Surface finish is a thin layer of protective coating applied onto the surface of printed circuit boards to protect the copper pads and traces from corrosion, oxidation and to provide a level solderable surface. It also improves the bond strength between the PCB and electronic components. Surface finish is an essential step in PCB manufacturing, which enhances the overall quality and reliability of the electronic circuit. There are different types of surface finishes available, which can be selected based on the specific requirements of the PCB.
The primary purpose of surface finish is to protect the copper traces and pads from oxidation, which can lead to poor solderability and decreased electrical performance. During the PCB manufacturing process, copper is exposed to the air, which causes copper oxidation. Therefore, a surface finish is necessary to prevent copper from oxidizing and provide a protective layer to the PCB. A good surface finish also enhances the bond strength between the PCB and the components.
There are several types of surface finishes used in the PCB manufacturing process. Here are some popular types of surface finishes:
PCB with HASL Surface Finish
HASL is one of the most commonly used surface finishes. It involves the application of molten solder on the copper surface of the PCB to create a solderable surface. HASL is a cost-effective option but is not suitable for fine-pitch components.
Advantages of HASL:
Disadvantages of HASL:
PCB with Lead-free HASL
Lead-free HASL is a surface finish process used on PCBs to protect copper traces from oxidation and to promote solderability. As the name suggests, it is a lead-free version of the traditional HASL process, which uses a molten tin-lead alloy to create a protective coating on the board. Lead-free HASL, on the other hand, uses a tin-copper-nickel alloy that is free from lead, which is better for the environment and in compliance with RoHS (Restriction of Hazardous Substances) regulations.
Advantages of lead-free HASL include:
Environmentally friendly: As lead is a toxic substance, its elimination from electrical and electronic equipment contributes to a cleaner and safer environment.
Cost-effective: Lead-free HASL is a less expensive process compared to many other PCB surface finishes, such as ENIG and ENEPIG.
Good solderability: The lead-free alloy used in HASL provides good solderability, which is essential for components to be reliably attached to the board.
Disadvantages of lead-free HASL include:
Limited shelf life: The lead-free HASL process requires careful monitoring to ensure that the plating solution remains within specification. Once the solution degrades or becomes contaminated, it can no longer be used, which can be a wasteful and costly process.
Relatively thick coating: Lead-free HASL produces a relatively thick coating, which can lead to issues with fine-pitch components and potentially affect board thickness tolerances.
Susceptible to oxidation: The exposed copper pads on the surface of the lead-free HASL finish can oxidize if left uncoated, which can negatively impact solderability.
PCB With ENIG Surface Finish
ENIG is a widely used surface finish that offers a flat and uniform surface. It consists of a thin layer of nickel followed by a layer of gold. ENIG provides excellent electrical conductivity and is suitable for fine-pitch components.
Advantages of ENIG:
Good corrosion resistance: ENIG provides excellent corrosion resistance, making it ideal for applications where the PCB will be exposed to harsh environments.
Flat surface: ENIG produces a flat surface that is ideal for fine pitch surface mount components. This flat surface also means that components can be easily placed and soldered in the correct position.
Excellent conductivity: ENIG provides excellent electrical conductivity, which is important for the proper functioning of the PCB.
Good solderability: ENIG has good solderability, making it ideal for applications where the PCB will be subjected to high temperatures during the soldering process.
Disadvantages of ENIG:
Cost: ENIG is a relatively expensive finish. This cost may be prohibitive for some applications.
Thickness variation: There can be variation in the thickness of the nickel and gold layers, which can lead to problems with soldering or conductivity.
Black pad: ENIG can be susceptible to a defect called “black pad,” where the nickel layer is porous and corroded, leading to reliability problems.
Difficult to rework: ENIG can be difficult to rework compared to other finishes, such as HASL or OSP.
Application:
PCB Board with OSP Surface Finish
OSP is a surface finish that protects copper by applying a layer of organic material. It provides excellent solderability and is suitable for fine-pitch components. OSP is a cost-effective option, but it has a short shelf life.
Advantages of OSP:
Low Cost: OSPs are cost-effective in comparison with other surface finishes, such as ENIG and Immersion Tin.
Simple Process: OSPs are easy to apply, and only require one step to protect the copper surface.
Good Solderability: The thin layer of OSP does not affect the solderability of the surface, as it dissolves during the soldering process, and allows the solder to bond with the copper beneath.
Disadvantages of OSP:
Limited Shelf Life: OSP coatings have a limited shelf life, as they start to degrade over time, reducing the effectiveness of the coating.
