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PCB Surface Finish

PCB Surface Finish OSP

The PCB surface is the most crucial part. PCB surface finish protects the copper surface from being oxidized. It provides the coating or the protective layer to the PCB surface. The oxidation will quickly deteriorate the copper. This affects the electrical performance of the PCB, leading to a useless PCB. Thus, applying a suitable PCB surface finish during the PCB manufacturing process is crucial. This article will provide information on surface finish on PCB, its types, and the right PCB surface finish based on your requirements.

What is PCB Surface Finish?

Surface Finish in PCB refers to the coating or treatment applied to the copper surface of PCB. It protects them from oxidation like a solder mask on PCB and also improves their solderability. These two are the significant functions of PCB surface finish. In PCB manufacturing, PCB finish is one of the most critical factors. It influences the overall performance of the circuit board, including reliability and solderability. 

The PCB finish also protects the PCB from environmental factors. Those factors can degrade the copper surface over some time. The selection of surface finish depends on the specific requirements and the manufacturing process. 

PCB finish is used for various reasons, including:

  • To prevent oxidation at the surface of copper
  • To help in soldering the components quickly.

PCB Surface Finish Types

PCB Surface Finish Types

The most common types of PCB surface finish are given below:

Hot Air Soldering Leveling (HASL)

HASL is the conventional type of surface finish applied to PCB. It involves the coating of the PCB using molten solder. It uses hot air to remove the excess solder, and it then leaves a thin layer on the surface of copper. 

HASL Surface finish is of two types: lead-based solder and lead-free HASL. 

Pros

  • Cost Effective
  • More suitable for through-hole components

Cons

  • Uneven surface not suitable for fine-pitch components
  • Thermal stress during processing could affect some electronic components

Electroless Nickel Immersion Gold Plating (ENIG)

PCB Surface Finish ENIG

The RoHS compliances and the features of this ENIG made it one of the most popular choices as a PCB finish for PCB. It consists of a thick layer of nickel and Gold. The gold coating over the nickel protects the nickel from oxidation. Likewise, nickel protects the copper and helps to sell it to its surface securely. 

Pros: 

  • Durable and long shelf life
  • Can bear harsh environment 

Cons:

  • It is expensive since it has a gold immersion process
  • Signal loss for signal integrity

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

ENEPIG is the variation of ENIG. It includes three layers: electroless nickel, followed by a layer of palladium, and immersion gold on the copper surface. The combination of this layer provides the strongest bondable option. Although it is quite expensive, it offers better solderability with a flat surface. It is mainly used for high-frequency applications. 

Pros:

  • Excellent for wire bonding.
  • Suitable for fine-pitch components.
  • Extremely Flat Surface

Cons:

  • It is a complex and costly process.
  • Challenges in thickness control.

Immersion Silver (IAg)

Immersion Silver includes a thick layer of silver on the copper surface. It provides good solderability and a flat surface for the fine-pitch components.

Pros:

  • Good solderability.
  • Flat surface suitable for fine-pitch components.

Cons:

  • Susceptible to tarnishing.
  • Limited shelf life.

Immersion White Tin (IWT):

Immersion White Tin includes a thin layer of white tin. It provides a good option for solderability. It is one of the lead-free surface finishes and is very thin and flat. It is the perfect choice for small components and fine-pitch technology. 

Pros:

  • Good solderability.
  • Flat surface suitable for fine-pitch components.

Cons:

  • Tin whisker formation over time.
  • Limited shelf life.

Organic Solderability Preservative (OSP)

PCB Surface Finish OSP

It is an organic coating used in PCB copper traces to provide good solderability and protection for oxidation. It contains an organic acid that forms a protective layer on the PCB surface after reacting with copper traces. It thus prevents the copper from getting oxidized from the air. 

Pros

  • Effective for surface mount technology due to its flat surface
  • Lower thermal stress on PCB 
  • Provides uniform coating thickness across the PCB surface

Cons

  • Limited Shelf Life around 6-12 months
  • At higher humidity levels, it could absorb moisture
  • Oxidation ability can decline over time

Hard Gold

Hard Gold includes the gold layer, which is plated over the layer of nickel coating. It is a gold alloy with iron, nickel, and cobalt. It provides a tough friction resistance compared to other surface finishes. This finish is a perfect option when a PCB is intended to be inserted into another board, like RAM or expansion cards. Based on the application, the thickness of this finish could vary.

