Comprehensive Guide to PCB Surface Finishes and Conformal Coatings

What are PCB Surface Finishes and Conformal Coatings?

PCB surface finishes act as defensive layers that sit between the bare printed circuit board and a component. Copper-based PCBs devoid of a finish are more susceptible to oxidation and deterioration.  In addition to protecting the surface and circuitry of the PCB, surface finishes ready the surface for component soldering during the assembly process. 

Manufacturers apply surface finishes after the PCB fabrication process – before the assembly process. Once they’ve placed the components and completed the PCB assembly process, they can then apply what is known as a conformal coating. 

The conformal coating protects the PCB assembly from moisture, dust, dirt, extreme temperature, and ultraviolet light and enhances electrical insulation. 

Now, let’s take a look at the specific surface treatments and the differences between them.

Types of PCB Surface Finishes

There are seven main types of PCB surface finishes, each with a specific set of strengths and weaknesses. You must be aware of these properties to select the best coating for your PCB project.

Hot Air Solder Leveling (HASL)

Because of its affordability, HASL is the most common surface finish. Its application process involves dipping the PCB in a solution of molten solder consisting of lead and tin alloys. A hot air knife then blows off any residue.

Advantages

In addition to its affordability, HASL’s other benefits are:

• It allows for a larger processing window
• Compatibility with a wide range of PCBs
• Facilitates excellent solderability

Disadvantages

As mentioned, the application of this surface finish involves dipping the board in a solution of molten solder made of lead and tin. This makes it unsuitable for panels that demand RoHS compliance. This shortcoming is tied to other disadvantages like:

• Results in a substantial difference in thickness between large and small pads
• It is unsuitable for HDI products 
• Increases the risk of bridging in fine-pitch components
• Unsuitable for Ball Grid Array (BGA) components and surface mount devices (SMDs) measuring less than 20 mils in diameter.

Use Cases

HASL’s weaknesses make it unsuitable for use in harsh environments. Instead, manufacturers should only use it for inexpensive consumer electronics and other budget-constrained projects. This could include educational projects and cheap electronics.     

Immersion Tin (ISN)

A lead-less coating consisting mainly of tin. People sometimes refer to it as a “white tin” because of its frost-like appearance. Manufacturers apply ISN by initiating a chemical displacement reaction with the PCB’s copper surface. 

Advantages

Immersion tin comes with a wide range of advantages. They include:

• Results in an even surface, unlike lead-free HASL
• Reworkable
• It can be substituted for reflowed solder 
• The most affordable immersion method
• Is very reliable
• It is ideal for a wide range of use cases

Disadvantages

Metal diffusion bonding is inevitable because tin and copper (in the PCB) share a strong affinity. The result is a shorter shelf-life. Additionally, it may negatively impact the PCB’s performance. Other shortcomings include:

• Not ideal for Plated Through-Hole (PTH) Technology 
• It can be reworked but to a limited extent
• It can result in tin whiskering
• Can damage the solder mask 
• It is difficult to handle and can result in a wide range of damages
• It may be unhealthy for handlers as it contains a carcinogen known as thiourea

Use Cases

Immersion Tin is ideal for fine-pitch, planar, backplane, and press-fit components. This makes it highly effective for PCBs in compact devices like calculators and other handheld devices. PCBs with immersion tin coatings have also been found to be extensively used in computer peripherals, automotive electronics, and communication devices.

Immersion Silver (IAG)

Immersion Silver has been widely popular since the WEEE and RoHS directive was passed. It is considered an excellent alternative to Electroless Nickel Immersion Gold for several reasons. Chief among them is that it is ideal for fine pitch. 

Advantages

One of the main advantages of immersion silver is that it contains an Organic Solderability Preservative (OSP), which prevents tarnishing. In addition to this, other advantages of immersion silver include:

• Compliance with RoHS directives and requirements

• Suitable for fine-pitch components
• Features a moderate shelf life of about 12 months
• Highly stable when compared to other surface finishes
• Ideal for planar components
• It is affordable and cost-effective

Disadvantages

Because of the presence of OSP, the manufacturer must package the PCB immediately after application. This is because OSP is sensitive to contaminants. 

Other shortcomings include:

• It may result in silver whiskering
• It is not suitable for compliant-pin interaction as it has a high fraction coefficient.
• Some of its systems cannot throw into micro vias aspect ratios of 1:1 

Use Cases

Immersion silver is mainly used for aluminum wire bonding, membrane switches, and EMI shielding.

Electroless Nickel Immersion Gold (ENIG)

Electroless Nickel Immersion Gold has become one of the most popular surface finishes in the PCB industry. This is because it overcomes many of the significant shortcomings associated with other surface finishes. 