Sensitive to Environment: OSP coatings are sensitive to humidity and temperature and need to be stored in controlled atmospheric conditions, or they can become ineffective.
Limited Thickness: OSP coatings are usually very thin, which means that they offer limited protection against wear and tear and corrosion compared to other surface finishes.
PCB With Immersion Tin Surface Finish
Immersion tin is a surface finish that consists of depositing a layer of tin on the copper surface of the PCB. It provides excellent planarity and solderability. The immersion tin is suitable for fine-pitch components.
Advantages of Immersion Tin:
Good solderability: Immersion Tin provides a flat and even surface that allows for good solderability.
Surface Finish: Immersion Tin offers a smooth and uniform surface finish that results in good electrical performance.
Environmentally friendly: Immersion Tin is considered environmentally friendly since it is lead-free compared to other surface finishes.
Cost-effective: Immersion Tin is less expensive than other surface finishes such as ENIG, OSP, and HASL.
Compatibility with a variety of components: Immersion Tin can be used with a wide range of electronic components, including surface mount technology (SMT) components, through-hole components, and connectors.
Disadvantages of Immersion Tin:
Storage difficulties: Immersion Tin has a limited shelf life, and the material must be stored carefully to prevent oxidation.
Limited thermal resistance: Immersion Tin has relatively poor thermal resistance, so it may not be suitable for high-temperature applications.
Brittle nature: Immersion Tin has a relatively brittle nature, which can lead to cracking and flaking during handling and assembly, particularly in thin-film designs.
Process sensitivity: The Immersion Tin process is sensitive to various process parameters and requires tight process control to ensure consistency.
Flat surface requirement: Immersion Tin requires a flat and uniformly finished copper surface to ensure good adhesion and prevent defects and failures.
PCB With Immersion Silver Surface Finish
Immersion silver is a surface finish that involves the deposition of a thin layer of silver on the copper surface of the PCB. It provides excellent electrical conductivity and is suitable for high-frequency applications.
Advantages of Immersion silver:
Good solderability: Immersion silver provides excellent solderability, which is required for attaching electronic components to PCBs.
Flatness: The surface of immersion silver is flat, which ensures good contact between the electronic component and the PCB surface.
Good electrical conductivity: Immersion silver has good electrical conductivity, which is necessary for the proper functioning of electronic devices.
Reliability: Immersion silver provides a more reliable finish compared to other finishes, such as OSP or HASL.
Disadvantages of Immersion silver:
Cost: Immersion silver is more expensive compared to other surface finishes such as HASL and OSP.
Corrosion susceptibility: Immersion silver is more susceptible to corrosion compared to other finishes such as ENIG and ENEPIG.
Silver whisker growth: Silver whisker growth can occur on the immersion silver surface, which can lead to short circuits and device failures.
Shelf life: Immersion silver has a limited shelf life, and the PCBs may need to be reprocessed if they are not used within a certain timeframe.
Circuit Board with ENEPIG Surface Finish
ENEPIG stands for Electroless Nickel Electroless Palladium Immersion Gold. It is a type of surface finish used in the electronics industry to protect PCBs from corrosion and oxidation. ENEPIG consists of multiple layers of electroless nickel, electroless palladium, and immersion gold, which provide superior oxidation resistance, conductivity, and surface flatness. This type of finish has gained popularity due to its ability to withstand multiple thermal cycles and provide a reliable interconnection between components on the PCB.
Advantages of ENEPIG:
High Reliability: ENEPIG is known for its high reliability and excellent electrical properties, which make it a popular choice for high-end electronic applications.
Flat Surface Finish: The immersion gold layer in ENEPIG provides an extremely flat surface, which is essential for fine-pitch surface mount technology components.
Good Solderability: The palladium layer in ENEPIG provides good solderability, which improves the solder joint strength.
Good Corrosion Resistance: The Nickel layer in ENEPIG provides good corrosion resistance to the PCB.
Multiple Thermal Cycles: ENEPIG has the ability to withstand multiple thermal cycles, making it an ideal choice for devices that need to operate under extreme conditions.
Disadvantages of ENEPIG:
High Cost: ENEPIG is generally more expensive compared to other surface finishes, such as HASL and OSP.
Complex Process: The process of ENEPIG involves multiple steps, which require high precision and increase the overall processing time.
Palladium Availability: Palladium is a rare metal, and its availability in the market can affect the cost of ENEPIG.
