Introduction to PCB Surface Finishes

Printed Circuit Boards (PCBs) are essential components in modern electronic devices, providing a platform for electrical connections and mechanical support for various components. The copper layer on a PCB is crucial for establishing electrical connections, but it is prone to oxidation and contamination when exposed to the environment. To protect the copper layer and enhance the PCB’s performance, various surface finishes are applied to the exposed copper surfaces. These surface finishes not only protect the copper but also improve solderability, corrosion resistance, and the overall reliability of the PCB.

One of the key aspects of PCB surface finishes is the formation of an Intermetallic Layer between the copper and the surface finish material. This intermetallic layer plays a vital role in ensuring a strong bond between the copper and the surface finish, as well as providing additional protection against oxidation and contamination.

In this article, we will explore the importance of the intermetallic layer in PCB surface finishes, discuss the various types of surface finishes available, and examine their properties, advantages, and limitations.

The Role of the Intermetallic Layer

What is an Intermetallic Layer?

An intermetallic layer is a thin layer formed at the interface between two dissimilar metals, in this case, the copper layer of the PCB and the surface finish material. This layer is created through a chemical reaction or diffusion process, resulting in a new phase with distinct properties from the original metals.

The formation of an intermetallic layer is crucial for several reasons:

1. Bonding Strength: The intermetallic layer provides a strong metallurgical bond between the copper and the surface finish, ensuring excellent adhesion and preventing delamination.

2. Diffusion Barrier: The intermetallic layer acts as a barrier, slowing down the diffusion of copper atoms into the surface finish material and vice versa. This helps maintain the integrity of both the copper layer and the surface finish over time.

3. Corrosion Resistance: The intermetallic layer can enhance the corrosion resistance of the PCB by preventing the direct exposure of the copper layer to the environment.

Factors Affecting Intermetallic Layer Formation

The formation and growth of the intermetallic layer depend on several factors:

1. Temperature: Higher temperatures accelerate the formation and growth of the intermetallic layer. This is particularly relevant during the soldering process, where the PCB is exposed to elevated temperatures.

2. Time: The thickness of the intermetallic layer increases with time, as the diffusion process continues. However, excessive growth of the intermetallic layer can lead to brittleness and reduced reliability.

3. Surface Finish Material: The choice of surface finish material influences the formation and properties of the intermetallic layer. Different surface finishes have varying affinities for forming intermetallic compounds with copper.

Types of PCB Surface Finishes

There are several types of PCB surface finishes available, each with its own advantages, limitations, and intermetallic layer characteristics. Some of the most common surface finishes include:

Hot Air Solder Leveling (HASL)

HASL is one of the oldest and most widely used surface finishes in the PCB industry. In this process, the PCB is dipped into a molten solder bath, and the excess solder is removed using hot air knives. The resulting surface finish is a thin layer of solder (typically tin-lead or lead-free) on top of the copper.

Advantages:
– Low cost
– Excellent solderability
– Good shelf life

Limitations:
– Uneven surface due to the dipping process
– Not suitable for fine-pitch components
– Potential for thermal shock during the dipping process

Intermetallic Layer:
In HASL, the intermetallic layer is formed between the copper and the solder. The most common intermetallic compounds are Cu6Sn5 and Cu3Sn, depending on the solder composition and the process parameters. The growth of the intermetallic layer is relatively rapid due to the high temperatures involved in the HASL process.

Electroless Nickel Immersion Gold (ENIG)

ENIG is a two-layer surface finish that consists of an electroless nickel layer plated onto the copper, followed by a thin immersion gold layer. The nickel layer provides a barrier between the copper and the gold, while the gold layer ensures excellent solderability and protects the nickel from oxidation.

Advantages:
– Excellent solderability
– Good corrosion resistance
– Suitable for fine-pitch components
– Flat and uniform surface

Limitations:
– Higher cost compared to HASL
– Potential for “black pad” defect due to excessive phosphorus content in the nickel layer
– Gold can dissolve into the solder during multiple reflow cycles

Intermetallic Layer:
In ENIG, the intermetallic layer is formed between the copper and the electroless nickel. The most common intermetallic compound is Ni3P, which grows slowly and provides a stable barrier between the copper and the nickel. The gold layer does not form a significant intermetallic layer with the nickel.