It features a two-part coating process. First, the manufacturer applies a nickel layer. It will shield the copper and act as a surface for components to be soldered onto. Next, the manufacturer applies a layer of gold which protects the nickel layer.

Advantages

The most significant advantage of ENIG is that it is ideal for the new and upcoming generation of complex surface components. This includes flip chips and BGAs. 

Older surface finishes have limitations that may make them unsuitable for use in these newer board types. Other advantages of ENIG include:

• It is ideal for flat surfaces as the layers of nickel and gold as thin and even
• It does not contain lead
• Ideal for PTHs
• It has a long shelf life
• Features excellent electrical performance
• Supports low signal loss
• Results in reliable solder joints
• Prevents solder bridging 

Disadvantages

Although ENIG has a long shelf-life, it has also been associated with an issue known as “black pad”. This occurs when phosphorus builds up between the layers of nickel and gold, resulting in fractures as well as faulty board connections. Other disadvantages include:

• Unsuitable for reworking
• Features a complex application process
• Comparatively more expensive than other surface finishes

Use Cases

As mentioned, because of its compatibility with modern complex packages and PCBs, ENIG is suitable for:

• High-frequency signal designs
• Fine Pitch Components
• Medical Devices
• Telecommunications
• Automotive Industry
• Applications that require RoHS Compliance 

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) PCB surface finish is an upgrade of ENIG. In ENIG, immersion gold has been shown to slowly degrade the layer of nickel. ENEPIG introduces an additional layer of palladium plating between the gold and nickel layers. 

ENPIG’s application process consists of four stages:

• Copper activation: The manufacturer uses a displacement reaction to selectively activate parts of the PCB’s copper layer – the parts that require protection. Essentially, the electroless nickel will ultimately “stick” to the activated portions of the copper. 
• Application of electroless nickel: The manufacturer deposits electroless nickel onto the copper to prevent it from reacting with the gold (and other metals).
• Application of electroless palladium: This step involves applying palladium, which then interacts with nickel to form a thin layer. 
• Application of Immersion Gold: In the last step, the manufacturer applies immersion gold plating to the PCB, which dissolves and combines with the palladium to form an outer layer.  

Advantages

People refer to ENEPIG as the ” universal finish” because of its high compatibility with most PCBs. It comes with a range of advantages that include:

• Does not struggle with black pads
• Results in an even flat surface
• Result in low contact resistance
• Easy to process
• Doesn’t have a toxic effect on human skin 
• Free from lead
• Ideal for multiple reflow cycles
• It is highly compatible with Tin-Silver-Copper (Sn-Ag-Cu)-based solder
• It has a long shelf life

Disadvantages

Despite being a marked improvement over ENIG, ENEPIG isn’t perfect. Some of its cons include:

• More expensive than other surface finishes (including ENIG)
• Features a complicated application process
• Its thickness can interfere with surface mount technology soldering
• Features a longer wetting period
• Failing to apply it correctly can lead to reliability issues.

Use Cases

Because of its high compatibility, ENEPIG is suitable for a variety of PCBs in a wide range of applications. However, its high cost may prevent manufacturers from using it in productions where cost is a factor. ENEPIG is most suitable for applications in:

• Military and aerospace
• Wire-bonding
• Medical Devices
• High-frequency electronics
• Telecommunications
• Automotive industry
• Manufacturing equipment  

Organic Solderability Preservative (OSP)

Although it’s a compound found in other surface finishes, manufacturers can use OSP on its own. It is an organic, water-based surface finish. OSP contains an acid that forms a protective shield when it comes into contact with a PCB’s copper layer. 

This is a mostly aqueous process that requires very little equipment to apply. The application process results in very little chemical waste, making OSP one of the most environmentally friendly options available. 

Advantages

OSP is easy to apply. Moreover, the application process is relatively short and straightforward compared to other surface finishes. Its other advantages include:

• Provision of an even, co-planar surface
• Low equipment and maintenance requirements
• It is lead-free
• Repairable

Disadvantages

OSP is organic and water-based. This makes it susceptible to damage during handling. Because of this, OSP has the following shortcomings:

• Features a short shelf life (6 to 12 months)
• Unsuitable for PTH
• Its protection against oxidation declines over time
• It can be hard to inspect because of its see-through/clear color, which blends with copper
• Has comparatively subpar copper bonding
• Susceptible to moisture in high-humidity environments
• Features limited reflow cycles

Because of these limitations, it’s important to store and handle OSP with care. 

