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74 PCB007 MAGAZINE I MAY 2020 Nickel palladium gold (ENEPIG) surface fin- ish is being referred to as the "universal fin- ish." The finish is an excellent soldering sur- face, forming a Ni/Sn intermetallic (IMC). It is a wire-bondable surface for both gold and aluminum wire. It also serves as a good con- tacting surface. ENEPIG was also the answer to the nickel corrosion "black pad" occasionally encountered with electroless nickel/immer- sion gold (ENIG) deposits. As the finish gained more market share due to its flexibility—particularly as a gold wire bonding surface—occasional bond failure was observed in cases where the dwell time in the immersion gold was extended in an attempt to meet design requirements of > 3.0 µins (0.075 µm) of gold. The failure was manifested as wire bond lifts. Failure analysis of the failed bonds showed a separation at the Ni/Pd interface. The nickel surface was black, and was clear- ly corroded "black pad." Nickel corrosion in ENIG occurs in the immersion gold deposition step and is usually the result of a compromised nickel surface (uneven), in combination with an aggressive immersion gold bath (low gold concentration, low pH) and an extended dwell time in the gold bath. The extended dwell time is used in instances where the design require- ments demand a thicker immersion gold. How can the nickel capped with an electroless palla- dium layer corrode when it is theoretically not available to the immersion gold step? Can 'Nickel Corrosion' Occur in ENEPIG? The Plating Forum by George Milad, UYEMURA

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