Pros

  • Durable and tough 
  • Longer shelf life
  • Lead-free PCB finish

Cons

  • Expensive with poor solderability
  • Requires more processing time
  • It provides an uneven surface

Comparison of PCB Surface Finish

Requirements

HASL

ENIG

ENEPIG

ImAg

ImSn

OSP

Cost-Sensitive Product

Yes (Economical)

No (Relatively Expensive)

No (On the Pricier Side)

Yes (Budget-Friendly)

Yes (Cost-Efficient)

Yes (Economical)

High Volume Required

No (Complex)

No (Complex)

No (Complex)

Yes (Efficient)

Yes (Efficient)

Yes (Efficient)

Cosmetics of Surface Finish

Yes (Aesthetic)

No (Plain)

Yes (Sleek)

No (Plain)

Yes (Sleek)

Yes (Sleek)

Lead-Free Wave Solder

Yes (Compatible)

Yes (Compatible)

Yes (Compatible)

Yes (Compatible)

No (Not Ideal)

No (Not Ideal)

Fine Pitch Components Used

No (Not Ideal)

Yes (Ideal)

Yes (Ideal)

Yes (Ideal)

Yes (Ideal)

Yes (Ideal)

Wire Bonding Requirement

No (Not Needed)

Yes (Required)

Yes (Required)

Yes (Required)

No (Not Needed)

No (Not Needed)

High Yield ICT

Yes (Robust)

Yes (Robust)

Yes (Robust)

Yes (Robust)

Yes (Robust)

No (Not Ideal)

Lead-Free Shock/Drop

Yes (Durable)

No (Less Durable)

No (Less Durable)

Yes (Durable)

Yes (Durable)

Yes (Durable)

Corrosion Failure Possibility

Yes (Resistant)

Yes (Resistant)

Yes (Resistant)

No (Susceptible)

Yes (Resistant)

Yes (Resistant)

 

Factors to Consider While Selecting PCB Surface Finish

  • PCB Finish on the PCB determines the long-term reliability and working of the PCB. Thus, before selecting the PCB finish based on your requirements, you need to consider various things, including:
  • Understand your requirements, such as environmental conditions, temperature requirements, etc. 
  • Check the cost of various surface finishes, ensuring it balances the quality and quantity. 
  • Choose a PCB finish accommodating the PCB design’s component density and fine-pitch requirements.
  • Evaluate the solderability. HASL provides better solderability without compromising the cost. 
  • ENIG and ENEPIG suit your PCB, requiring aluminum and gold wire bonding.
  • Check the reworking option of the surface finishes. Many surface finishes, except immersion tin, can be used for reworking options. 
  • Consider the compatibility of the PCB finish with the specific assembly processes used in PCB manufacturing.
  • Evaluate the long-term durability of the PCB finish. Consider factors like corrosion resistance and wear resistance.
  • Consider the shelf life of the PCB finish and storage conditions.
  • Consider the thickness of the PCB finish and its impact on the planarity of the PCB.
  • Refer to industry standards and manufacturer recommendations for specific surface finishes.
  • Consider the capabilities of the PCB manufacturer or supplier in applying and handling the chosen surface finish.

Conclusion

Surface Finish comes in various types with their pros and cons. Considering the right PCB surface finish is essential in the PCB design process. Before selecting the right PCB surface finish, you must know your requirements. The surface finish will determine your PCB’s durability and reliability. Thus, by considering the application, reliability, environmental factors, signal integrity, and cost, you can ensure the dependability and performance of your electronic devices. Thus, not only is the selection of the right PCB surface finish, but you also need to choose the proper PCB manufacturer to have a reliable PCB for your needs. 

ElePCB is one of the leading manufacturers of PCBs. From prototype to production, Elepcb ensures quality products and customer satisfaction. So, if you have any queries or are confused regarding PCB surface finish, contact ElePCB with your requirements. We will ensure your PCB surface finish meets your needs.

FAQs

Which is the most common surface finish for PCB?

Electroless Nickel Immersion Gold (ENIG) is one of the most commonly used surface finishes. Its layer of Gold protects the nickel and provides a lower resistance for storing the thin Gold. This lead-free option results in a long-lasting and durable surface finish. ENIG surface finish is growing with the growth and use of RoHS regulations. It is also the best option for RF PCB.

What is the thickness of the PCB surface finish?

The typical thickness of PCB surface finish is 100 micro inches. The thickness of surface finish can vary depending on the types of surface finish and applications. HASL, for example, has a thickness of 1-40 micrometers. However, it depends based on the specific requirements. On the other hand, ENIG has a thickness of 3-6 micrometers, and the gold layer has a thickness of 2-6 microinches. Thus, always refer to the specifications provided by your PCB manufacturers to ensure the consistency of your surface finish. 

Which is the cheapest PCB surface finish?

HASL is the cheapest surface finish of all types. It is commonly available and very economical too. However, it has some drawbacks, like not being RoHS compliant and providing an uneven surface. Besides, it has a limited shelf life. 

CONTACT ELE

ELE PCB has got the IS09001:2015, IS013485:2016, ROHS and FCC certifications. We can offer all kinds of services, including PCB manufacturing and PCB assembly, sample orders and batch orders. For PCB assembly, utilizing 7 high-speed SMT PCBA lines from Yamaha and Sony, to meet our customers’ needs. Our extended services include PCB design &PCB Layout, hardware design engineering, firmware &software development, and personalization. ELE company is honored as an excellent supplier from any company all around the world. We deeply believe that our good service and experience will completely meet your needs. Integrality, value and innovation are the forces that drive our success.

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About Michael

Michael is the general manager of ELEPCB, a leading PCB design and manufacturing company based in China. He has over 10 years of experience in the PCB industry, and has been involved in various projects.

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