What are PCB surface finishes? Types of PCB surface finishes.
Editorial Team - PCB Directory
May 31, 2023
PCB surface finishes protect exposed copper on a PCB from oxidation and ensure a reliable connection between components and the board. They are crucial for the performance, reliability, and lifespan of the PCB, and impact manufacturing quality. They also impact the manufacturing process, including the quality of the soldering and assembly.
Different types of PCB surface finishes
In this article, we will explore the concept of surface finishes, what they are, their benefits, applications of using them in your PCB, and the different types of surface finishes used in PCB manufacturing.
Immersion Silver (ImAg)
Immersion Silver (ImAg) PCB Surface Finish is a type of coating applied to the copper on a printed circuit board (PCB). The ImAg process involves immersing the PCB in a solution containing silver ions, which are reduced to elemental silver on the surface of the copper.
Immersion Silver PCB surface finish
Advantages of ImAg surface finish
Disadvantages of ImAg surface finish
Excellent Solderability: ImAg provides a highly conductive surface that is easy to wet and spread with solder.
Susceptible to Tarnishing: ImAg is susceptible to tarnishing and can become discolored or lose its conductivity over time.
Good Wire Bonding Performance: ImAg is suitable for use in applications that require wire bonding, such as medical devices.
Limited Shelf Life: ImAg has a limited shelf life and can degrade over time.
Environmentally Friendly: ImAg does not contain any hazardous chemicals or heavy metals, making it a more environmentally friendly option.
Requires Careful Handling: ImAg requires careful handling during the manufacturing process to avoid contamination.
High Surface Energy: The high surface energy of ImAg makes it easier for components to adhere to the PCB surface.
Thin Layer: The ImAg layer is relatively thin, which may not provide adequate protection against certain types of corrosion or wear.
Low Cost: ImAg is relatively low cost compared to other surface finishes.
ImAg may not be suitable for certain applications that require a higher thickness or a different level of protection.
Immersion Gold (ENIG)
Immersion Gold (ENIG) PCB surface finish
Immersion Gold (ENIG) PCB Surface Finish is a type of coating applied to the copper on a printed circuit board (PCB). The ENIG process involves depositing a thin layer of gold over the copper, followed by the immersion of the PCB in a solution containing nickel ions. The nickel chemically displaces the gold, forming a nickel-gold intermetallic compound.
Advantages of ENIG surface finish
Disadvantages of ENIG surface finish
Good Corrosion Resistance: ENIG provides excellent corrosion resistance due to the protective nickel layer.
Higher Cost: ENIG is more expensive than other surface finishes, such as HASL.
Uniform Thickness: ENIG provides a uniform layer thickness, which is important for fine-pitch surface mount technology.
Sensitive to Handling: ENIG requires careful handling during the manufacturing process as nickel is sensitive to oxidation.
Excellent for Wire Bonding: ENIG is an excellent surface finish for wire bonding applications, as it provides a highly conductive surface that is easy to bond to.
Limited Thermal Shock Resistance: ENIG is susceptible to thermal shock, which can cause the nickel layer to crack and peel.
Flat Surface: ENIG provides a flat surface that is suitable for chip-scale packages (CSPs).
Difficult to Inspect: The nickel layer of ENIG is difficult to inspect visually.
Suitable for Multiple Reflow Cycles: ENIG is suitable for multiple reflow cycles.
Not Suitable for High-Temperature Applications: The nickel-gold intermetallic compound can break down at elevated temperatures.
Immersion Tin (ImSn)
This PCB Surface Finish is a type of coating applied to the copper on a printed circuit board (PCB). The I Sn process involves immersing the PCB in a solution containing stannous ions, which are reduced to elemental tin on the surface of the copper.
Immersion Tin (ImSn) PCB surface finish
Advantages of ImSn surface finish
Disadvantages of ImSn surface finish
Flat Surface: ImSn provides a flat surface with good planarity, which is useful for SMT components.
Poor Wire Bonding Performance: ImSn is not suitable for wire bonding applications due to its brittleness.
Good Solderability: ImSn provides good solderability and can be easily wetted by solder.
Limited Thermal Resistance: ImSn has limited thermal resistance.
Long Shelf Life: ImSn has a long shelf life.
Susceptible to Tin Whiskers: ImSn is susceptible to tin whisker growth, which can lead to short circuits.
Cost-Effective: ImSn is relatively cost-effective compared to other surface finishes.
Sensitive to Handling: ImSn is sensitive to handling and requires careful treatment.
Limited Corrosion Resistance: ImSn provides limited corrosion resistance and may not be suitable for harsh environments.
