How to Choose the Appropriate PCB Surface Finish

A few factors must be considered when choosing the appropriate PCB Surface Finish. Essentially, the purpose, components used, working climate, and funding are all aspects of the Surface Treatment decision.

Among the most widely used surface treatments include HASL, ENIG, OSP, Immersion Tin, Immersion Silver, and Hard Gold. Choosing the type of finish will rely on the particular needs of the PCB. Each finish has benefits and drawbacks.

Generally, a surface treatment with excellent electrical characteristics, such as ENIG or Immersion Silver, may be chosen if the intended use includes high-speed digital data or high-frequency circuitry. On the contrary, a coating with excellent rust protection, such as Hard Gold or OSP, may be preferable if the PCB is subjected to severe conditions.

Overall, the right PCB Surface Finish is the one that meets the application’s requirements for efficiency and dependability while remaining within the allocated funds. As a result, this article will tackle the different PCB Surface Finish in detail.

What is PCB Surface Finish?

In order to prevent rust and guarantee excellent solderability during construction, a PCB’s uncovered copper lines and terminals are coated with a substance known as PCB Surface Finish.

After the board has been pierced, coated, and solder masking has been completed, the surface finish is usually applied. Several methods, including electroless plating, electrochemical plating, and chemical coating, are used to apply the polish.

In a PCB, the surface polish fulfills some crucial functions. First, it guards against copper rust, which can happen when exposed to oxygen and dampness, lowering copper’s electrical conductivity.

In addition, it offers a surface that is simple to glue during construction, guaranteeing trustworthy electrical contacts between the PCB and the components. The PCB Surface Finish also protects against external elements like rust and erosion, increasing the PCB’s general dependability and longevity.

Each Surface Treatment has benefits and drawbacks, including varying solderability, price, electrical characteristics, and longevity. The dependability and general performance of the PCB assemblage can be significantly impacted by choice of the proper surface treatment, which is a crucial component of the board’s layout.

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

The abbreviation “HASL” (Hot Air Solder Leveling) refers to a procedure to add a surface polish to a PCB. The PCB is submerged in a stream of liquid solder to cover the visible copper terminals and lines. In order to leave a consistent coating of solder on the surface, the extra solder is cleared by running the PCB over several hot air blades.

Since lead harms the environment, information was typically present in the glue used in HASL, which prompted environmental worries. Lead-free HASL, which replaces lead with a combination of tin, copper, and other metals, was developed to solve this problem.

Advantages

• Affordability: Given that the ingredients are readily available and the procedure is straightforward, HASL is an affordable PCB Surface Finish.
• Enhanced Solderability: HASL offers a surface with excellent solderability suitable for use with lead-based and lead-free solders. As a result, it is a well-liked option for PCBs that will be built with various variables.
• Outstanding Thermal Stability: The solder coating created by HASL has superb thermal endurance, meaning it can endure high temps during construction and use.
• Excellent Wire Bonding: The uneven surface texture of HASL offers exceptional adherence for wire bonding, making it a standard option for wire bonding uses.

Disadvantages

• Sporadic Surface Finish: Since hot air blades eliminate extra solder, HASL can produce an irregular surface finish. Further, high-speed impulses and fine-pitch components may experience issues as a result.
• Constrained Store Life: HASL must be used immediately after application because its store life is comparatively brief compared to other surface treatments.
• Environmental Concerns: Since lead is poisonous, using lead-based glue in HASL creates ecological issues. Lead-free HASL has been designed as a substitute but has drawbacks.
• Thermal Shock: The HASL procedure includes a thermal shock that can harm the PCB, mainly if it contains delicate components.

In general, HASL and Lead-Free HASL are affordable, offer good solderability, and provide good temperature stability. Still, their constraints should be carefully examined when choosing a PCB Surface Finish.

ENIG (Electroless Nickel Immersion Gold)

A PCB Surface Finish called ENIG (Electroless Nickel Immersion Gold) is employed to produce PCBs. It is a two-layer metallic covering comprising a thin layer of Gold (Au) on top of the nickel (Ni) layer applied over the copper terminals or traces of the PCB. To keep copper from diffusing into the gold layer and to improve the gold layer’s ability to be soldered, the nickel layer acts as a buffer between the copper and the gold coating.

