The Plating Forum: Can ‘Nickel Corrosion’ Occur in ENEPIG?

Nickel palladium gold (ENEPIG) surface finish is being referred to as the “universal finish.” The finish is an excellent soldering surface, forming a Ni/Sn intermetallic (IMC). It is a wire-bondable surface for both gold and aluminum wire. It also serves as a good contacting surface. ENEPIG was also the answer to the nickel corrosion “black pad” occasionally encountered with electroless nickel/immersion 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 clearly 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 requirements demand a thicker immersion gold. How can the nickel capped with an electroless palladium layer corrode when it is theoretically not available to the immersion gold step?

Immersion gold deposition is a displacement reaction where one atom of nickel metal is oxidized to the nickel ion giving up two electrons. The two electrons are picked up by two positively charged gold ions in solution and, in turn, are reduced to the metal and deposited on the surface of the nickel substrate.

The driving force of the oxidation-reduction displacement reactions can be derived from the electromotive series. The electromotive series is a listing of chemical species (atoms, molecules, and ions) in the order of their tendency to gain or lose electrons (be reduced or oxidized, respectively), expressed in volts and measured with reference to the hydrogen electrode—which is taken as a standard and arbitrarily assigned the voltage of 1.0 v.

To read this entire column, which appeared in the May 2020 issue of PCB007 Magazine, click here.

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2020

The Plating Forum: Can ‘Nickel Corrosion’ Occur in ENEPIG?

05-25-2020

Nickel palladium gold (ENEPIG) surface finish is being referred to as the “universal finish.” ENEPIG was also the answer to the nickel corrosion “black pad” encountered occasionally with electroless nickel/immersion gold (ENIG) deposits. In this column, George Milad answers the question, "Can 'nickel corrosion' occur in ENEPIG?"

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The Plating Forum: EPIG—A Nickel-free Surface Finish for Next-generation Products

03-11-2020

In recent years, electronic devices, such as smartphones and tablet PCs, have been miniaturized. Chip-size package (CSP) used inside the electronic devices have been miniaturized as well, and the spacing between the lines continues to diminish every year. Some of the latest packages have spacing as little as 15 µm or less. If electroless nickel electroless palladium immersion gold (ENEPIG) is used with an EN thickness of 5–6 µm, only 5 µm of spacing would be left, increasing the risk of shorts between the traces. George Milad explains.

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The Plating Forum: New Developments in ENIG

01-08-2020

ENIG has been around the printed circuit industry for more than 25 years. George Milad provides an update and explains how although the occurrence of corrosion was recognized, a better understanding of the defect has led to a series of improvements over time.

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2019

The Plating Forum: Update on IPC-4552 ENIG Specification Revisions

10-20-2019

George Milad's columns will cover PCB plating, IPC specifications, and more. In this debut installment, he gives us an update on the IPC-4552 ENIG specification, including Revision A and B.

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2014

The Plating Forum: Wire Bonding to ENIG

03-05-2014

The IPC-4552 ENIG specification was written in 2002, but the committee is currently updating and revising the document. The thickness of the immersion gold layer is being revised with the intent of reducing the minimum thickness from 2.0 µin to 1.6 µin. A series of studies were conducted to find out if this reduction is possible.

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The Plating Forum: ENIG and the Plating Process

01-07-2014

ENIG continues to gain market share due to its versatility in a wide range of component assembly methods including solder fusing, wave soldering, and wire bonding. The plating of ENIG is a complex multi-step process. Each process step is carefully designed and must be well understood and controlled to produce the desired end product. George Milad reports.

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2013

Acid Copper Plating for High Aspect Ratio and Via Fill

07-16-2013

To meet new specification requirements, board shops are forced to seek new and advanced processes in every department. Acid copper plating comes under heavy scrutiny, as it is the process that forms the traces and the through-hole connectivity that conveys the signal from end-to-end of the final device. George Milad, a new columnist for The PCB Magazine, explains.

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