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

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

The reactions start simultaneously with the introduction of the substrate (nickel, palladium, or copper). Because the immersion reaction is a displacement reaction, it diminishes over time as the substrate becomes less available for deposition. The electroless reaction continues, relying on the reducing agent present in the electrolyte, rather than on substrate availability. RAIG gold is limited in how much gold it can deposit, as compared to a pure electroless gold. It is ideally suited for deposits of 3–6 µin.

Curbing the immersion reaction and allowing gold thickness to build autocatalytically expands the operating window by allowing thicker gold to deposit without nickel corrosion. A thicker gold layer (3–6 µin), is beyond the capability of standard immersion gold electrolytes, but is desirable for many applications, as it widens the operating window for gold wire bonding.

Uyemura_RAIG.jpg

Figure 1: The effect of gold thickness on the operating window of the bonding force.

Figure 1 shows the relationship between gold thickness and the strength of the wire bond. Two thicknesses of gold, 0.05 mm (2.0 µin) and 0.2 mm (8.0 µin) were bonded using a 0.25 mm gold wire at three different bonding forces (25, 50 and 75g force). Ultrasonic power (mW) and time (sec) were held constant. The wires were then pulled and the break point in the wire was recorded. Bond lifts (E) and brakes at the heel of the bond (D) indicated weak or non-reliable bonds. Break points (B and C) were indicative of a reliable bond.

The data shows that both thicknesses could produce robust joints. However, the thinner (0.05 mm) gold required a higher gram force to form a reliable bond. The thicker gold (0.2 mm) produced reliable bonds at lower gram force, opening the operating window for the bonding parameter.

For gold wire bonding applications, designers prefer to specify a gold thickness of 3–5 µin, which is beyond the capability of immersion gold. Increasing dwell time in an immersion gold bath is the most common cause of nickel corrosion.

RAIG Applications
RAIG gold is an immersion gold replacement for ENIG, ENEPIG and EPIG. It meets the design requirement for thicker gold with no nickel corrosion, in a single plating step.

ENIG
ENIG is a common gold surface finish that is solderable and aluminum wire bondable. It is also used as a contacting surface. ENIG is non-electrolytic and does not require bussing or connectivity during plating. It is ideal for SMT pads. The choice of chemicals and the conditions of plating are important to avoid corrosion of the nickel. Nickel corrosion, also referred to as “black pad,” occurs in the gold bath. Immersion gold deposition is, in essence, a corrosion or displacement reaction. If conditions are not controlled, nickel may go into solution at the expense of hydrogen ions without gold deposition, creating nickel corrosion.

Under controlled plating conditions, nickel corrosion does not occur. It can occur, however, if the nickel deposit is uneven, the gold bath pH is too low, the gold concentration is below spec or the dwell time in the bath is too long. Such conditions are usually avoidable, except when the design requires thicker immersion gold (in excess of 2.5 µin).

Some manufacturers attempt to achieve this thickness by extending the dwell time in the gold bath, however, immersion gold baths are not designed to deposit more than 2.5 µin. If higher gold (3–5 µin immersion gold (RAIG).

Electroless gold is not common at PWB manufacturing sites. Electroless gold requires an immersion gold strike prior to electroless deposition. Adding another gold bath is a costly proposition. RAIG gold is a single bath that can deposit 3–5 microinches of gold without corrosion.

ENEPIG
This finish is gaining a lot of traction. ENEPIG forms the most reliable solder joint with lead-free solder and is also a gold-wire-bondable surface. In a previous column, I discussed the possibility of nickel corrosion beneath the palladium layer under certain condition, one being extended dwell time in an immersion gold bath to achieve higher gold thickness. The use of RAIG gold allows for a thicker deposit without nickel corrosion.

EPIG vs. EPAG
EPIG (electroless palladium/immersion gold) and EPAG (electroless palladium autocatalytic (electroless) gold) are nickel-free surface finishes that prevent the signal loss associated with electroless nickel deposits. They are widely valued for high frequency RF applications.

As discussed, thicker gold (greater than immersion gold) increases the wire bond operating window. EPAG is promoted as the answer to the thickness limitations of immersion gold. Autocatalytic gold requires an immersion gold underlay and is a difficult bath to control. RAIG gold deposits a higher gold thickness than immersion gold in a single gold plating step. The hybrid EPIG/EPAG using RAIG gold produces the desired thickness without the complexity of an additional electroless bath.

The concept of reduction-assisted immersion gold RAIG baths is coming into its own as more manufacturers and OEMs recognize its benefits, most notably the ability to deposit a thicker layer of gold (3–6 µin) with no corrosion of the underlying nickel, in a single gold plating step.

This column originally appeared in the March 2021 issue of PCB007 Magazine.

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2021

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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