An entirely organic procedure creates the electroless nickel coating; electricity is unnecessary. A fine nickel layer is formed on the PCB surface through a self-catalytic reduction of the nickel atoms in the plating solvent. Changing the plating solution’s content and formation duration can manage the nickel layer’s thickness.

When the PCB is submerged in a gold-ion-containing fluid, the immersion gold layer is created on top of the nickel layer. A fine coating of Gold is formed on the nickel surface after reducing the gold atoms. The Gold solution’s content and soaking duration can be changed to alter the thickness of the gold coating.

Due to its exceptional rust protection, smooth mounting surface, and good solderability for Surface Mount Technology (SMT) portions, ENIG is a well-liked surface finish for high-density PCBs used in electrical devices such as cellphones, iPads, and laptops.

Advantages

• Exceptional Corrosion Resistance: The PCB interface is more rust-resistant by acting as a buffer between the copper and gold layers and the nickel layer in ENIG.
• Flat Surface for Mounting Components: Since SMT components have a reduced size and need a flattened surface, the ENIG surface finish offers an even surface that is perfect for mounting them.
• Outstanding Solderability: ENIG’s gold coating offers superb solderability, essential for the PCB’s dependability throughout production.
• Consistent Deposition: Steady electrical performance is ensured throughout the PCB because of ENIG’s homogenous layering of the copper layers.

Disadvantages

• Costly: Due to the price of the Gold used in the procedure, ENIG is one of the priciest Surface Treatments.
• Thickness Constraints: Due to the Gold’s tendency to diffuse into the nickel layer and alter the PCB’s electrical characteristics, ENIG is subject to width restrictions.
• Brittle Gold Sheet: Since ENIG’s gold layer is comparatively thin, it is prone to splitting and fracturing when put under mechanical strain.
• Long Working Period: The ENIG method requires several stages, making it take longer to complete than other PCB Surface Finish.

ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)

In the manufacture of PCBs, ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) is a surface treatment. A layer of nickel (Ni) is applied over the copper terminals or lines of the printed circuit board (PCB), then comes an additional coating of palladium (Pd) on top of the nickel, and finally, a tiny amount of Gold (Au) on the cover of the palladium. The palladium layer is a diffusion barrier between the gold and nickel layers. In contrast, the nickel layer is a barrier between the copper and palladium sections.

A chemical procedure creates the electroless nickel coating without electrical current. The nickel atoms in the plating solution undergo reduction through a self-catalytic process, forming a fine nickel coating on the PCB surface. The thickness of the nickel coating can be managed by varying the percentage of the plating solution used and the application period.

A similar procedure places the electroless palladium layer on the nickel layer. After the plating solution’s palladium atoms are diminished, a fine palladium coating is created on the nickel surface. The percentage of the plating fluid and the formation period can be changed to alter the palladium layer’s width.

When the PCB is submerged in a gold-ion solution, the immersion gold layer is created on top of the palladium layer. A fine coating of Gold is formed on the palladium exterior by reducing the gold atoms. By varying the Gold solution’s content and the duration of soaking, the thickness of the gold coating can also be managed.

Due to its excellent anti-corrosion properties, flat mounting surface, and superior solderability for Surface Mount Technology (SMT) parts, ENEPIG is a well-liked PCB Surface Finish for high-reliability applications in electronic devices, such as those used in the aerospace, military, and medical industries. Furthermore, compared to other surface treatments like ENIG, it provides better wire soldering capabilities.

Advantages

• Better Wire Bonding Capacity: ENEPIG performs better than other surface treatments, making it the best option for high-reliability uses.
• Superb Corrosion Resistance: ENEPIG’s nickel and palladium layers act as a buffer between the copper and gold layers, which improves the PCB surface’s ability to withstand rust.
• Smooth Interface for Attaching Components: Since SMT components have a reduced size and need a flattened surface, the ENEPIG surface offers a smooth finish perfect for attaching these components.
• Superior Solderability: This is an excellent feature of ENEPIG’s gold layer, essential for the PCB’s dependability during construction.
• Thermal and Electrical Capabilities: ENEPIG offers better thermal and electrical stability than other surface treatments because it uses palladium as a medium for diffusion shield.

