PCB Surface Finish Types: Choosing ENIG, HASL, or Others

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A printed circuit board (PCB) surface finish has two main functions: To prevent the copper from oxidizing and to provide a solderable surface.

Some PCB surface finishes, however, are more popular and versatile than others. As you review your options, you&#;ll likely consider durability, cost, and the materials used within the finish, such as tin-lead, lead-free, various gold finishes, and silver. Additionally, you may have concerns about the finish compatibility with the subsequent termination process &#; such as wire bonding or welding. But how do you know which surface finish is best for your next PCB project?

We&#;ve compiled a comprehensive list of the nine different surface finishes. In this article, we&#;ll explain the purpose and common uses of each surface &#; plus the advantages and disadvantages &#; so you can find the best option for your next project.

The 9 Different PCB Finishes
How to Choose the Best PCB Finish for Your Project

The 9 Different PCB Surface Finishes

Electroless Nickel Immersion Gold (ENIG)

ENIG is the most popular option and is used on about 80% of all PWBs. This finish provides a thin, gold, solderable layer that protects the copper traces with a nickel barrier between it and the copper. ENIG is a good lead-free option that results in a durable, long-lasting finish.

Advantages:

• Protects copper from passivation
• Fast, easy process
• Lead-free
• Long-lasting and durable
• Restriction of Hazardous Substances (RoHS) compliant
• Reasonably low cost
• Very good solderability

Hot Air Solder Leveling (HASL)

With HASL, the printed circuit board is held vertically by clamps and dipped into a flux bath and then a molten solder hot vat. The finish, which contains tin/lead, is leveled off with hot air knives and the printed circuit board panel is raised out of the molten solder vat.  This ensures a homogenous thickness over the entire board. It was once one of the most popular finishes but is now mainly used only on military and aerospace applications.

Advantages:

• Meets military requirements
• Low cost

Disadvantages:

• Cannot be used in High Density Interconnect (HDI) structures
• Extremely high temperatures can affect other materials in the PWB
• Process is messy, hot and smelly
• The resulting topography of the HASL finish is not flat enough for many very dense, high pin-count, tight-pitch components surfaces

Lead-Free (HASL)

The properties and uses of this finish are the same as HASL &#; except there is virtually no lead in the alloy. Instead, the alloy is a tin and copper, tin and nickel, or tin, copper, nickel and germanium alloy. This finish is not widely used.

Advantages:

• Contains no Lead
• RoHS compliant

Disadvantages:

• Cannot be used in HDI applications
• May not meet military and aerospace requirements
• Possible formation of &#;tin whiskers&#; (thin conductive filaments that can cause short circuits and a variety of other issues)

Immersion Tin

With this finish, a layer of tin is deposited onto the copper surface using a chemical process. This is frequently used in high-speed circuits. Immersion tin works well to protect underlying copper from oxidization, but copper and tin in such close proximity can create diffusion from one metal into the other, resulting in challenges.

Advantages:

• No nickel
• Optimal for faster circuitry
• Works well to protect underlying copper from oxidization

Disadvantages:

• Not ideal for two-sided assembly
• Not ideal for long storage periods

Organic Solderability Preservative (OSP)/Entek

A thin layer of material is applied over the copper to protect it prior to soldering. Though this water-based organic surface finish was once very popular, it is not widely used today.

Advantages:

• Eliminates metal finishes
• Protects the copper for soldering
• Lead-free
• RoHS compliant

Disadvantages:

• Not commonly used
• Not a long shelf life

Hard Gold

Hard gold is plated over the nickel, which holds up well and doesn&#;t break down easily compared to other finishes. Because of its durability, this type of PCB plating is used in applications that involve mechanical contact, such as buttons and keypads.

Advantages:

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• Extremely durable
• Long-lasting
• Lead-free
• RoHS compliant

Disadvantages:

• Poor solderability
• Expensive

Immersion Silver

Similar to tin, this PCB finish is used in high-speed signals that don&#;t contain any nickel. Immersion silver, however, does require unique storage as it will tarnish if exposed to air.

Advantages:

• Nickel-free
• Faster signal speed than tin
• RoHS compliant and environmentally friendly

Disadvantages:

• Becomes contaminated easily
• Not ideal for two-sided assembly
• More expensive
• Requires the use of sulfur-free paper
• Will tarnish if exposed to air
• Not good for long storage periods

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

This three-layer metallic finish is used for printed circuit boards that also require wire bonding. The electroless nickel protects the copper and prevents it from coming into contact with the other metals. Palladium provides another anti-oxidation insurance layer and prevents the nickel from corroding, and the final gold layers adds more of a barrier against oxidation &#; plus it completely covers the PCB.

