The Plating Forum: Plating in Electronic Applications

Plating is the deposition of a metal layer on a substrate to modify its properties. It occurs when the metal ion in an aqueous solution is reduced to the metal:

M+ metal ion +  e- reducing electron = M0 deposited metal

Plating can achieve numerous functional and aesthetic goals, including:

  • Improve solderability
  • Inhibit corrosion
  • Increase hardness/durability
  • Decorate objects (jewelry)
  • Reduce friction
  • Alter conductivity
  • Improve IR reflectivity
  • Provide radiation shielding

The reducing electron e- can be supplied from these deposition reactions:

  • Electroplating
  • Electroless plating
  • Immersion plating

Electroplating
This is a galvanic reaction that occurs when current is applied to an anode and a cathode immersed in the metal electrolyte. The anode is composed of the desired metal for deposition, typically copper, tin, or nickel. When voltage or current is applied, the metal at the anode will dissolve as a positive ion, leaving a negative charge at the anode.

At the cathode, the positive ion will be reduced to the metal, taking up electrons from the cathode which is rendered positive. The deposited weight depends on the current and the time, and is governed by Faraday’s law. Electroplating requires connectivity. It is used to plate metal alloys, and in dispersion plating. The latter involves the uniform dispersion of a non-conductive entity in the electroplated metal.

Electroless Plating
In electroless plating, the negative electron is supplied by a reducing agent present in the electrolyte. The reducing agent is oxidized, and the metallic target ion is reduced to the metal. Plating continues as long as the reducing agent and the target ion concentration are maintained through replenishment. Electroless plating is independent of connectivity. Examples are electroless copper and electroless nickel, electroless gold, and electroless palladium.

Immersion Plating
In immersion plating, the reducing electron is supplied by the substrate where the deposition is occurring. The substrate metal is oxidized to the metal ion giving up an electron. The electron reduces the target ion to the metal for deposition. Only metals higher in the EMF series can replace metals below them. Immersion deposition is a displacement reaction and is limited to the availability of the substrate being plated. Examples include immersion silver, immersion tin, immersion gold, and others.

The general principles of all three methods are well understood and have been used for decades in electronics manufacturing. Refining the plating process to meet specific electronics requirements is the challenge faced by chemical manufacturers as complexity and miniaturization continue to increase. The following examples demonstrate how plating processes are modified and adapted to meet specific design requirements.

Copper Plating
Copper is a highly conductive non-precious metal. It is the conduit that carries the flow of current through printed wiring boards and semiconductors (integrated circuits) devices. Electroplated copper is the dominant method for plating traces and through-holes.

The challenge for electroplated copper in electronics is achieving consistent quality and thickness distribution throughout the part. Controlling the quality of the deposited copper (tensile strength and elongation) ensures that the deposit will not crack or fracture with the thermal excursion the parts are exposed to during manufacture and throughout their life cycle. Thickness distribution is critical for controlling impedance throughout the device.

The use of additives (brightener, suppressor, and leveler) in the electrolyte, plating current density, and plating cell geometry are designed and manipulated to achieve a specific outcome, such as via filling, plating high aspect-ratio holes, and plating fine lines and spaces.

Surface Finishes
Surface finishing of electronic products is an integral part of manufacturing. It serves many functions, the most prominent of which is to create a solderable surface. It is also used as a corrosion barrier and an electrical contacting surface, as well as a wire bonding surface. One or more of the three methods of plating are used to create the desired surface. The layer may be a single component such as immersion silver, tin, gold, or palladium, or multiple consequential layers such as electroless nickel/immersion gold (ENIG), electroless nickel/electroless palladium/immersion gold (ENEPIG) and electroplated nickel/gold.

The choice of what thickness of metal to plate and what plating method to use is always based on meeting specific design requirements with the lowest cost. For example, solderable and gold wire-bondable finishes limit the choice of deposit. At the moment, the leading finish for this application is ENEPIG. However, if the part is used for high RF (>20 GHz) signal propagation, nickel must be eliminated to avoid signal loss. This makes electroless palladium/immersion gold (EPIG) the appropriate alternative.

Automotive
As electric vehicles continue to grow in popularity, there is great demand for EV surface finishes that provide:

  • Wear resistance and corrosion resistance
  • High number of plugging cycles (10K+…)
  • High electrical and thermal conductivity 
  • Elimination of lubricant 
  • Long-term, stable hardness (1000h → end of life) 
  • Temperature-stable hardness (150°C → 180°C)
  • Extended contact/connector life

Silver is fundamental for electronic components used in electric vehicles, charging stations, and power grids, but to deliver the capabilities required, silver must be modified. Studies of various alloys found none were able to meet the criteria listed above.     