Disadvantages

• High Price: Due to the high expense of the palladium used in the procedure, ENEPIG is one of the costliest surface treatments.
• Thickness Limitations: In light of the Gold’s tendency to diffuse into the palladium layer and alter the PCB’s electrical characteristics, ENEPIG has width restrictions.
• Long Production Time: ENEPIG requires several stages, which can make it take slower to process than other Surface Treatments.
• Possibility for Black Pad: The palladium coating in ENEPIG may be vulnerable to the black pad, a flaw brought on by leftover chlorine from the refining procedure.

Hard Gold

The PCB Surface Finish, known as Hard Gold, creates PCBs. Usually, a layer of nickel is placed across a layer of Gold to act as a dispersion buffer between the copper layer of the PCB and the gold coating.

In contrast to other gold finishes used in PCBs, such as soft Gold or Electroless Nickel Immersion Gold (ENIG), the gold coating in a hard gold surface finish is usually bulkier. Hard Gold is typically used on the PCB where needed, like edge connections or places with frequent touch and high wear characteristics.

Hard Gold is usually applied using the electroplating technique, in which a coating of Gold is deposited onto the nickel layer using an electrical current. By varying the coating duration and current intensity, the width of the gold covering can be managed.

Advantages

• Remarkable Resilience to Wear: Hard Gold has a high wear resistance, making it perfect for connections and other uses requiring frequent contact.
• Minimal Interaction Impedance: The Hard Gold material has a low contact resistance, which enhances the PCB’s efficiency and reduces data loss.
• Outstanding Reusability: Hard Gold’s dense coating of Gold offers a high level of reusability for repetitive use, which is critical for services requiring long-term endurance.
• Tarnishing and Oxidation Resistance: Hard Gold is steadier and more dependable as a Surface Finishes over time because it is less susceptible to corrosion and rust than other gold treatments.

Disadvantages

• Expensive: The extent of the gold coating and the expense of the refining procedure make Hard Gold more costly than other surface treatments.
• Limitation on Thickness: During the coating process, Gold may diffuse into the nickel layer, which may impact the electrical characteristics of the PCB. This can result in a restriction on the thickness of the gold layer in Hard Gold.
• Not Suitable for All Applications: Hard Gold is frequently used in particular PCB locations, such as edge connections or other places that need frequent touch and high wear resistance. For some PCB uses, it might not be appropriate or required.

ImAg (Immersion Silver)

Another Surface Finishing, Immersion Silver (ImAg), is applied when PCBs are made. A fine coating of silver is deposited onto the PCB surface using a chemical casting procedure. The silver coating is applied to the PCB surface using the ImAg method instead of electroplating, which applies metallic to the surface using an electric current.

When high-frequency efficiency and minimal insertion loss are necessary, ImAg is frequently used in those applications. Due to its capacity to create a solid connection with lead-free solders is also a well-liked PCB Surface Finish for uses requiring lead-free materials.

In the ImAg procedure, the PCB surface is cleaned, the copper layer is activated, and the PCB is submerged in a silver substance. The silver coating is chemically fixed to stop rusting and lengthen the PCB’s storage life.

Advantages

• Highly Effective in High-Frequency Applications: It has minimal insertion loss and good signal fidelity, making Immersion Silver (ImAg) an excellent surface treatment for high-frequency uses.
• Cost-Efficient: ImAg is a more affordable choice for PCBs than other surface treatments like Hard Gold because of its cost-effectiveness.
• Suitable for Lead-Free Operations: ImAg is well-suited for lead-free uses because it solidly bonds with lead-free solders.
• Consistent Surface Texture: ImAg creates a stable and smooth surface, which can help the PCB work better and be more reliable.

Disadvantages

• Prone to Corrosion and Oxidation: ImAg creates a stable and smooth surface, which can help the PCB work better and be more reliable.
• Durability: ImAg may not be as enduring as surface coatings like Hard Gold or Electroless Nickel Immersion Gold (ENIG).
• Special Handling Required: ImAg may need specific treatment and storage to protect the silver coating.
• Limited Control over Thickness: ImAg’s silver coating thickness can be difficult to manage compared to other PCB Surface Finishes.

