Balancing environmental exposure, component type, shelf life, production volume and regulatory compliance – and cost.

While printed circuit board designers often focus on the circuit layout and picking the right components, one key detail short on attention is the surface finish on the PCB. This thin coating significantly impacts a board’s performance, durability and reliability.

Surface finishes come in many styles, each designed for different conditions, budgets and compatibility needs. Here we explore those finishes, their types and how to select an appropriate one for a PCB.

Before choosing, understand what a surface finish is and why it matters. A surface finish consists of the thin layer of coating applied to copper surfaces on a PCB. Without it, exposure to the elements would cause the copper to oxidize and corrode. This can lead the PCB to develop catastrophic faults. This finish serves several crucial functions:

• Shields the copper from oxidation and corrosion, for board longevity.
• Provides a surface suitable for soldering components.
• Maintains good electrical conductivity for component connections.
• Enhances the PCB’s visual appeal.

Figure 1. A side-by-side view of several surface finish options.

Picking the right finish affects all these outcomes, so it’s a decision worth getting right during manufacturing. Some of the most popular PCB surface finish options available in the market include:

Hot air solder leveling (HASL). The HASL process involves dipping (typically) the PCB into a bath of molten solder to cover all exposed copper surfaces with solder. Excess solder is removed by passing the PCB between hot air knives.

Advantages:

• Excellent solderability; the solder coating improves the ability of components to be soldered to the board.
• Longer shelf life compared to other finishes like OSP.
• Relatively inexpensive and readily available.
• Provides a consistent, even layer of solder, promoting reliable solder joints.

Disadvantages:

• Unevenness of HASL finish can be a problem for fine-pitch surface mount technology (SMT) components, making it unsuitable for certain applications.
• Traditional HASL uses a tin-lead alloy, which can run afoul of certain environmental regulations. (Lead-free HASL options are available.)
• Thermal stress; heat involved in HASL process can potentially stress the laminate.
• Not suitable for high-frequency applications.
• Not suitable for PTH (plated through-hole) components.

Electroless nickel immersion gold (ENIG). ENIG consists of an electroless nickel plating covered with a thin layer of immersion gold, which protects the nickel from oxidation. This combination provides corrosion resistance, solderability and good electrical properties. ENIG is commonly used to protect copper pads and plated through-holes from oxidation and to facilitate soldering.

Advantages:

• Excellent surface planarity makes it particularly suitable for fine-pitch components.
• Nickel layer acts as a robust barrier to prevent copper from oxidizing, while the thin layer of gold protects nickel from oxidation.
• Good for high-frequency applications.
• Excellent for wire bonding applications because it provides a highly conductive surface that is easy to bond to.
• Suitable for multiple reflow cycles, making it a good choice for applications that require repeated assembly.
• Long shelf life (more than1 year in control condition).

Disadvantages:

• Higher cost relative to other surface finishes such as HASL or OSP (organic solderability preservative). ENIG process involves deposition of both nickel and gold layers, which increases cost.
• Potential for black pad, a defect that can occur during electroless nickel deposition. This defect is characterized by a brittle, nonuniform nickel layer, which can lead to poor solder joint reliability and potential PCB failure.
• Susceptible to thermal shock, which can cause the nickel layer to crack and peel, leading to poor adhesion and other problems.
• Subpar for rework and makes PCB repair difficult.
• Less suitable for high-current applications.

Immersion tin (ImSn). Immersion tin is a metallic finish deposited by a chemical displacement reaction that is applied directly over the basis metal of the circuit board, copper. ImSn protects the underlying copper from oxidation over its intended shelf life. It’s more affordable than ENIG and immersion silver, and is RoHS-compliant. A typical thickness range for immersion tin is 20-50µm. Due to interaction between tin and copper, they eventually diffuse into one another.

Advantages:

• Good solderability; the tin finish readily wets and solders just like bare copper, unlike HASL or ENIG.
• Flat surface suitable for fine-pitch components.
• More affordable than electroless nickel-immersion gold.
• Preferable for press fit pin insertion.

Disadvantages:

• Shorter shelf life than ENIG due to tin whisker growth.
• Potential for copper dissolution during the plating process.
• Not ideal for multiple reflow cycles/assembly processes.
• Can lead to tin whiskers.

Immersion silver (ImAg). Immersion silver is applied directly to the base metal of a PCB via chemical displacement. It’s more affordable than ENIG and is RoHS-compliant. A typical ImAg thickness is 4-12µm. Due to the way copper and silver interact, they eventually diffuse into one another.

