PCB Surface Finishes: HASL, OSP and ENIG

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When creating surface finishes for printed circuit boards (PCBs), you can choose from either organic or metallic materials. Knowing which types of surface finishes exist is simple enough, but how do you determine which is best for your PCB? They share similarities, but each comes with its own benefits, disadvantages and technical considerations.

If you&#;re wondering how to select the right finishing technique, this article can help you learn more about three common types &#; Hot Air Solder Leveling (HASL), Organic Solderability Preservative (OSP) and Electroless Nickel Immersion Gold (ENIG).

The Importance of Your PCB Surface Finish

The surface finish you choose for your PCB will have a significant impact on its quality and usefulness within its parent part. The finish prevents the PCB&#;s copper layer from oxidizing, which would otherwise decrease its solderability. Applying a finish protects the board from oxidizing before the components are added, ensuring that you can solder the additional elements as needed. Because the PCB&#;s electrical connections rely on adequate copper conductivity, it&#;s essential to prevent oxidation and adhere the components as necessary.

Selecting the appropriate finish will depend on various factors, such as the finishing process, the PCB&#;s design and the quality of the final result. Not all finish types are appropriate for every PCB. Below are a few characteristics to keep in mind when considering the different ones:

1. Solderability

Avoiding soldering issues is essential to creating a PCB that will operate as intended. Smooth surfaces are necessary to ensure a connection that functions adequately within its given environment. Consider whether the surface finish can solder directly to the copper, such as in the case of immersion tin, or if it&#;s a layered technique, like ENIG.

Good wire bonding is also crucial, considering that different metals require unique manufacturing techniques and behave differently even in the same environments. Wires may consist of materials like aluminum, gold and copper, with each surface finish type being compatible or incompatible with these elements.

2. Processing Time

The processing window for a specific finish may be large or small &#; finishes like HASL have larger processing windows.

How much time the process takes depends on how complex the assembly is. Some surface finishes, like OSP, have limited thermal cycles and can&#;t withstand numerous soldering processes. After a few cycles, it will disappear, and the PCB will lose its protection against oxidation. However, an OSP surface finish for a PCB can be reworked during fabrication. The same applies to immersion silver.

ENIG is a more complex finishing process and takes longer to complete, which may make it better to use for PCB production lines that don&#;t need to output a large number of PCBs in a short time.

3. Reliability

How well will the chosen surface finish stand up to its environment? If your PCB will need to meet specific reliability requirements, you will want a finish that accommodates these. Although cost is only one component of the many other elements mentioned here, you should consider how high the cost of failure will be if the PCB&#;s finish doesn&#;t work as intended and fails to protect the surface.

The IPC offers a set of standards under its TM-650 Test Methods Manual that details different ways to test PCBs for reliability and quality. These include techniques such as dimensional verification, chemical resistance, copper ductility and amount of signal loss. One test uses chemicals such as sulfuric acid and isopropanol to determine the effects these substances have on PCB dielectric materials.

4. Corrosion

Some surface finishing types, like silver, can be more vulnerable to creep corrosion than others. This kind of corrosion can occur with essentially any type of finish you use, but it&#;s most common with immersion silver. As the corrosion of metal elements spreads along the PCB&#;s surface, it can cause short circuits due to interacting with neighboring features on the board. This issue has become more common as lead-free finishes have increased in widespread use, as lead was notably effective at providing resistance to corrosion.

Creep corrosion is also more common in humid, sulfur-rich environments. Consider what kind of settings your PCBs will be exposed to before settling on any one surface finish. The environment may have a major hand in determining your products&#; life cycles.

Other finishes like immersion tin are prone to developing whiskers, although anti-migration agents can reduce this issue.

Factors That Affect the PCB Surface Finish You Use

A range of factors will affect which type of PCB finish you decide to use. It&#;s essential to consider all of these factors as an integrated whole. Though they all hold weight within the process, your finished PCB may not turn out as intended if you favor one characteristic greatly over the others. For example, you may choose lead-free HASL because of its cost-effectiveness but later realize that its co-planarity is lacking compared to other finishing processes.

Below are some crucial components to keep in mind when investigating the various PCB surface finishes.

1. Cost

How much you spend on surface finishing is determined by the type you choose. Finish quality also factors into the cost. HASL finishes are more affordable than ENIG types, but they may not meet the precise quality standards you expect.

The price of fabricating the board itself will also matter. After the initial fabrication, will there be room in your budget to select more advanced finishing methods such as ENIG? PCBs for industries like consumer electronics are decidedly less expensive to manufacture, which can allow you to utilize more costly finishes for higher-quality results.

2. Volume

Production volume can significantly impact which type of surface finishing you use. For example, a surface finish consisting of immersion tin will begin tarnishing soon after being deposited onto the copper PCB. However, using a high volume of PCBs can help you avoid tarnishing. If you have a small batch, it may be better to go for a finish like immersion silver.

3. Cosmetics

Processes like HASL and OSP tend to offer a better aesthetic value than ENIG, which is another component that could influence which kind you select. Do you need a shiny surface that&#;s less susceptible to creep corrosion, whiskers and similar issues? You may want to avoid dealing with materials like silver and tin and choose other options that are less likely to cause significant corrosion.