Organic Solderability Preservatives (OSPs)
Organic Solderability Preservatives (OSPs) are a type of surface finish used in printed circuit boards (PCBs) that provides a layer of organic material on the copper pads. OSP is a water-based, organic compound that forms a protective layer over the copper.
Advantages of OSP surface finish
Disadvantages of OSP surface finish
Flat Surface: OSP provides a flat surface finish on the PCB.
Short Shelf Life: OSP has a short shelf life and can degrade over time.
Cost-Effective: OSP is one of the most cost-effective surface finishes available.
Susceptible to Damage: OSP is easily damaged during handling and can be affected by contaminants.
Simple Process: The OSP process is relatively simple compared to other surface finishes.
Not Suitable for High-Temperature Applications: It can break down and lose its protective properties.
Environmentally Friendly: OSP is an environmentally friendly option since it doesn't contain heavy metals.
Limited Reworkability: OSP is difficult to rework since the organic layer can be damaged during the rework process.
No Additional Thickness: Unlike other surface finishes, OSP does not add any additional thickness to the PCB.
Limited Corrosion Protection: OSP provides limited protection against corrosion.
LF-HASL Surface Finish
LF-HASL (Lead-Free Hot Air Solder Leveling) is a surface finish used in the production of printed circuit boards (PCBs). This process involves coating the exposed copper surfaces of a PCB with a thin layer of molten solder, which is then leveled off by hot air knives. LF-HASL is a lead-free alternative to the traditional HASL process, which used a tin-lead alloy that was banned due to environmental concerns.
Advantages of LF-HASL surface finish
Disadvantages of LF-HASL surface finish
Low Cost: LF-HASL is a relatively low-cost process compared to some other surface finishes.
Not Suitable for High-Frequency Applications: The relatively thick layer of solder used in LF-HASL can cause issues in high-frequency applications due to its impedance and signal loss characteristics.
Good Solderability: The thin layer of solder provides good solderability.
Not Suitable for Tight Tolerance Applications: The thickness of the solder layer can vary and cause deviations from the design specifications.
Good Shelf Life: LF-HASL has a long shelf life and can be stored for extended periods.
Good for Fine-Pitch Components: LF-HASL is well-suited for use with fine-pitch components.
Uniform surface: The hot air leveling process creates a uniform surface, which helps to prevent component placement issues.
HASL (Tin-Lead) PCB Surface Finish
HASL (Tin-Lead) PCB Surface Finish, which stands for Hot Air Solder Leveling, is a common surface finish used in printed circuit boards (PCBs). This process involves coating the copper surface with a layer of tin-lead alloy, followed by hot air leveling to create a flat and uniform surface for soldering.
Advantages of HASL surface finish
Disadvantages of HASL surface finish
Good Solderability: HASL provides a reliable and robust solder joint, making it ideal for through-hole components and wave soldering.
Not Environmentally Friendly: HASL contains lead, which is hazardous to the environment and human health.
Cost-Effective: HASL is a relatively low-cost surface finish compared to other alternatives.
Uneven Surface: HASL can create an uneven surface due to the hot air leveling process, making it unsuitable for fine-pitch components and surface mount technology.
Widely Available: HASL is widely available and can be applied to a wide range of PCBs.
Thickness variation: The thickness of the HASL coating can vary depending on the size and shape of the PCB.
Long Shelf Life: HASL has a long shelf life and can maintain its properties for an extended period.
Cost: The cost of HASL can be higher than other surface finishes, such as OSP or ENIG.
RoHS compliance: HASL with tin-lead is not RoHS compliant.
Comparison Table
Property
Immersion Silver (I Ag)
OSP
LF-HASL
HASL (Tin-Lead)
Cost
High
Medium
Lowest
Low
Estimated Shelf Life (in months)
12
6
Thermal Shock
Very Low
Wetting Performance
Very Good
Good
Best
Fine Pitch
Excellent
Nice
Integrity
Wire Bonding
Yes
No
RoHS Compliant
Conclusion
To summarize, PCB surface finishes are essential for protecting the exposed copper on a PCB from oxidation and ensuring reliable connections between components and the board. They impact the performance, reliability, and lifespan of the PCB, as well as the manufacturing process. The four main types of PCB surface finishes discussed in this article are Immersion Silver (ImAg), Immersion Gold (ENIG), Immersion Tin (I Sn), and Organic Solderability Preservatives (OSPs). Each of these finishes has its advantages and disadvantages, and the selection of the finish depends on the specific application and requirements of the PCB. It is important to carefully consider the factors such as cost, solderability, wire bonding, corrosion resistance, and shelf life while choosing the right PCB surface finish.
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