The Plating Forum: Surface Finish Evolution from Conventional to Advanced

Surface finish is a critical component in PCB design and functionality. Surface finishes form the interface between the component and the circuitry. As its most essential function, the final finish process is intended to provide exposed copper circuitry with a protective coating to preserve solderability. Select surface finishes are also used for wire bonding or as an electrical contacting surface. Perhaps no other step in the PCB manufacturing process has undergone more change in the era of surface mount manufacturing than the final finish chemical process.

Electrical signals are transmitted through copper circuitry connecting the different components throughout the finished board. Copper is a highly conductive metal but one of its shortcomings is that it oxidizes when exposed to ambient air. An oxidized copper surface will not solder and is a poor conductor with no possibility of wire-bonding. Surface finish is applied to ensure no exposed copper is in the final product. Soldering, wire bonding, and contacting are all made with the finished surface.

Today there is a wide variety of finishes that board designers can specify to meet the desired functionality and intended use of their circuit boards:

  • A reliable solder joint with leaded and lead-free solder interface
  • Bonding surface Al, Au, Cu, Cu/Pd
  • Contacting surface insertion
  • A finish that will not impede RF signal transmission

When soldering of through-holes was the only desired attribute, three SFs dominated in manufacturing facilities. These were hot air solder leveling (HASL) and reflowed tin/lead for surface contacting (insertion) and electrolytic tab plating of nickel/gold as needed. As solder mask over bare copper (SMOBC) made its debut, reflowed tin/lead fell out of favor.

In the next generation (lighter, smaller, and faster), two major manufacturing developments dominated PCB designs, namely surface mount technology (SMT) and ball grid array (BGA) to meet the requirements of newer designs.

SMT and BGA created challenges at assembly. Coplanarity was an absolute must. HASL formed a meniscus that interfered with the application of solder paste on surface mount pads.

This requirement made organic solderability preservative (OSP) and electroless nickel/immersion gold (ENIG) come to the forefront for these applications.

The next major evolution in PWB manufacturing was the elimination of lead from solder. A new generation of lead-free (LF) solder—tin/silver/copper (SAC) alloys—were the prevailing replacement for tin/lead. The SAC family of alloys have a melting point of 217–219oC, with a peak liquidus temperature of 240oC for complete wetting and for forming a consistent intermetallic compound (IMC), as compared to tin/lead which averaged 187oC for a melting point and a peak soldering temperature of 215oC.

To accommodate the elimination of lead from solder, equipment makers made design changes to accommodate the higher reflow temperature of LF alloys. Although HASL only provides a soldering surface, it remains a viable surface finish today for products that have adequate spacing between pads and do not require contacting or bonding. In the same way, OSP suppliers were able to produce the next generation, namely OSP-HT (high temperature). OSP and OSP-HT are widely used worldwide.

For designs that require high temperature soldering and coplanarity, immersion silver and immersion tin filled that need. However, neither could meet all the demands of solder joint reliability and long shelf life. Silver is susceptible to tarnishing and creep corrosion, and tin requires a thick immersion coating to retain its solderability. Over time, copper will diffuse into the immersion tin, forming a non-solderable Cu/Sn IMC. Both immersion silver and immersion tin remain viable surface finishes with allowances made to overcome their shortcomings.

ENIG, although it is a more complex and more costly process compared to immersion silver and immersion tin, was successful in filling the need for a surface finish that is solderable with LF solder, aluminum wire bondable, and a good contacting surface with an extended shelf life. ENIG had a challenging start when it was first introduced; there were incidents of nickel corrosion under the immersion gold. The corrosion, if excessive, would interfere with IMC formation and the affected part would fail to form a reliable solder joint.

IPC ENIG Specification 4552 Rev B, issued in 2021, spelled out a method to evaluate and measure the extent of ENIG nickel corrosion. Now that there is a method to measure and quantify nickel corrosion, the defect is on its way to being eliminated. “You can’t fix what you can’t measure.”

ENIG remained a popular finish for parts that required its attributes. The next challenge was the need for an additional attribute and that was gold (Au) wire bonding. ENIG is not Au wire bondable because of the possible diffusion of nickel into the thin immersion gold layer. A diffusion barrier was needed to prevent the nickel from reaching the surface. Electroless palladium was the answer and ENEPIG (electroless nickel/electroless palladium/immersion gold) was the right finish when the desired attributes included Au wire bonding.

As military and aerospace boards continued to advance into RF signal propagation, there was a need for a new class of finishes that did not include electroless nickel. RF signals travel along the surface of the trace and the presence of electroless nickel would interfere with high frequency (RF) signal propagation.

One way to achieve this was to dramatically reduce the thickness of the nickel layer in ENEPIG to below 0.1 micron. Other finishes, including EPIG or EPAG (electroless palladium/immersion gold or electroless palladium/autocatalytic gold), eliminated the nickel completely. These finishes relied on catalyzing the copper surface with immersion palladium; a more advanced system to achieve the same goal is using immersion gold to catalyze the copper surface. IGEPIG (immersion gold/electroless palladium/immersion gold) was shown to provide a more reliable solder joint.

Still another option was to also eliminate the palladium and immerse gold directly on copper, immersion gold (DIG). A new development in DIG is reduction-assisted immersion gold. RAIG deposits a thicker gold layer (6-8 µins) that prevents the diffusion of copper into the gold wire bonding surface.

Newer and advanced surface finishes are constantly being researched, tested, and implemented. It is important to keep in mind that all the SFs mentioned above are presently used in board fabrication. More advanced finishes are beginning to make headway as the higher technology boards continue to use high RF signals in their designs. Stay tuned.

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

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2022

The Plating Forum: Surface Finish Evolution from Conventional to Advanced

11-15-2022

Surface finish is a critical component in printed circuit board (PCB) design and functionality. Surface finishes form the interface between the component and the circuitry. As its most essential function, the final finish process is intended to provide exposed copper circuitry with a protective coating to preserve solderability. Select surface finishes are also used for wire bonding or as an electrical contacting surface. Perhaps no other step in the PCB manufacturing process has undergone more change in the era of surface mount manufacturing than the final finish chemical process.

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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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