HASL vs ENIG: An Ultimate Guide on Surface Finish

What is the Surface Finish?

PCB surface finish refers to a thin layer of metallic material applied to the copper traces of a printed circuit board (PCB) to protect them from oxidation and facilitate the soldering process. Surface finishes help improve electrical performance and prevent corrosion. Some common types of PCB surface finishes include HASL (hot air solder levelling), ENIG (electroless nickel immersion gold), OSP (organic solderability preservatives), and immersion silver. The preference for surface finish depends on the intended application of the PCB and its specific requirements. In this article, NextPCB is discussing HASL vs ENIG in detail to help you select the best possible surface finish for your circuit board.

HASL Definition and Finish

HASL is an acronym for “Hot Air Solder Leveling”. It is a widely used surface finish process in printed circuit board manufacturing. This process involves applying a layer of molten solder to the exposed copper traces and pads and then using hot air to level and smooth the surface. HASL is the second most used surface finish. It provides a reliable and cost-effective surface finish option for PCBs.

In the HASL surface finishing process, the PCB is dipped in a container containing molten solder so that the exposed bare copper surface gets covered with a coat of solder. Of course, this process leaves an uneven layer of solder on the exposed copper areas, which is later levelled y applying hot air. However, it is crucial to ensure that the molten solder is equally levelled, covers all the exposed copper areas and does not create solder bridges. 

Types of HASL Surface Finish

As stated above, the HASL is a method of covering bare copper with molten solder. It raises a very common question of whether the solder is leaded or lead-free. The answer to this question makes us familiar with the two types of HASL surface finish.

• Leaded HASL Surface Finish: It is a conventional type of surface finish with solder that contains lead with a proportion of 37% and 63% Tin in its formulation.
• Lead-Free HASL Surface Finish: It is not a common type of HASL surface finish. Lead-free HASL is available only on demand. It contains almost 99.3% Tin and 0.6% Copper in its formulation. The performance of Lead-free HASL is outstanding under high-temperature and high-humidity areas.

ENIG Surface Finish

ENIG is an acronym for Electroless Nickel Immersion Gold PCB surface finish. It is the most commonly used surface finish for intense working and enduring circuit boards. ENIG consists of a thin layer of gold plated over an electroless layer of nickel to protect it from oxidation. The combination of nickel and gold enhances the quality of the circuit board. As gold offers a protective and corrosion-resistant behavior while electroless nickel improves the solderability.

ENIG due to its feature might be the best option for circuit boards that have to operate under harsh environmental conditions, such as military, aerospace, marine, and medical industry. 

HASL vs ENIG: Advantages and Disadvantages

When it comes to PCB surface finishes, we have several options to select from. But two of the most popular surface finishes are HASL and ENIG both have some advantages over each other. If one is cheaper compared to the other, the other one is durable. In this section of the article, NextPCB is discussing some advantages and disadvantages of HASL vs ENIG. So, you can select a better surface finish for your next circuit board.

Advantages of HASL Surface Finish

The quality, cost, and availability of the Hot Air Solder Levelling HASL make it the most commonly used surface finish for electronic products. To help you build a better understanding of HASL, here we are enlisting some of its advantages.

• Easy to Apply: HASL is a conventional surface finish process and does involve a lot of technical processes. The process involves dipping the circuit board into a molten solder container and then levelling the unequal layer of solder through a hot air knife. 
• Easily Available: HASL is the most commonly used budget-friendly surface finish. Hence, it is available at every PCB manufacturer.
• Wettability: Leaded HASL provides excellent wettability, which helps in making enduring joints between the circuit board and the components. 
• Reworking: As we know, HASL contains solder as the main component in its formulation. It makes resoldering a lot easier and hassle-free while maintaining the soldering quality and providing a brighter surface.
• Cost-Effective: HASL is a budget-friendly surface finish compared to ENIG and immersion Silver. Its cost-efficiency makes it a top priority for hundreds of household and commercial electronic products. 

Disadvantages of HASL Surface Finish

HASL is the most commonly used surface finish and has numerous advantages. Despite its merits, it also has some disadvantages. In this section of our article, we will discuss some of its highlighting drawbacks and how they impact performance and reliability.

1. Thicker than Others: HASL is a thicker surface finish compared to ENIG and immersion silver. 
2. Not a Good Choice for SMD: As we know that HASL creates a relatively thicker layer of metal on the bare copper. This thicker layer of solder has the potential to make it challenging to achieve consistent and reliable solder joints on tiny SMD components.
3. Increased Threat of Sodler Bridges: Because HASL is applied by dipping the whole circuit boards into molten solder and then levelling with a hot knife, there is a greater risk of solder bridge formation in closely placed pads. Hence, circuit boards require a keen visual inspection after the process.
4. High-Temperature: The application process of HASL includes a high temperature, which can cause damage to the circuit.
5. Strict Parameters: HASL surface finish has some strict parameters for the circuit boards that are thinner or thicker than the normal ones. These parameters may reject your circuit board for this specific surface finish. So, in the case of customized thickness, it is preferred to contact your manufacturer.
6. Line Binding: Not appropriate for line binding.