Advantages:

• Excellent solderability.
• Good for high-frequency applications.
• Less expensive than immersion gold.
• Flat surface suitable for fine-pitch components.

Disadvantages:

• Prone to oxidation and tarnishing.
• Requires careful handling and storage.
• Higher cost compared to HASL.
• Typical short life is short – six to 12 months under dry storage conditions. After this period, solderability may become a concern.
• Left unprotected from environmental influences, silver reacts with sulphur compounds, like those found in air, to form a black coating of silver sulphide (Ag₂S). This reaction causes silver to tarnish or become dull. This phenomenon is known as corrosion, and specifically for silver is called tarnishing.

Organic solderability preservative (OSP). OSP is an organic, water-based compound. It offers a flat finish and provides the advantage of being lead-free. As the name indicates, this organic coating covers the copper traces of a PCB to protect them from oxidation and maintain solderability. OSP contains an organic acid that reacts with the copper surface to form a protective layer only a few atoms thick. This layer blocks air from reaching the copper and prevents oxidation.

Advantages:

• Has a small tolerance compared with HASL. Is used for fine-pitch PCB assembly and BGA PCBs.
• Good reflow solderability. OSP is removed completely, and bare PCB pads are directly soldered with surface-mount components.
• Is the least expensive surface finish and applies easily at room temperature.
• Environmentally friendly, is lead-free and contains no toxic substances, making it environmentally friendly.
• Good for high-frequency applications.
• Provides good contact resistance for ICT testing, Probe pins do not damage soft OSP coating.

Disadvantages:

• Limited shelf life (three to six months).
• Unsuitable for applications requiring multiple reflow cycles.
• Cannot be used for plated through-holes.
• Less robust than metal finishes.

Hard gold. A hard gold finish, also known as electrolytic hard gold, is an electroplated gold layer that is thicker and more durable than finishes like ENIG. It is alloyed with metals such as nickel or cobalt to enhance hardness and wear resistance, for high-contact areas that require long-lasting electrical connections.

Hard gold applications are extremely durable and enjoy a long shelf life. They’re commonly reserved for components that expect a substantial amount of use. It’s not often used for soldering points, due to poor solderability. Common applications include edge connectors, battery contacts, test boards and keyboard contacts, all of which require robust, high-wear connections for data transmission and electrical continuity.

Advantages:

• Extremely durable.
• Excellent for applications requiring multiple insertions (e.g., edge connectors).
• Very good corrosion resistance.
• RoHS compliant.
• An excellent conductor, ensuring reliable electrical connections.

Disadvantages:

• The most expensive option.
• Overkill for many standard applications.
• Can cause solderability issues if not properly applied.
• Hard gold is not easily soldered, making it less suitable for applications requiring solderability.
• Hard gold is not recommended for plated through-holes (PTH).

ENEPIG. ENEPIG (electroless nickel, electroless palladium, immersion gold), a three-layer coating on the copper pads, consists of a nickel base, followed by a palladium layer and a top layer of gold, all applied through electroless chemical processes. It is similar to ENIG (electroless nickel immersion gold), with a palladium layer added to the mix. The combination of gold and palladium is more cost-effective than pure gold and more durable than other finishes.

Advantages:

• Palladium layer enhances wire bonding capabilities over ENIG, especially for gold and aluminium wire bonding.
• Multiple reflow soldering capability.
• Excellent solderability.
• Combination of nickel, palladium and gold provides robust corrosion resistance, protecting the underlying copper from environmental factors such as humidity and chemical exposure (crucial for PCBs used in harsh environments).
• Long shelf life due to corrosion resistance provided by palladium and gold layers.
• Lead-free and RoHS-compliant.

Disadvantages:

• Generally more expensive than other surface finishes such as HASL and OSP.
• ENEPIG process involves multiple steps and requires precise control of plating thickness and uniformity. This can lead to higher production costs and longer lead times compared to simpler surface finishes.

EPIG/EPAG. EPIG (electroless palladium/immersion gold) and EPAG (electroless palladium autocatalytic gold) are nickel-free PCB surface finishes designed to avoid signal loss from nickel layers, making them ideal for high-frequency RF applications. Each uses a palladium base layer topped with gold, but differs in gold deposition methods: EPIG uses immersion gold, resulting in a thinner layer, while EPAG employs autocatalytic gold, permitting a thicker gold coating. This thicker layer in EPAG supports more demanding uses like gold wire bonding and soldering, making it more versatile. Both finishes are particularly suited for markets requiring nonmagnetic bonding such as military, aerospace, and deep-sea RF applications.