Types of PCB Surface Finishes

Though many types of PCB surface finishes exist, we&#;ll be covering three well-known ones here &#; HASL, OSP and ENIG.

1. HASL and Lead-Free HASL

A HASL surface finish offers high-quality solderability and accommodates multiple thermal cycles while also being one of the more affordable options. It once served as the industry standard, although standards under the Restriction of Hazardous Substances (RoHS) have caused HASL to fall out of compliance. In turn, lead-free HASL has become the more acceptable option in terms of its environmental and health impacts. Although HASL has a long industry history and is well-known among engineers, lead-free HASL is safer to use and better fits the needs of directives like RoHS.

HASL finishes are created by dipping the board into a tin and lead or tin and nickel solder and holding it there for some time. Once the PCB is removed, hot bursts of air called air knives remove the excess finish. HASL finishing allows for a large processing window, but various factors can affect its evenness and therefore its solderability. The angle of the air knives, the air pressure and the speed of the PCB board&#;s entry and removal from the solder all influence the finish&#;s quality.

You&#;ll find HAL and lead-free HASL surface finishes used for applications such as:

• Electrical testing: HASL finishes provide automatic protection for the test pads and vias during electrical testing of the circuit boards.
• Hand soldering: HASL finishes are a suitable option for hand soldering processes, as the joints are easily formable.
• High-performance electronics: HASL is often a great choice for high-performance and high-reliability applications, like aerospace and military devices, because of its ability to form strong joints.

2. OSP

An OSP surface finish is an example of an organic PCB finish. There are no toxins involved in the process, making it environmentally friendly while still retaining its protective and anti-corrosive properties. Because of the absence of harmful chemicals, OSP boards are also RoHS compliant. This water-based finish provides a flat surface for attaching additional PCB components, and like the HASL process, it is cost-effective.

OSP can be used as an effective replacement for lead-free HASL due to its co-planarity. When you need a PCB surface finish that will provide sufficient flatness while offering a simple manufacturing process, OSP is arguably the best choice.

Applying an OSP surface finish to a PCB usually involves a conveyorized chemical method or a vertical dip tank. The process generally looks like this, with rinses between each step:

1. Cleaning: The copper surface of the PCB is cleaned of oil, fingerprints and other contaminants that could affect the flatness of the applied finish.
2. Topography enhancement: The exposed copper surface undergoes micro-etching to increase the bond between the board and the OSP. This process also reduces oxidation. To achieve adequate film thickness, the micro-etching must be kept at a consistent speed.
3. Acid rinse: The PCB undergoes an acid rinse in a sulfuric acid solution.
4. OSP application: At this point in the process, the OSP solution is applied to the PCB.
5. Deionization rinse: The OSP solution is infused with ions to allow for easy elimination during soldering. This rinse should be used before preservatives build to avoid tarnishing from the presence of other ions in the OSP solution.
6. Dry: After the OSP finish is applied, the PCB must be dried.

OSP offers a simple and affordable process, but it&#;s also important to keep in mind that it&#;s extremely sensitive to handling and can easily retain scratches, which can degrade its solderability. Additionally, its shelf life is shorter than that of ENIG or HASL.

For more ENIG Automotive PCB Boardinformation, please contact us. We will provide professional answers.

Common uses for OSP include:

• Fine pitch devices: This finish is best to apply to fine pitch devices because of the lack of co-planar pads or uneven surfaces.
• Server boards: OSP&#;s uses range from low-end applications to high-frequency server boards. This wide variation in usability makes it suitable for numerous applications. It&#;s also often used for selective finishing.
• Surface mount technology (SMT): OSP works well for SMT assembly, for when you need to attach a component directly to a PCB&#;s surface.

3. ENIG

Despite the higher pricing of ENIG surface finishing, it has a high success rate for producing high-quality products. It holds up under multiple thermal cycles, showcases great solderability and is a suitable option for wire bonding. As the name suggests, it consists of two coating layers &#; nickel and gold. The nickel protects the base copper layer and enables secure attachment of electrical components, while the gold serves as an anti-corrosion measure for the nickel.

ENIG can be used in situations where tight tolerances are required for PCB elements like plated holes, as HASL is not as effective for this. Like OSP, it offers excellent flatness and is ideal for fine pitch devices.

Applying the ENIG coating requires nickel to be deposited onto a copper surface catalyzed with palladium. The immersion gold stage causes the gold to adhere to the nickel by way of a molecular exchange. ENIG is similar to OSP in its inclusion of micro-etching and rinsing between each stage &#; the process includes these steps:

1. Cleaning.
2. Micro-etching.
3. Pre-dipping.
4. Applying the activator.
5. Post-dipping.
6. Applying the electroless nickel.
7. Applying the immersion gold.

Standard applications for an ENIG surface finish include:

• Complex surface components: Because of its flat surface, this finish type is a preferable choice for complex PCB components requiring smooth surfaces, like ball grid arrays (BGAs) or quad flat packages (QFPs).
• Wire bonding: ENIG finishes allow for minimal wire bonding for aluminum wires, although they are not compatible with gold.
• High reliability applications: You will often find ENIG surface finishes used for PCBs in industries such as aerospace, military, medical and high-end consumers. Its quality makes it suitable for applications where precision and durability are vital.

