The Plating Forum: Immersion Plating Reaction in Electronics Manufacturing

Plating or metal deposition is a key component in the manufacturing of electronic packages (circuit boards and integrated circuits). Plating occurs when a metal ion in solution (electrolyte) is reduced to the metal. The reduction takes place when electrons are supplied to the ion:

milad1.jpg

The source of electrons differs with the type of plating as follows:

  • Electrolytic plating: Electrons supplied by an external power supply (rectifier)
  • Electroless plating: Electrons are supplied by a reducing chemical agent that is present in the electrolyte
  • Immersion plating: Electrons are supplied by the oxidation of the (metal) substrate

This column will be dedicated to the immersion reaction.

Immersion reactions occur between metals according to their location in the electromotive (EMF) series. The EMF series is a listing of the elements according to their half potentials or tendency to lose electrons (get oxidized), or their tendency to gain an electron (get reduced), measured in voltage. Elements that lose electrons (reducers) have negative voltage values, and elements that gain electrons (oxidizers) have positive Eo voltage values.
miladT1.jpg

Table 1 shows the half potentials Eo measured in volts of common elements, and metals are ranked with respect to their inherent reactivity. The metals located at the top of the series are considered the noblest with the highest level of positive electrochemical potential. 

The electromotive force that drives an immersion reaction is determined by the difference in Eo (half potential) between the reduced and oxidized elements. The reduced element is the element receiving electrons and is derived as follows:
milad2.jpg

If the cell potential is positive, the reaction is spontaneous. A negative voltage indicates that no reaction will take place. 

In printed circuit fabrication, immersion plating plays a key role in the following processes:
Immersion gold in electroless nickel/immersion gold (ENIG)

  • Corrosion in ENIG
  • Immersion gold in electroless nickel/electroless palladium/immersion gold (ENEPIG)
  • Palladium catalyst on copper
  • Immersion silver
  • Immersion tin

Immersion Gold in ENIG
The deposition of gold on the electroless nickel substrate is an immersion reaction. The presence of nickel metal in an electrolyte containing gold ions creates a spontaneous deposition reaction. The EMF of the cell comes up to + 1.75 v, and the difference between the half potentials of gold +1.5 v (reduced species) and nickel –0.25 v (oxidized species) can be expressed as follows.
ENIG reaction:
milad3.jpg
Corrosion in ENIG
Corrosion may occur when the electrons released by the oxidation of nickel reduces the hydrogen ion present in solution releasing hydrogen gas. The EMF of the cell is + 0.25 v and is the difference between the half potentials of hydrogen (0.00 v) and nickel (–0.25 v).

Electroless nickel/hydrogen ion corrosion reaction:
milad4.jpg
The EMF driving force for this reaction is only 14% of the EMF for the gold deposition reaction and would only occur if the availability of the gold ion is interfered with. An example would be localized areas (crevices) where gold is depleted.

Nickel corrosion can be mitigated by:

  • Eliminating crevices in the nickel 
  • Reducing the acidity of the electrolyte (reduced hydrogen ion availability)
  • Reducing the half potential of nickel by increasing its phosphorous content

Immersion Gold in ENEPIG
Electroless palladium/immersion gold reaction:
milad5.jpg
In this galvanic cell, the gold is the reduced species, and the palladium is the oxidized species. Comparing the EMF of this cell to the EMF of the nickel/gold cell, it is clear that this reaction is less driven and would proceed slower than the deposition of gold on nickel. This creates a problem if the underlying nickel is accessible to the gold electrolyte. In this case, the gold would exchange with the nickel layer under the palladium and nickel corrosion would occur. For mitigation of nickel corrosion, refer to my previous column titled Can ‘Nickel Corrosion’ Occur In ENEPIG?

Palladium Catalyst on Copper
For an immersion palladium on copper reaction:
milad6.jpg
Immersion palladium on copper is an integral part of electroless nickel (EN) deposition. For EN to initiate on the copper substrate, the copper surface must be catalyzed. The catalyst is immersion palladium. In this galvanic cell, the palladium is the reduced species, and the copper is the oxidized species. Palladium  
sits below gold and above copper in the EMF series. Immersion palladium on copper is a spontaneous reaction driven by + 0.72 v. A palladium deposit is specific to the copper substrate and will not deposit on laminate or solder mask.

The uniformity of the palladium catalyst layer is critical. The uniformity depends on the pre-treatment of the copper surface, which must be free of oxidation or any contaminants. An uneven palladium distribution would lead to uneven EN initiation, generating crevices in the EN deposit, which are potential sites for nickel corrosion.

Immersion silver on copper:
milad7.jpg
The driving force for this reaction is + 0.46 v and would proceed spontaneously. Although the reaction is straight forward, the design of the electrolyte for immersion silver is a differentiating point from one supplier to the other and may include anti-tarnish components.

Immersion tin on copper:
milad8.jpg
Replacement reaction between Cu and Sn2+ cannot occur in a standard electrolyte because the EMF voltage is negative (- 0.48 v). The half potential of Cu is + 0.337 v, which is much higher than that of Sn is – 0.136 v. 

For tin to immerse on copper, the half potential for copper must be reduced below the half potential of tin. The addition of a copper ion complexing agent, such as thiourea, will decreases the half potential of copper to – 0.620 v. 
(E0 for Cu[SC(NH2)2]4  = – 0.620 v).

In the presence of thiourea, the EMF for the immersion reaction is positive and would proceed spontaneously:
milad9.jpg
A good understanding of the principles of immersion plating goes a long way in eliminating costly defects that may occur during fabrication. It is a powerful tool in analyzing failure, as well as the subsequent assignable cause, and recommending corrective action.   

This column originally appeared in the September 2020 issue of PCB007 Magazine.

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2020

The Plating Forum: Immersion Plating Reaction in Electronics Manufacturing

09-16-2020

Plating or metal deposition is a key component in the manufacturing of electronic packages (circuit boards and integrated circuits). Plating occurs when a metal ion in solution (electrolyte) is reduced to the metal. The reduction takes place when electrons are supplied to the ion. George Milad dedicates this column to the immersion reaction.

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The Plating Forum: Minimizing Signal Transmission Loss in High-Frequency Circuits

07-06-2020

All PCB materials have both conduction and dielectric RF signal losses. In this column, George Milad highlights resistive conduction losses by the copper layer used in the board.

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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: Eliminating Waste From Electrolytic Acid Copper Plating

03-15-2020

Acid copper plating in most shops is done in vertical plating tanks. Acid copper solutions are not dumped but are continuously used with occasional carbon treatment to remove organic build-up from the additives and from dry film leaching. George Milad explains.

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

01-11-2020

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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2019

The Plating Forum: New Developments in ENIG

12-08-2019

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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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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