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32 I-CONNECT007 MAGAZINE I JUNE 2026 particular metal) influence attenuation loss at higher frequencies. Metal finishes have different conduc- tivity 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 fre- quencies, engineers are seeking final finishes that satisfy both electrical and assembly requirements, and several alternative finishes are now commer- cially 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 solder- able finishes such as Electroless Nickel-Immersion Gold (ENIG) or Electroless Nickel-Palladium-Immer- sion 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 cir- cuit designs • They support wire bonding and solderability T RO U B L E I N YO U R TA N K Table 1: Approximate conductivity of metals METAL CONDUCTIVITY VALUE Silver 6.301 x 10 7 S/m Copper 5.817 x 10 7 S/m Gold 4.520 x 10 7 S/m Nickel 1.500 X 10 7 S/m Tin 0.870 x 10 7 S/m