Advantages of ENIG Surface Finish

Electroless Nickel Immersion Gold (ENIG) is one of the premium surface finishes. It bears excellent electrical and environmental properties. This surface finish has many advantages compared to HASL, immersion silver and others. In this section of the article, we are stating some of the advantages of ENIG.

• Availability: ENIG is a combination of immersion gold over electroless nickel. It has a wide demand in the market. Hence, most of the time, it is easily available. But, if you have a fixed manufacturer it is convenient to ask them if they provide ENIG or not. 

• Durable: Immersion gold over nickel plating protects underlying copper from corrosion and harsh environmental conditions which increased the shelf life of the PCB.

• Solderability: ENIG is well known for its solderability. It exhibits better wettability and even surface compared to HASL. Hence ENIG provides excellent joints between the components and the circuit board. 
• LEAD-FREE Surface Finish: Unlike HASL ENIG is a lead-free surface finish which makes it compatible with the guidelines of the Waste Electrical and Electronic Equipment Directive and Restriction of Hazardous Substances Directive against lead-free electronic products. 
• Electrical Conductivity: Due to gold ENIG provides better electrical and thermal conductivity than HASL and immersion silver.

Disadvantages of ENIG Surface Finish

Like everything else, ENIG has some disadvantages that should be taken into account before considering it to use as a surface finish for your circuit board. Some of the highlighting disadvantages of ENIG include.

• Cost: The first and foremost disadvantage of ENIG is its high cost. The main reason for this is the contribution of gold in its formulation. However, when compared to the properties of other surface finishes, ENIG justifies its cost.

• Gold Wire Bonding Issues: ENIG is a sensitive surface finish. It is prone to high temperatures during soldering. If not handled carefully, it can lead to gold wire bonding issues, which makes Quality Check hard to pass.  

• Reworking: With ENIG resoldering is a dream. The gold layer over nickel is extremely thin if not handled properly, it can cause surface defects. While hand-soldering melting and suction of solidified solder can uproot the gold layer.

• Signal Loss: Nickel has a high electric resistivity. It can cause signal loss at higher frequencies.

HASL vs ENIG: Variants of HASL and ENIG

Variants of HASL

Leaded HASL

It is the most commonly used variant of HASL. It uses solder with a composition of  63% Tin and 37% Lead.

Lead-Free HASL

Lead-free HASL is quite popular on the European and American sides due to the guidelines of the Waste Electrical and Electronic Equipment Directive and Restriction of Hazardous Substances Directive. It uses non-eutectic solder with a composition of 99% Tin and 1% copper or silver.

Organic Solderability Preservatives HASL

This variant of HASL provides extra protection to the bare copper. It is a combination of Organic Solderability Preservatives and HASL. The OSP is applied directly on the bar copper to prevent copper from oxidation and enhance solderability.

Flash Gold HASL

Flash Gold HASL uses a thin layer of flash gold over bare copper before the application of solder. It enhances the circuit board’s ability to resist corrosion and increase solderability.

Variants of ENIG

EPIG

Electroless Palladium Immersion Gold is a technique of depositing a small amount of palladium between the nickel and immersion gold layers. Palladium has a higher melting point compared to gold it behaves as a barrier between nickel and gold which reduces the diffusion of gold into the nickel.

ENEPIG

Electroless Nickel Electroless Palladium Immersion Gold ENEPIG involves setting a thin layer of palladium between nickel and immersion gold. It enhances the corrosion resistance of the circuit to an extent that it can survive in the worst environmental conditions. ENEPIG is a longer and more complex process than the EPIG

Hard Gold ENIG

This variant of ENIG bears a relatively thicker gold layer over nickel. A hard gold surface finish usually has a hardness of around 130-200 HK25.

Soft Gold ENIG 

It bears a thinner layer of gold and has a hardness of about 20-90 HK25.

Summary

PCB surface finish refers to a thin layer of metallic material applied to the exposed copper traces of a printed circuit board (PCB) to protect them from oxidation and facilitate the soldering process. There are different types of surface finishes, including HASL and ENIG. HASL involves dipping the PCB into molten solder to cover exposed copper traces and pads and then using hot air to level and smooth the surface. It is a budget-friendly surface finish that is easy to apply and readily available. However, it is thicker than other surface finishes, making it not suitable for tiny SMD components. ENIG, on the other hand, is the most commonly used surface finish for intense working and enduring circuit boards. It consists of a thin layer of gold plated over an electroless layer of nickel to protect it from oxidation. ENIG is a durable and reliable surface finish. It is suitable for circuit boards that have to operate under harsh environmental conditions, such as military, aerospace, marine, and medical industries. Despite its advantages, ENIG is relatively expensive than HASL.

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