ImSn (Immersion Tin)

A distinctive PCB Surface Finish utilized in PCB production is Immersion Tin. (ImSn). An excellent tin coating is chemically deposited onto the PCB surface during this procedure. In contrast to the conventional Hot Air Solder Leveling (HASL) method, which coats the PCB’s surface with liquid solder, the ImSn method employs a non-toxic metal alloy instead.

In the ImSn procedure, the PCB surface is cleaned, the copper layer is activated, and the PCB is submerged in a tin substance. The tin coating is chemically fixed to stop decomposition and lengthen the PCB’s lifespan during storage.

ImSn is used in many uses, such as power electronics, high-speed digital circuitry, and vehicle electronics. Due to its ability to avoid solder bridging between terminals that are near together, it is also utilized in situations where Surface Mount Technology (SMT) components are employed.

Advantages

• Even Surface Texture: Immersion Tin (ImSn) creates an even and consistent Surface Finish that can enhance the PCB’s functionality and dependability.
• Suitable for Lead-Free Operations: ImSn is a standard option for lead-free uses because it creates a solid connection with lead-free solders.
• Ideal for High-Speed Digital Circuits: ImSn has minimal insertion loss and good signal fidelity, making it an excellent surface finish for high-speed digital electronics.
• Excellent for SMT Components: ImSn is a superior PCB Surface Finish for SMT components because it helps avoid solder coupling between terminals that are near together.

Disadvantages

• Susceptible to Whisker Growth: ImSn may be prone to bristle development, resulting in short circuits and other issues with the PCB.
• Effectiveness: ImSn’s tin coating erodes over time, making it unsuitable for connections and other uses that must be used repeatedly.
• Specified Handling Specification: ImSn may need to be handled and stored in a specific way to protect the tin coating.
• Limited Availability: ImSn may not be as broadly accessible as other Surface Treatments, making it more challenging to find and use in some situations.

OSP (Organic Solderability Preservatives)

A Surface Finish, known as Organic Solderability Preservatives (OSP), is employed in producing PCBs. A fine coating of organic substance is laid down to shield the copper surface of the PCB from rust and other external variables.

Essentially, OSP ensures that the copper traces and terminals on the PCB stay pristine and unoxidized during manufacturing. The OSP is covering aids in preventing the growth of a copper oxide layer on the copper’s surface, which may hinder the solder’s ability to adhere to the metal during construction.

Advantages

• Cost-Efficient: As opposed to other Surface Treatments, like Electroless Nickel Immersion Gold (ENIG) or Immersion Silver, OSP is a more affordable option.
• Ecologically Favorable: OSP is an environmentally friendly choice because it doesn’t contain harmful or heavy metals.
• Simple to Integrate: Compared to other Surface Treatments, OSP is comparatively simple, cutting production time and expense.
• Excellent for Fine-Pitch Constituents: OSP is an outstanding PCB Surface Finish for smaller and more intricate PCBs because it can be employed for fine-pitch items.

Disadvantages

• Sensitivity to Handling: OSP needs cautious storing and Handling to retain its efficacy because it is more susceptible to Handling than other surface treatments.
• Limited Shelf Life: OSP can degrade over time and has a finite shelf life. It might be necessary to reapply the OSP covering if it is not used within a predetermined time frame.
• Humidity Sensitivity: Humidity and other external variables may generally influence OSP’s performance.
• Decreased Durability: OSP is less resistant to wear and strain than other surface treatments, which can lessen its efficacy over time.

OSP is an inexpensive and ecologically favorable choice for PCB surface treatments; it has drawbacks and must be handled and stored carefully to keep working correctly.

Choosing the Perfect Surface Finish for your PCB

After individually discussing the frequently used PCB Surface Finish, we have compiled a checkbook wherein one can assess whether a particular Surface Polish is appropriate for their desired purpose and application through a table.

Summary

In summary, selecting the highly suitable PCB Surface Finish for one’s application is essential since it will dictate its performance in safeguarding the copper traces and pads from the PCB. Apart from that, PCB Surface Finish is primarily responsible for ensuring the solderability of the board is exceptional. Thus, it must be thoroughly evaluated and assessed.

If you’re considering applying a quality PCB Surface Finish to your application, look no further because MV Flex Circuit can perform this for you. Furthermore, we can satisfy any specifications you may have without settling for mediocrity in production.

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