What Is Surface Finish in PCB Manufacturing?

Without question, the easiest way to think about PCB manufacturing (printed circuit boards) is to think about the big parts, such as the circuits, connections, and components. But there's one crucial aspect that plays a major role in how well your PCB performs: the surface finish.

In PCB manufacturing, the copper of the board is exposed and needs protection. Why? For some reason (possibly because it's copper), copper tends to oxidize, so it doesn't perform too well and can interfere when you're trying to solder it later. 

This copper exposed by laser had a surface finish because it needed to prevent that copper from oxidizing and making the soldering of components onto the board practically impossible. In this guide, we will show you the most common types of PCB surface finishes and shall assist you with which one might be the right fit for your project.

What Is a Surface Finish?

Before anything further, it's worth understanding what surface finishes are. The protective layer refers to a surface finish over copper pads, forming a surface finish layer on PCB. For example, the copper pads of the PCB are locations at which electronic components, i.e., resistors, capacitors, microchips, etc, are soldered. The surface finish keeps the copper from ever reacting with the air to begin the reaction cycle of corrosion. It also ensures that components can indeed be soldered on properly.

Why Is the Surface Finish So Important?

Here’s the thing: you might wonder why anyone would put so much work into the surface finish when it comes to making a PCB. Isn't it more important what the circuits are designed like, or the quality of the components? That’s important too but if that surface finish’s not good the overall performance of the PCB could suffer.

Prevents Oxidation: We mentioned that copper is very oxidizable when exposed to air. Copper can then be oxidized, creating a layer that prevents good electrode conductivity on the copper.

Makes Soldering Easier: A good surface finish is achieved by a smooth, even surface for soldering. The PCB components are attached by soldering.

Improves Reliability: In the long run a PCB with a good surface finish applied will be more reliable. The finish protects the board from environmental factors, wear and tear, and damage during the assembly process.

Durability: Surface finishes are developed to encourage the durability of the PCB. The finish provides protection to the copper from oxidation and wear to allow the PCB to handle multiple soldering cycles and have good performance even in different environments.

Types of Surface Finishes in PCB Manufacturing

Once you know why the surface finishes matter, we will look at the most common finishes used for PCB manufacturing. There are pros and cons to each type and the optimal choice depends on what your project needs.

HASL (Hot Air Solder Leveling) and Lead-Free HASL

One of the oldest and most popular PCB manufacturing surface finishes is Hot Air Solder Leveling (HASL). In HASL ( Hot Air Solder Leveling), the PCB is dipped into molten solder containing either tin and lead or just tin (lead-free HASL).

Advantages of HASL:

Cost-effective: The cheapest of all surface finishes, HASL is commonly used in budget-friendly projects.

Good shelf life: It has a long-lasting finish that makes it well-suited for mass production.

Easily reworkable: HASL is easy to fix if any mistakes are made during assembly in the stock compared to other surface finishes.

Disadvantages of HASL:

Uneven surface: Since it is applied in a molten state, it leaves an uneven surface, unsuitable for high-density boards or small components.

Not suitable for fine-pitch components: Because of its uneven surface, HASL can potentially cause problems when the components are very small or close together.

Not RoHS compliant (if leaded): However, lead-based HASL is not RoHS compliant, with available lead-free versions.

ENIG (Electroless Nickel Immersion Gold)

ENIG is a premium surface finish where nickel is applied, followed by a thin layer of gold over the copper pads. The output is a smooth, flat surface that is ideal for complex PCBs with small components and then runs through a controlled process.

Advantages of ENIG:

Flat, smooth surface: ENIG provides one of the smoothest surfaces, required to have low-cost high-density PCB with close or small components.

Excellent corrosion resistance: The gold over the copper and nickel prevents corrosion over time, and makes the board more reliable.

RoHS compliant: ENIG contains no lead, so it is a safe and green option.

Disadvantages of ENIG:

More expensive: If costs are tight, ENIG may be a little more expensive than HASL and other basic finishes.

Black pad issue: An ENIG problem is a "black pad" where the nickel layer oxidizes and leads to soldering problems. Proper quality control during manufacturing allows you to avoid this.

ENIGPIG (Electroless Nickel Electroless Palladium Immersion Gold)

The improved version of ENIG is ENEPIG. In this finish, an atomic layer of palladium is deposited between the nickel and gold layers. As well as excellent protection from corrosion, this also makes the finish perfect for wire bonding, an industrial process that is particularly common in high-performance applications.

Advantages of ENEPIG:

Great for wire bonding: The use of ENEPIG in high-frequency applications where wire bonding is used (PCR/COG applications), gives it an advantage compared to OSP-coated PCBs.

Very reliable: ENEPIG is one of the most durable and, therefore, reliable finishes due to the added palladium layer.

No black pad issue: The ENEPIG process differs from ENIG in that it doesn’t have the black pad problem and, therefore, is a safer choice for long-term reliability.

Disadvantages of ENEPIG

High cost: Because ENEPIG is one of the most expensive surface finishes available, they are usually reserved for high-end or specialized PCBs.

Less common: ENEPIG is not an option with all manufacturers because it’s a more advanced finish.

ENIG vs ENIGPIG vs HASL

OSP (Organic Solderability Preservative)

It’s an organic finish that protects the copper pads with a thin organic layer. It’s the least expensive finish and it’s often used in consumer electronics where there will be a long-term durability disadvantage.

Advantages of OSP:

Very affordable: Of all finishes available, OSP is one of the cheapest and is the perfect answer to a budget-conscious project.

Flat surface: Smooth organic surfaces which are useful for fine-pitch components.

Environmentally friendly: OSP is a lead-free organic finish, making it an environmentally friendly product.

Disadvantages of OSP:

Short shelf life: OSP wears down quicker than metallic finishes and therefore, works best for PCBs that will be assembled soon after they're produced.

Not suitable for complex designs: While not ideal for boards needing multiple soldering cycles, OSP is not particularly difficult to do due to organic layer degradation during the process.

Immersion Silver （Im Ag）

A Metal finish called Immersion Silver is a thin layer of silver over the copper pads. The price and performance are well in line with this middle range: good electrical conductivity and a smooth surface.

Advantages of Immersion Silver:

Good for high-frequency applications: Immersion Silver exhibits very good conductivity, which makes it suitable for PCBs in high-frequency applications.

Flat surface: Providing a flat, smooth surface, Immersion Silver is like ENIG and is ideal for fine-pitch components.

RoHS compliant: No lead is used in Immersion Silver, making it environmentally friendly.

Disadvantages of Immersion Silver:

Tarnishing: When left out in the open, there are issues where silver tarnishes, which can lead to problems if the board isn't assembled in the short term.

Not as durable: Immersion Silver is not as durable as many of our other finishes, so it is not always the best finish for boards that you need to last for a long time.

A comparison chart of PCB surface finish

HASL vs ENIG vs ENIGPIG vs OSP vs Im Ag vs Im Sn

Choosing the right surface finish for your PCB is a critical step in the manufacturing process. Whether you’re looking for a cost-effective option like HASL or a high-performance finish like ENEPIG, understanding the pros and cons of each type will help you make the best decision for your project.

With the right surface finish, your PCB will not only perform better but also last longer, ultimately contributing to the success of your final product.