Use Cases 

Due to its short shelf life, OSP is best for the following applications and use cases: 

• Low volume prototyping
• Small-batch PCB manufacturing
• Automated assembly
• Simple single and double-sided boards
• Fabrication that requires quick turnarounds
• Manual soldering

Hard Gold (Plating)

Hard gold is the most expensive type of surface finish. It consists of a gold alloy layer bound to a coat of nickel, cobalt, or iron. Manufacturers typically reserve it for components that will experience heavy use.

As such, manufacturers commonly use hard gold finishes for edge connectors, test boards, interconnected carrier boards, keypads, and contacts.

The thickness of the hard gold finish/plating can range from 25 to 100 micrometers, depending on the eventual application of the PCB. For instance, electronics for industrial and military use cases require PCBs with thicker hard gold plating.  

Advantages

Because of its robustness, hard gold has several advantages, including: 

• Exceptional durability
• Provides excellent conductivity
• Offers superior resistance to corrosion

• The application process results in a uniform metal deposit
• Lead-free
• RoHS Compliant

Disadvantages

Some disadvantages of hard gold plating include: 

• High-cost
• Requires specially skilled labor
• Due to poor solderability, it’s unsuitable for soldering points
• Not suitable for wire bonding 

Use Cases

Hard gold’s biocompatibility makes it ideal for medical devices that will come into contact with human skin. In addition to this, hard gold plating is suitable for use in:

• PCB edge connectors and contact fingers
• Relays
• Switch contacts
• Automotive components
• Aerospace electronics

Carbon Ink

As the name implies, the carbon-ink surface finish consists of conductive ink. The application involves depositing the carbon ink onto the exposed copper pads of the PCB using a silk screen-like process. The thickness of the carbon ink layer can range from 10 to 50 micrometers. 

Advantages

Carbon ink is worth using because it is:

• High conductivity
• Affordable
• Suitable for solder joints and wire connections

Disadvantages

Carbon ink’s main disadvantages include:

• Features a short shelf life
• Low protection against wear and tear
• Susceptibility to environmental conditions such as humidity and temperature
• The application process can be very complicated

Use Cases

Manufacturers apply carbon ink to form protective contact surfaces for electronic components such as switches, LCDs, and jumpers.

Carbon ink surface finishes are typically used for applications such as:

• RF Shielding
• Components for keyboards and keypads
• Control panel PCBs
• LCDs and other touch-screens

Bare Copper

The term “bare copper finish” is slightly misleading. In truth, bare copper means that the manufacturer has not added any additional surface treatments or finishes to the PCB, leaving the traces and pads bare.

Advantages

The advantages of using bare copper are few. While manufacturers should avoid using it, it is: 

• Inexpensive
• Simple

Disadvantages

As you may have expected, using bare copper has more disadvantages than advantages. Some of these advantages include:

• Short shelf life
• Limited to a few use cases, such as prototyping
• This leaves the board susceptible to oxidation
• Not well-suited to harsh environments
• Bare copper PCBs require careful and considerate handling 

Use Cases

Bare copper PCBs aren’t ideal for harsh environments as they lack protection against corrosion. However, they can still find usage in: 

• Prototyping
• Educational electronics
• Specific low-cost productions

Types of PCB Conformal Coatings

There are two main types of conformal coating: traditional and modern. The following guide will touch on the various conformal coatings that fall underneath these categories.  

Traditional Conformal Coatings

Traditional conformal coatings consist of acrylic, epoxy, or silicone resins. Manufacturers can apply them using brushes, sprays, or immersion. Traditional conformal coatings aren’t fully hermetic. 

This means they don’t fully seal and protect the electronics. Moreover, we can easily dilute them using solvents and, in some cases, water. The most common are:

Acrylic Resin (AR)

These acrylic polymers are soluble in solvents, providing general protection and good elasticity. 

Advantages 

Some advantages of AR include:

• Affordability
• No shrinkage during curing
• Easy to repair and rework
• Easy to apply and remove

Disadvantages 

Some of AR’s shortcomings include:

• Low abrasion resistance
• Not suitable for harsh environments 

Use Cases

Acrylic resins have wide general uses in bathtub liners, aquarium windows, and cellphone display screens. AR-based conformal coatings are ideal for PCBs that will their way into:

• General electronics
• Consumer electronics
• Automotive electronics
• Medical devices
• Industrial equipment
• Reworkable applications 

Epoxy Resin (ER)

These are mainly available in a two-part compound and tend to create some of the hardest PCB coatings. Unlike the alternatives, they are hard to remove once the curing process is over. 

