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Trouble in Your Tank: At 40 GHz, Everything Matters, Part 2

In Part 1 of this article, we examined alternative solderable finishes for 5G/6G applications and reviewed literature on their performance at high frequencies.

We found that certain solderable finishes are more susceptible to signal loss as frequency increases. These attenuation losses become increasingly significant at frequencies approaching and exceeding 40 GHz. Because of their high conductivity, silver and copper generally outperform finishes such as nickel and palladium. Taken together, these findings underscore the importance of finish selection in high-frequency applications.

Part 1 described, however, that modified nickel (higher phosphorous content, thinner deposit, smoother topography) mitigates some of the signal loss when compared to conventional electroless nickel deposits. But this may not be a sufficient solution as designers are faced with 1.6T/3.2T 6G. We are seeing frequencies above 77 GHZ and beyond. So, what is the answer?

Immersion Silver-Immersion Gold (ISIG)
Immersion silver has been used commercially as a solderable finish for more than 25 years. Its high conductivity makes it especially well-suited for high-frequency applications, where signal integrity becomes increasingly important.

Alkaline Immersion Silver
Most immersion silver processes are acid-based. The process described here uses an alkaline formulation as an alternative approach. This chemistry was developed to support high-frequency performance while addressing some of the limitations associated with conventional acid-based systems.

Development of ISIG
To further enhance the performance of alkaline immersion silver, a specialized immersion gold process was developed for deposition over the silver layer. This combined finish, immersion silver-immersion gold (ISIG), was designed to improve signal integrity and reduce attenuation loss in 5G applications.

The initial development effort focused on immersion silver alone, since silver is already recognized for its ability to improve high-frequency performance because of its superior conductivity. However, concerns about silver tarnishing through environmental interaction led to the addition of a thin immersion gold overcoat, creating a more robust dual-finish system.

The alkaline formulation of immersion silver has mitigated one of the most annoying issues associated with acidic immersion silver processes: solder mask interfacial attack. This is often known as trench etch (Figure 1). The interfacial attack is often blamed for the genesis of creep corrosion, which is often related to exposure of the circuits to harsh environments.

Nitrate-based formulations are overly aggressive on copper. This leads to voiding along the surface of the copper during the galvanic reaction. As copper is displaced, the silver deposits on the copper surface, forming a silver “roof” over the voided area. Upon assembly, the silver dissolves, leaving microvoids as shown in Figure 2. This issue was named “champagne bubbles.”

The alkaline-based process will not attack the copper as aggressively as any nitrate-based process. In addition, the silver ions in the alkaline silver process are chelated. This allows for more controlled deposition of silver. This results in a denser, less porous silver deposit.

While gold is slightly less conductive than silver, the benefits of an immersion gold deposit would outweigh any additional signal loss due to the presence of gold over silver. In addition, gold provides additional protection for the silver and enhances wire-bonding performance. With a thin coating of immersion gold, the underlying silver is protected from tarnishing.

In general, the ISIG process presents a more controllable and efficient final finish for 5G/6G. The result of a nitrate-free immersion silver process is a deposit with insertion loss identical to copper, and now providing protection very close to electroless nickel. This makes nitrate-free immersion silver a viable process for 5G and millimeter-wave applications well into the future.

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