Thickness Control: Controlling the thickness of the individual layers in ENEPIG can be difficult, leading to inconsistencies and reliability issues.
Application:
Choosing the right surface finish for your PCB is crucial for ensuring the reliability and performance of your electronic device. Here are some factors to consider when choosing the right surface finish:
Solderability: The surface finish of your PCB should have good solderability. It should form a strong bond between the solder and the copper pad without any issues like solder bridging or solder balls.
Corrosion resistance: Your PCB surface finish must offer resistance to corrosion, as it will be exposed to a variety of harsh environments during its lifetime.
RoHS compliance: With the increasing regulatory pressure, it is important to choose a surface finish that is RoHS compliant.
Cost: The choice of a surface finish can affect the overall cost of your PCB manufacturing.
Shelf life: Shelf life is important, especially if you don’t manufacture PCBs regularly. Some surface finishes tend to deteriorate over time, so it’s necessary to choose a finish that ensures a longer shelf life.
Processing time and complexity: Some surface finishes require additional processing steps, which can add to the overall manufacturing time and cost. Hence, it is essential to choose a surface finish that is easy to process and suits your manufacturing needs.
By considering these factors, you can choose the right surface finish that suits your application and manufacturing requirements.
The selection of surface finish for PCBs is a crucial step in the PCB fabrication process due to its direct impact on process yields, rework numbers, field failure rate, test capability, scrap rate, and cost. Therefore, it is essential to consider all the important aspects of assembly during surface finish selection to ensure high-quality and high-performance end products.
Having introduced each type of surface finish, the most critical elements that determine the selection standard are some of their attributes. To aid in the selection process, the table below shows the attributes that each type of surface finish possesses or lacks. By considering the specific requirements and features of PCB products, you can use this table to select the ideal surface finish option that matches your needs.
| Items | HASL | ENIG | ENEPIG | Immersion Silver | Immersion Tin | OSP |
|---|---|---|---|---|---|---|
| Cost | Low | High | High | High | High | Low |
| Lead Time | Short | Moderate | Moderate | Moderate | Moderate | Short |
| Solderability | Good | Excellent | Excellent | Excellent | Excellent | Good |
| High volume required | No | No | No | Yes | Yes | Yes |
| Cosmetics of surface finish | √ | x | √ | x | √ | √ |
| Lead-free wave solder | √ | √ | √ | √ | x | x |
| Fine pitch components used | x | √ | √ | √ | √ | √ |
| Requirement of wire bonding to surface finish | x | √ | √ | √ | x | x |
| High yield ICT | √ | √ | √ | √ | √ | x |
| Lead-free shock/drop | √ | x | x | √ | √ | √ |
| Possibilities of corrosion failure | √ | √ | √ | x | √ | √ |
Regarding surface finishes for PCB manufacturing, HASL, and OSP tend to be the least expensive options, while ENIG and ENEPIG are typically the most costly. As for corrosion resistance, HASL and Immersion Tin demonstrate the highest capabilities, whereas Immersion Silver falls short. Regarding ICT, OSP typically scores the lowest, but the others perform similarly well. Finally, when it comes to hole fill, both HASL and ENIG outperform other types of surface finishes. Overall, selecting the ideal surface finish involves weighing costs, corrosion resistance, ICT performance, and hole-fill capability to ensure the best outcomes for any given PCB manufacturing project.
When placing an order for PCBs, there are several important factors to consider such as PCB substrate material, solder mask, silkscreen, board size and thickness, copper thickness, blind and buried vias, through-hole plating, SMT, panels, tolerances, and surface finish. Among these factors, surface finish is of utmost importance as it significantly contributes to the reliability of electronic products. The copper layer on PCBs can easily oxidize, resulting in a reduction in soldering quality that can ultimately decrease the validity and reliability of the end products. Selecting the right surface finish is essential to prevent oxidation and ensure excellent solderability and electrical performance.
With environmental issues becoming increasingly prevalent in the electronics industry, RoHS has been published by the EU to limit the use of hazardous materials in electrical and electronic products. All applicable products in the EU market must pass RoHS compliance standards since July 1, 2006. Hence, surface finishes with lead-free solder will have more followers in the future.
Choosing the right PCB surface finish can be challenging, but JHYPCB is here to help. We are your best-in-class solution for PCB prototype, fabrication, and assembly. With over four decades of experience, we have worked with almost every leading technology company and industry worldwide. Our experienced sales team is always ready to help you select the best surface finish for your project. Contact us today to learn more.
