practical alternative is to use a thinner electroless nickel deposit with a smoother topography.

Electroless Nickel-Immersion Gold (modified)
If one follows the IPC-4552 ENIG standard, the electroless nickel plating thickness is specified as 118.1–236.2 µin (3–6 microns).

There is a proven alternative to ENIG, with several commercially available final finish combinations designed to reduce signal loss. These include Direct Immersion Gold (DIG) over copper, Electroless Palladium-Autocatalytic Gold (EPAG), and Electroless Palladium-Immersion Gold (EPIG).

It’s notable that these processes eliminate the nickel deposit from the stack. Electroless nickel is often considered a significant contributor to signal loss. Other considerations include the nickel thickness as required by IPC-4552 and the overall topography of the nickel deposit. However, the various processes described above have process complexity and cost issues.

So, simply eliminating the nickel deposit is not the answer. Nickel, plated from a specially formulated process, will provide a thinner deposit (which is non-magnetic), a higher phosphorous content, and a near smooth topography. These three attributes improve conductivity and reduce skin effect signal loss due to the smooth topography. Figure 1 shows the comparison of the grain boundaries of the modified nickel formulation as compared to the conventional process. Note the roughness (often called cauliflower) of the conventional nickel.

As others have pointed out, nickel metal contributes to conductor signal loss due to its lower conductivity as compared to copper. However, the research hypothesis stated that if the nickel deposit morphology could be minimized and the magnetic effect eliminated, skin-effect signal loss at higher frequencies would be reduced.

A second hypothesis considered the overall plating thickness of the electroless nickel deposit. Previous experiments showed that a lower nickel thickness would improve the deposit’s conductivity. Nickel plating uniformity across the circuit is also improved with a lower metal thickness. This adds further improvement in signal integrity,² and, in turn, reduces lossy characteristics to a manageable level.

Signal integrity testing showed that the modified electroless nickel deposit reduces signal attenuation loss. Figure 2 compares the signal loss of the modified nickel deposit (THP) with that of conventional nickel and copper.

The modified ENIG (thinner deposit, smooth topography), while not as effective as immersion silver, nonetheless offers an alternative to conventional ENIG. This is due to the thinner nickel deposit, higher phosphorous content of the nickel deposit, and its non-magnetic properties. Another advantage is that the nickel deposit requires only 30–40 µins of plating thickness. That is much lower than conventional electroless nickel processes. The data compares insertion loss results on a microstrip test vehicle to a grounded coplanar waveguide (GCPW) designed test vehicle.

Summary

As AI and the Internet of Things (IoT) drive rapid technological change, processes and materials must keep pace. The electroless nickel-immersion gold process has served the industry well for many years, and a simple modification enables its continued use in higher-frequency applications.

In a future column, I will explore other final finish processes and materials for 40 GHZ and beyond.

References

1.  “The Effects of PCB Fabrication on High Frequency Electrical Performance,” by John Coonrod, IPC APEX EXPO 2015.
2.  “Insertion Loss Performance Differences to Plated Finish and Different Circuit Structures,” by John Coonrod, IPC APEX EXPO 2023.

This column originally appeared in the June 2026 issue of I-Connect007 Magazine.