Advantages 

Some of the advantages of using ER include:

• Excellent humidity resistance
• Good dielectric properties
• High chemical and abrasion resistance

Disadvantages 

ER’s shortcomings are as follows: 

• Complex mix ratios
• Difficult to remove and rework

Use Cases

Especially useful in PCBs that will be used in harsh environments like building and construction. It’s also ideal for military and industrial applications. 

Silicone Resin (SR)

These single-component compounds are often ideal for electronics in extreme temperature conditions. They are also common in the making of coatings and paints. 

Advantages 

You should consider using a silicone resin conformal coating in your PCB because it:

• Performs well under extreme temperatures
• Excellent chemical resistance
• Good corrosion and humidity resistance 

Disadvantages 

SR’s greatest strengths result in some of its weaknesses. For instance, it is: 

• Very difficult to remove
• Allows only localized repairs 

Use Cases

SR can withstand temperatures above 120°C. Its flexibility makes it ideal for flexible PCBs. PCBs with SR conformal coating can be found in:

• Automotive electronics
• Aerospace
• Industrial applications

Polyurethane Resin (UR)

Polyurethane Resin features excellent chemical and moisture resistance. Additionally, polyurethane resin is abrasion-resistant, making it very hard to remove.

Advantages

UR features:  

• Excellent dielectric properties
• Excellent moisture resistance 
• Abrasion-resistance
• Good resistance to moisture

Disadvantages 

• Negative environmental impact
• Takes a long time to cure

Use Cases

UR is ideal for aerospace applications where exposure to fuel vapor is standard.  

Modern Conformal Coatings

Modern conformal coatings consist of more advanced or recently discovered compounds such as Parylene and Fluoropolymer. These conformal coatings offer superior protection to traditional ones. Many of them are fully hermetic.

Modern conformal coatings require more complex application techniques, such as vapor deposition. This provides a more even and uniform coating. 

Let’s take a closer look at the most common conformal coating:

Parylene (XY)

Parylene offers superior protection against chemicals, temperature, dust, and moisture.

To apply XY, the manufacturer must first heat it into a gas. After cooling, it polymerizes and subsequently turns into a thin film coating the PCB. This is the process of vapor deposition. 

Advantages 

It’s worth replacing your traditional conformal coating with XY because it is: 

• Colorless and transparent
• Does not need to be cured
• High dielectric strength 
• Easily withstands extreme temperatures

Disadvantages 

Although it offers marked improvements over traditional conformal coatings, XY is: 

• Difficult to remove 
• Not suitable for long-term outdoor exposure
• Difficult to apply
• Expensive
• Susceptible to adhesion issues
• Only suitable for limited batch sizes
• Costly

Use Cases

Parylene is suitable for PCB assemblies that will find their use in: 

• Automotive electronics
• Aerospace devices
• Medical devices
• Consumer electronics
• High-frequency electronics
• Micro-Electro-Mechanical Systems (MEMS)
• Optical devices  

Comparative Analysis

It’s good to have choice but too much of it can lead to decision paralysis. Even if you have all the specs of these finishes and coatings laid in front of you, it can be hard to compare them. 

Performance and Durability

First, you need to consider how the conformal coating or surface finish aids or diminishes the performance of the PCB. 

Although they’re pricey, modern conformal coatings offer the best performance. Parylene is non-permeable, offers superior protection against corrosion, and has a long shelf life.

ENEPIG is considered the best-performing surface finish. This is due to its superior solderability, excellent surface planarity, and superior corrosion resistance. ENEPIG causes very little signal loss or interference.

Cost Considerations

The problem with high-performance conformal coatings and surface finishes is that they can be costly. Moreover, their application processes can be complex and expensive, too. The use case will also be a factor in determining which coatings or finishes you can use.

For instance, developing and manufacturing high-density interconnect PCBs for use in military applications will always be more expensive than standard PCBs.

Nevertheless, if you’re looking for a conformal coating that balances cost and performance, it’s best to select acrylic resin (AR). It offers decent abrasion resistance, high dielectric strength, and good overall protection. It’s also easy to rework and repair. 

HASL is still the most cost-effective surface finish. It also offers good solderability and protection.

Environmental Impact

The environmental impact of the conformal coating and surface finish is an important consideration. Not only does it affect your company’s reputation, but it will also affect how compliant your PCBs and electronics are with the relevant regulations.

Once again, acrylic resin (AR) is a good environmentally friendly option for conformal coating. This is because acrylics are water-based and devoid of corrosive or harmful chemicals. AR’s removal process has a low environmental impact.  

While it’s the least-performing surface finish, OSP is the most environmentally friendly option. It’s water-based and free from solvents and other harmful chemicals. However, its use should be limited to low-cost, low-complexity PCBs.  