HASL involves applying a thin layer of molten solder onto the exposed copper pads, while ENIG involves depositing a layer of nickel and gold onto the copper pads using a chemical process.
Yes, immersion tin/silver surface finishes are compatible with lead-free soldering processes.
PCB gold fingers refer to the gold-plated contacts or connectors on the edge of a printed circuit board (PCB). They are typically used as an interface for plugging in other circuit boards or connectors. The gold plating is applied to these fingers because gold is corrosion-resistant, which helps ensure good contact conductivity over time. Gold fingers can be found in many types of electronic devices, including computer motherboards, graphics cards, and memory modules.
Hard gold is a particular type of gold plating used for printed circuit boards (PCBs). It refers to a thick coating of pure gold, usually ranging from 30 to 50 micro-inches, that is electroplated onto the surface of the PCB for the purpose of making contacts. The term "hard" refers to the durability and wear resistance of the gold plating, which can withstand repeated pressure and friction without wearing down over time. Hard gold plating is typically used for contacts and connectors exposed to harsh environments or frequent use, such as connectors on memory modules, edge connectors, or gold fingers on PCBs.
Soft gold is a type of surface finish used in PCB manufacturing that provides a relatively thin layer of gold coating to protect copper traces from oxidation and to provide a reliable contact surface for components. The gold layer in soft gold finish is typically composed of roughly 99.9% pure gold, with a few other trace elements to adjust its characteristics. Compared to hard gold finish, soft gold has a lower resistance to wear, but it is more malleable, which makes it easier to use in applications where there might be some mechanical stress or deformation. Soft gold is commonly used in applications where low-voltage, low-current signals are transmitted, such as in RF or microwave circuits.
ENIG and ENEPIG are two popular surface finishing methods used in PCB manufacturing. Here are the main differences between them:
Nickel thickness: ENIG (Electroless Nickel Immersion Gold) is a two-layer process consisting of a thin layer of nickel (typically 2-6 µm) and a layer of gold (0.05-0.15 µm) on top of the nickel. In contrast, ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) has an additional layer of palladium (0.15-0.25 µm) plated over the nickel before applying the gold layer.
Shelf life: ENIG has a relatively short shelf life compared to ENEPIG. ENIG surface finish deteriorates more quickly due to the presence of organic compounds in the nickel layer. ENEPIG, on the other hand, has a longer shelf life due to the use of palladium, which is more resistant to oxidation and corrosion.
Solder joint integrity: ENEPIG is known to have better solder joint integrity than ENIG due to the presence of the additional layer of palladium. Palladium acts as a barrier between the nickel and gold layers, preventing intermetallic formation, and helping to improve solder joint strength.
Lead-free soldering compatibility: ENEPIG is generally considered to be more compatible with lead-free soldering processes than ENIG due to its superior solder joint reliability and barrier properties.
HASL and Lead-free HASL are two different methods of coating a printed circuit board (PCB) for protection and to improve solderability. HASL stands for Hot Air Solder Leveling, which involves dipping the PCB in molten solder and then using hot air knives to remove the excess solder, leaving a thin, even coating.
Lead-free HASL is similar, but it uses a lead-free solder, typically made of tin, silver, and copper. The use of lead-free solder is important due to environmental concerns regarding the toxicity of lead.
The main difference between HASL and lead-free HASL coating is the type of solder used. Lead-free HASL is considered to be environmentally friendly and is becoming increasingly popular in the PCB industry. However, lead-free HASL is often more expensive than traditional HASL due to the higher cost of the lead-free solder materials.
The shelf life of immersion Tin depends on several factors, including the type and quality of the immersion Tin used, as well as storage conditions. In general, immersion Tin has a shelf life of approximately six to twelve months when stored properly.
When stored in a dry, cool, and dark environment, immersion Tin can last for up to twelve months. However, exposure to moisture, high temperatures, and bright light can reduce its shelf life. It is important to store immersion Tin in a sealed container or bag with a desiccant to help prevent moisture exposure. It is also recommended to periodically test the quality of immersion Tin to ensure that it remains effective and suitable for use.
Conformal coating is a protective coating applied to the surface of printed circuit boards in order to protect them from environmental factors such as moisture, dust, dirt, and other contaminants that may cause the board to malfunction. The conformal coating material can be either a liquid or a spray, and can be applied to the PCB surface in several different ways, such as brushing, dipping, or spraying. The coating conforms to the shape of the PCB, hence the name "conformal." The coating provides insulation against harsh environments, helps prevent corrosion, and improves the lifespan and reliability of the PCB.
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