EPIG advantages:

• Nickel-free, beneficial for high-frequency applications.
• Suitable for fine-pitch designs and high-density interconnects (HDI) as it eliminates nickel layer.
• Usable for soldering and wire bonding.
• High reliability and uniform thickness.

EPIG disadvantages:

• Immersion gold layer may be thinner compared to autocatalytic gold.

EPAG advantages:

• Offers a thicker gold layer compared to EPIG, beneficial for applications requiring more robust connections.
• Suitable for wire bonding and soldering.
• Usable in demanding applications like military, aerospace, and deep-sea RF due to its nonmagnetic properties.

EPAG disadvantages:

• Autocatalytic gold process may be more complex and require careful control.

The key differences between EPIG and EPAG include:

• Gold thickness. EPAG typically has a thicker gold layer.
• Process complexity. EPIG is generally simpler to implement.
• EPAG is often preferred for applications requiring thicker gold layers and more robust connections, while EPIG is suitable for fine-pitch designs and high-frequency applications.

Figure 2. RF boards are best suited to nickel-free PCB surface finishes like EPIG and EPAG to avoid signal loss from nickel layers.

Factors to Consider

When selecting a PCB surface finish, review these six factors.

Application requirements. The primary factor guiding selection should be the intended application of the PCB. For instance:

• High-frequency applications typically benefit from finishes such as ENIG or ImAg.
• For boards that will undergo multiple reflow cycles, ENIG or HASL may be more suitable.
• Edge connectors and other areas subject to wear call for a hard gold finish.
• EPIG without the nickel layer improves signal performance in high-frequency and RF PCB design.
• EPAG and ENEPIG are typically superior for wire bonding due to thickness and uniformity.

Component technology. The nature of the components utilized will impact the finish choice.

• Fine-pitch components require a flat surface, making ENIG, ImSn or OSP ideal options.
• Smooth and durable surface finishes, such as gold or immersion tin, are generally preferred for press-fit connectors.
• Any surface finish is typically compatible with through-hole components.
• Bottom-terminated components (BTC) such as BGAs and QFNs perform optimally with ENIG.

Environmental conditions. Consider the environment in which the PCB will operate.

• Corrosion-resistant finishes like ENIG are advisable for high humidity conditions.
• Hard gold and ENIG prove most suitable for applications exposed to extreme temperatures, due to their superior durability and conductivity.
• Use finishes with high corrosion resistance, like ENIG and hard gold, when exposure to chemicals or solvents is anticipated. ImSn and OSP may not withstand harsh environments as effectively.

Production volume and cost. Volume and budget constraints play a significant role.

• For high-volume production, HASL or OSP might be more cost-effective.
• For prototypes or low-volume production, ENIG or ImSn could be suitable despite higher per-unit costs.

Shelf-life requirements. For long-term PCB storage, consider the following.

• ENEPIG and hard gold are superior options for prolonged shelf life.
• ENIG provides an extended shelf life and is suitable for a wide variety of applications.
• Alternative choices include ImAg and HASL.
• Store materials in a humidity-free environment and follow established guidelines to achieve optimal results.
• OSP and immersion finishes generally have shorter shelf lives, typically not more than six months.

Regulatory compliance. Depending on the industry, particular compliance requirements might be in effect.

• PCB surface finish choices can significantly impact regulatory compliance, primarily concerning RoHS directives and other environmental standards. The type of finish, like HASL, ENIG or OSP, determines its ability to comply with restrictions on heavy metals and other substances. Some finishes, particularly those containing lead, may not comply, requiring alternative options.
• Industries such as medical or aerospace applications might have other finish requirements. Ensure the finish is appropriate per industry standards such as IPC and military specifications like MIL-PRF-50884 for aerospace.

Conclusion

Selecting the right surface finish for a PCB is a crucial decision that can significantly impact product performance, reliability, and cost. Understanding the characteristics of different finishes and carefully considering the specific requirements will inform the choice that optimizes the PCB for its intended application.

No single solution fits every project. Each application demands a different approach, and what works best for one might not be ideal for another. Stay informed about new developments in surface finish technology, as innovations in this field continue to offer new possibilities for PCB design and manufacturing.

Taking the time to select the best surface finish for the PCB is an investment in the quality and longevity of the end-product. Whether a high-frequency communication device, a rugged industrial controller, or a cutting-edge medical device, the right surface finish can make all the difference in achieving optimal performance and reliability.

Md. Imtiaz Uddin is deputy general manager at Napino Auto and Electronics (napino.com) with more than 26 years focused on SMT process failure analysis; m.imtiaz@napino.com.