Advantages:

• Provides a flat pad surface
• Allows more area to land wire as bond pads are flat
• Ideal replacement for soft bondable gold
• Palladium helps provide good solderability
• RoHS compliant
• Nickel corrosion &#; or  &#;black pad syndrome&#; &#; is not an issue

Disadvantages:

• Costs a little more than ENIG

Soft Bondable Gold

This PCB finish has been used for a long time and has the same wire bonding process as ENEPIG. The pad is a crowned surface, which means there is less area to land wires.

Advantages:

• Suitable for wire bonding
• RoHS compliant

Disadvantages:

• Difficult solderability

Key Considerations When Choosing Your PCB Finish

It&#;s important to pick the right PCB plating for your product. While there are a lot of available options, you want to consider the size and scope of your project &#; and budget.

For a solid overall option, ENIG offers a high quality finish and low-cost option with good solderability. You may, however, have to choose a finish based on a particular technology or requirement &#; such as HASL, which is military, aerospace, and space approved.

Selecting the right PCB surface finish can be a challenge, but we are here to help. Cirexx is your best-in-class solution for PCB layout, fabrication, and assembly. With more than four decades of experience, we have manufactured PCBs for almost every leading technology company &#; and we&#;ve worked with industries all over the world. Contact us today to learn more about what PCB surface finish you should choose for your next project or to connect with our experienced sales team.

Overview of Common PCB Surface Finishes

There are two major types of PCB surface finishes &#; metallic and organic. Common metallic finishes include:

• Hot Air Solder Leveling (HASL)
• Immersion Tin (Sn)
• Immersion Silver (Ag)
• Electroless Nickel Immersion Gold (ENIG)
• Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

While common organic finishes are:

• Organic Solderability Preservatives (OSP)
• Liquid Photoimageable Solder Masks (LPISM)

Metallic surface finishes can be further divided into reflowed finishes like HASL and plated finishes like ENIG. Reflowed finishes involve depositing solder on pads, then reflowing to form a coating. Plated finishes use electroplating or chemical deposition to coat the PCB surface with metal layers.

Below is an overview of each type of PCB surface finish:

Hot Air Solder Leveling (HASL)

HASL applies molten solder on PCB pads, which is then leveled by hot air knives. The result is a solder coating of 1-70 μm on pad surfaces. HASL provides excellent solderability and is the most cost-effective finish. However, it has several limitations:

• The thick and uneven deposit can lead to low assembly yields
• Solder masks can become damaged during board fabrication
• Not suitable for fine pitch components due to bridging risks
• The coating wears out over time, reducing solderability

Immersion Tin (Sn)

Immersion tin deposits a thin layer of tin (0.1-0.8 μm) by immersing PCBs into a heated tin-based solution. The tin coating offers superb solderability while being lead-free. It also prevents copper oxidation. But there are concerns over tin whisker growth and poor shear strength.

Immersion Silver (Ag)

Immersion silver involves dipping PCBs into a silver solution to form a thin silver coating (0.1-0.3 μm) on the copper traces. It provides excellent solderability, conductivity and corrosion resistance. However, the coating tarnishes over time. The process also requires careful maintenance of the silver bath.

Electroless Nickel Immersion Gold (ENIG)

ENIG is a dual plating process with electroless nickel (3-6 μm) plated first, followed by a thin immersion gold layer (0.05-0.15 μm). The nickel layer offers a diffusion barrier while gold provides excellent corrosion and wear resistance. But ENIG is relatively expensive. The thickness control of the gold layer is also critical.

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

ENEPIG is a three-layer plating with electroless nickel and palladium layers added below the immersion gold. The palladium acts as a barrier layer to prevent copper diffusion. It also minimizes gold embrittlement issues seen in ENIG. However, the cost is higher than ENIG.

Organic Solderability Preservatives (OSP)

OSP coatings are organic films applied by dipping or spraying PCBs with organic compounds containing azole heterocycles. The thin organic layer (0.1-0.2 μm) protects the copper from oxidation and preserves solderability. OSP is easy to apply and a low-cost process. However, it provides limited protection compared to metals.

Liquid Photoimageable Solder Masks (LPISM)

LPISM are UV-curable solder masks that act as a permanent organic coating on PCBs. It provides insulation, chemical and abrasion resistance while restricting solder flow. LPISM eliminates the need for secondary conformal coatings. But the material and processing costs are relatively high.

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