How to Choose the Right PCB Surface Finish or Coating

Picking the right surface finish and PCB conformal coating can be tedious, complex, time-consuming, and frustrating. There are a few considerations you must keep in mind as you make your selection. Some factors to consider include: 

Operating Environment

Surface finishes and conformal coatings protect PCBs from extreme temperatures, moisture, and dust, among other things. 

When selecting a finish, you also need to consider the PCB’s operating temperature range. This is because it will impact the chemicals in the conformal coating and finish. Some finishes and coatings have better thermal dispersions than others. 

Mechanical Stress

As mentioned, some surface finishes, such as OSP should be handled with care. How you plan to store and handle the PCB should be considered before you choose your conformal coating or finish. Ultimately, the surface finish or coating should be compatible with production, handling, and storage constraints. 

In addition to this, it’s also important to ensure that the conformal coating and finish do not cause undue stress to the PCB and its components. The nature of the coating and finish can change because of temperature.

Cost and Budget

The coating and/or finish should meet the budget limits. Not only the material costs but different application methods cost more than others. The surface finish or conformal coating should also be easy to rework – process and price-wise. 

Regulatory Compliance

Safety and environmental concerns are paramount when selecting the type of conformal coating material to use.

Some elements, once they become obsolete, may end up bringing some damaging effects to the environment.

Besides all these, there are several regulatory bodies tasked with the mandate of ensuring that PCB coating materials are environmentally friendly:

• Occupational Safety and Health Administration (OSHA): A statutory body in the US that ensures worker safety. Plenty of coatings are flammable, contain high toxicity levels, and emit fumes.OSHA ensures that all companies producing PCBs put safety measures in place to protect their workers. OSHA provides that all PCB manufacturers have personal protection equipment for their workers and that the company has enough ventilation.
  
• EPA (the United States Environmental Protection Agency)—Again, in the US, all PCB producers strictly adhere to EPA requirements. For instance, the EPA severely restricts chemicals that affect the ozone layer. Fortunately, most of the restricted chemicals aren’t readily available, so matters concerning ozone depletion appear not to be a big concern.
   
• CARB (California Aviation Review Board) and other regional regulations： For the longest time possible, the CARB, in conjunction with other local authorities, has continuously played more prominent roles to ensure that they enforce environmental restrictions aimed at the protection of the environment. Some of these restrictions include VOC (volatile organic compounds) or smog-producing chemicals. The latest environmental topic under discussion is the Global Warming Potential (GWP). 

Steps to Choosing Your Surface Finish or Conformal Coating

How do you go about choosing high-quality PCB conformal coating or surface finish material? What are some of the things that you need to watch out for the best PCB conformal coating material? Here is a summary of what you need to do to ensure you select the best materials :

1. Look for materials that feature excellent flexibility.
2. The material should have low toxicity levels.
3. Consider the cost of not just the material but its application process too.
4. The material should be easy for you to rework and repair.
5. Look for materials that support a wide range of operating temperature range.
6. At high frequencies and speeds, the material should be able to sit without producing chemical by-products.
7. The material must shield the board from low/high temperatures and mechanical impact.
8. The material must have a long shelf life.
9. Make sure that the material is easy to apply or that you at least have the means to apply it.

FAQs

What is the cheapest PCB surface finish?

Many consider bare copper to be the least expensive type of surface finish. This is because the bare copper finish process adds no additional surface treatments to the PCB.

If you’re looking for a surface finish that prevents oxidation and can increase, the next cheapest option is hot air solder leveling (HASL). 

How thick is a PCB surface finish?

The thickness of the PCB surface finish depends on the type. For instance, the thickness of an HASL-based surface finish can be as much as 50 um or as thin as 0.2 um, and gold plating can be as thick as 100 um.   

What is the difference between gold plating and immersion gold?

Gold plating tends to be thicker than immersion gold. As such, gold plating enables more full-bodied joints and is more suitable for harsh environments.

Immersion gold is slightly less expensive, and it offers a more consistent coating. It’s also ideal for fine-pitch SMT components.   

Conclusion

The above guide explores both surface finishes and conformal coating. To achieve the best result, it’s best to assess the properties of each material and application process against your PCB’s requirements. While ENEPIG is the closest to a one-size-fits-all solution for surface coatings, its high cost may not be ideal for your production. It’s also important to note that the industry will continue to make discoveries. Whether it’s 3D printing surface finishes or nano, UV curable, and hybrid conformal coatings – we’ll soon have even more options. That’s why it’s important to consult with an experienced PCB manufacturing partner when making important decisions regarding your project.