Immersion Tin (ISn)

Immersion tin is a single-layer surface finish that involves the deposition of a thin layer of tin onto the copper surface through a chemical displacement reaction. The tin layer provides good solderability and protects the copper from oxidation.

Advantages:
– Low cost
– Good solderability
– Suitable for press-fit connectors
– Flat and uniform surface

Limitations:
– Limited shelf life due to tin whisker growth
– Potential for copper dissolution during the immersion process
– Not suitable for multiple reflow cycles

Intermetallic Layer:
In immersion tin, the intermetallic layer is formed between the copper and the tin. The most common intermetallic compound is Cu6Sn5, which grows relatively quickly. The thickness of the intermetallic layer should be controlled to maintain good solderability and prevent excessive consumption of the copper layer.

Organo-Metallic Coatings (OSP)

OSP is a thin, organic surface finish that forms a protective layer on the copper surface. The organic compounds in OSP bond with the copper to create a barrier against oxidation and contamination.

Advantages:
– Very low cost
– Excellent solderability
– Suitable for fine-pitch components
– Environmentally friendly

Limitations:
– Limited shelf life due to the degradation of the organic layer
– Potential for poor adhesion if the copper surface is not properly prepared
– Not suitable for multiple reflow cycles

Intermetallic Layer:
OSP does not form a traditional intermetallic layer with the copper. Instead, the organic compounds in OSP bond with the copper surface, creating a thin, protective layer. This layer does not provide the same level of protection as metallic intermetallic layers but is sufficient for short-term storage and single reflow cycles.

Comparison of PCB Surface Finishes

The choice of PCB surface finish depends on various factors, such as the application, budget, component types, and manufacturing requirements. The following table compares the key properties of the surface finishes discussed in this article:

Selecting the Right PCB Surface Finish

When selecting a PCB surface finish, designers and manufacturers should consider the following factors:

1. Application Requirements: The surface finish should be compatible with the components and the assembly process used in the application. For example, fine-pitch components may require a flat and uniform surface finish like ENIG or OSP.

2. Cost: The budget for the project will influence the choice of surface finish. HASL and immersion tin are low-cost options, while ENIG is more expensive.

3. Shelf Life: If the PCBs need to be stored for an extended period before assembly, a surface finish with a longer shelf life, such as ENIG, should be considered.

4. Reflow Cycles: If the PCB will undergo multiple reflow cycles during assembly, a surface finish that can withstand multiple thermal exposures, such as ENIG, should be chosen.

5. Environmental Considerations: Some surface finishes, such as OSP, are more environmentally friendly than others. This may be a factor in the selection process, particularly for companies with strong environmental policies.

Conclusion

The intermetallic layer formed between the copper layer and the surface finish is a critical aspect of PCB surface finishes. This layer provides a strong bond, acts as a diffusion barrier, and enhances the corrosion resistance of the PCB. The choice of surface finish depends on various factors, including the application, cost, shelf life, and environmental considerations.

By understanding the role of the intermetallic layer and the properties of different surface finishes, designers and manufacturers can select the most suitable option for their specific requirements, ensuring the reliability and performance of their PCBs.

Frequently Asked Questions (FAQ)

1. What is the purpose of the intermetallic layer in PCB surface finishes?
   The intermetallic layer provides a strong metallurgical bond between the copper and the surface finish, acts as a diffusion barrier, and enhances the corrosion resistance of the PCB.

2. What factors affect the formation of the intermetallic layer?
   The formation and growth of the intermetallic layer are influenced by temperature, time, and the choice of surface finish material.

3. Which surface finish is the most cost-effective?
   HASL and immersion tin are the most cost-effective surface finishes, while ENIG is more expensive.

4. Which surface finish is best for fine-pitch components?
   ENIG and OSP are the most suitable surface finishes for fine-pitch components due to their flat and uniform surfaces.

5. What is the “black pad” defect in ENIG surface finish?
   The “black pad” defect is caused by excessive phosphorus content in the electroless nickel layer, which can lead to poor solderability and reduced reliability of the PCB.