One of the ways to achieve the desired performance is dispersive plating, specifically with graphite as the dispersion element due to its availability and lower cost compared to alternatives. Dispersing graphite in the silver deposit produced the desired tribology attributes (wear friction and lubrication), hardness and conductivity.

Chemical suppliers to the electronics industry are continuously adapting the plating processes described to achieve the appropriate thickness of metal or metals (alloy or dispersion metal) in single or multiple consecutive layers. Meeting the design functionality at the lowest cost will always control the choice of plating material and plating options.

This column originally appeared in the June 2022 issue of PCB007 Magazine.

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2022

The Plating Forum: Plating in Electronic Applications

07-06-2022

Plating is the deposition of a metal layer on a substrate in order to modify its properties. It occurs when the metal ion in an aqueous solution is reduced to the metal: M+ metal ion + e- reducing electron = M0 deposited metal. Plating can achieve numerous functional and aesthetic goals, including: improve solderability, inhibit corrosion, increase hardness/durability, decorate objects (jewelry), reduce friction, alter conductivity, improve IR reflectivity, provide radiation shielding.

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The Plating Forum: How the Pandemic Impacted PCB Manufacturing

03-30-2022

The coronavirus pandemic has had a major impact on PCB manufacturing and assembly. Thanks to its classification as an essential business associated with national security, PCB manufacturing in the U.S. was exempt from shutdowns; it was not, however, immune from supply chain disruption. Raw materials shortages set the stage for higher prices. Companies that relied on just in time (Kanban) inventory management held back product, further burdening the supply chain.

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2021

The Plating Forum: The Significance of IPC ENIG Specification 4552 Revision B

11-24-2021

The ENIG specification 4552 was issued in 2002. Since then, it has gone through a series of amendments and revisions in an attempt to meet the everchanging industry requirements. It started as a thickness specification that did not mention lead-free soldering or “nickel corrosion” and ended in the latest performance specification 4552B where all aspects of nickel corrosion were addressed. Suppliers now have a way to evaluate the performance of products in the field. They can increase the robustness of their products and service to ensure that customers can produce acceptable ENIG finishes in different manufacturing environments.

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The Plating Forum: The IPC Surface Finish Specifications

10-06-2021

Specifications are reference documents to be called out by OEM board designers in specifying the attributes of a surface finish. Designers may take exception with one or more items in the specification to ensure that the product meets the requirements of its intended use. The term “AAUBUS” (As Agreed Upon Between User and Supplier) is part of any specification.

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The Plating Forum: An Overview of Surface Finishes

09-06-2021

Surface finishes’ research and development departments on the supplier side have been very busy coming up with new finishes to meet the everchanging demands of the electronics industry. Today, designers have wide variety of finishes to choose from. George Milad breaks it down.

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The Plating Forum: DIG—The Next Generation

06-16-2021

DIG stands for “Direct Immersion Gold.” The acronym is used to specify direct deposition of gold on copper as a surface finish. It is a metallic solderable finish. At assembly, DIG forms a Cu/Sn intermetallic with the gold layer dissipating into the bulk solder.

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The Plating Forum: RAIG (Reduction Assisted Immersion Gold) for Gold Surface Finishes

04-05-2021

RAIG was introduced a few years ago to meet the requirements of newer designs. Since its inception, more gold finishes are finding RAIG gold to be a viable alternative to standard immersion gold. RAIG gold is a mixed reaction bath that functions as an immersion gold and with the added reducing agent it also functions as an electroless (autocatalytic) bath.

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2020

The Plating Forum: Training for Plating Processes in the Electronics Industry

12-24-2020

Plating is a very old industry and has been studied for many generations. Its basic principles are well understood and documented. However, when it comes to the intricate details of plating a circuit board, there is so much to learn and apply. George Milad explains.

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The Plating Forum: Via Plating for PWBs

11-19-2020

Vias are an integral part of PWB design and manufacturing. They are the means by which different layers of a board are connected. George Milad addresses the electroplating of vias, including the three main types of vias: through-hole vias, buried vias, and blind vias.

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The Plating Forum: The Critical Role of Pretreatment for Plating

10-22-2020

Pretreatment is usually customized to the incoming substrate and the plated metal. George Milad explains how it is a critical step and must be completed before plating to achieve the desired adhesion and to enhance the quality of the deposited metal.

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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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