Contact Millennium Circuits Limited for More Information on PCB Surface Finishes

With so many surface finishes to choose from, it can be overwhelming to figure out which is best for your desired application. Fortunately, Millennium Circuits Unlimited can assist you throughout the process.

MCL is a printed circuit board supplier delivering both small-batch and high-volume PCBs to consumers around the globe. As an ISO- certified PCB company, we have years of expertise on printed circuit boards and all their intricacies. Because PCBs and PCB accessories are all we do, we have the knowledge to help you with whatever questions or concerns you may have regarding this form of technology.

If you would like to learn more information about PCB surface finishes and explore your available choices, contact us today by filling out our form or calling us at 717-558-.

Immersion gold, also known as Electroless Nickel Immersion Gold (ENIG), is a common type of printed circuit board (PCB) surface finish. It involves depositing a thin layer of gold on the PCB's copper traces and pads using a chemical process that does not require an electrical current.

The thickness of an Electroless Nickel Immersion Gold (ENIG) coating on a printed circuit board (PCB) typically ranges from 0.05 to 0.2 microns for the nickel layer and 0.05 to 0.15 microns for the gold layer.

Immersion Gold (ENIG) PCB Coating

The thickness of the ENIG coating is critical to ensure optimal performance and reliability of the PCB. The nickel layer thickness is important for preventing the gold layer from diffusing into the copper, while the gold layer thickness is critical for ensuring good solderability and wire bonding. Immersion gold PCB surface finish is a popular choice for high-reliability PCB applications, such as aerospace, defense, medical, and telecommunications industries, where performance and reliability are critical.

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.

In this article, we will explore the concept of Immersion Gold (ENIG) as a PCB surface finish, what is it, and the benefits and applications of using it in your PCB.

Steps Involved in Coating Immersion Gold

The process of coating immersion gold to a printed circuit board (PCB) involves several steps:

Layers of Gold and Nickel deposited over a PCB

1. Cleaning: The PCB is first cleaned to remove any dirt, oil, or other contaminants from the surface. This is typically done using a combination of mechanical and chemical cleaning processes.
2. Etching: The PCB is then etched to remove any unwanted copper from the surface. This is typically done using an acid solution, which selectively dissolves the copper and leaves the remaining circuitry intact.
3. Electroless Nickel Deposition: The PCB is then immersed in a bath of electroless nickel solution. This solution contains a reducing agent and a source of nickel ions, which react to deposit a thin layer of nickel onto the exposed copper surfaces. The nickel layer serves as a barrier between the copper and the gold, preventing the gold from diffusing into the copper.
4. Rinse: The PCB is then rinsed to remove any excess nickel solution from the surface.
5. Immersion Gold Deposition: The PCB is then immersed in a bath of gold solution, which contains a reducing agent and a source of gold ions. The gold ions react with the nickel layer to deposit a thin layer of gold onto the surface of the PCB. The gold layer provides a reliable and durable surface for soldering and wire bonding.
6. Final Rinse: The PCB is then rinsed again to remove any excess gold solution from the surface.
7. Drying and Inspection: The PCB is then dried and inspected for any defects or irregularities in the coating.

Advantages/Disadvantages of ENIG Surface Finish

Here's some information on the advantages and disadvantages of using ENIG as a surface finish:

Advantages of ENIG:

• Good Corrosion Resistance: The nickel layer in ENIG provides excellent corrosion resistance, which is important for applications where the PCB may be exposed to harsh environments.
• Uniform Thickness: ENIG provides a uniform layer thickness, which is essential for fine-pitch surface mount technology. This ensures that there is no variation in the thickness of the coating, which can cause problems during assembly.
• Excellent for Wire Bonding: ENIG is an excellent surface finish for wire bonding applications because it provides a highly conductive surface that is easy to bond to.
• Flat Surface: ENIG provides a flat surface that is suitable for chip-scale packages (CSPs) and other applications where the surface must be flat.
• Suitable for Multiple Reflow Cycles: ENIG is suitable for multiple reflow cycles, making it a good choice for applications that require repeated assembly.

ENIG Properties

RoHS Compliance

Yes

Flat Surface

Yes

Aluminium Wire Bond

Yes

Solderability

Good

Shelf Life

One year or higher

Cost

High

Disadvantages of ENIG:

• Higher Cost: ENIG is more expensive than other surface finishes, such as HASL. The additional cost is due to the use of gold in the plating process.
• Sensitive to Handling: ENIG requires careful handling during the manufacturing process, as nickel is sensitive to oxidation. Any oxidation on the nickel layer can cause adhesion problems during assembly.
• Limited Thermal Shock Resistance: ENIG is susceptible to thermal shock, which can cause the nickel layer to crack and peel. This can lead to poor adhesion and other problems.
• Difficult to Inspect: The nickel layer of ENIG is difficult to inspect visually, which can make it challenging to detect defects during the manufacturing process.
• Not suitable for high-temperature applications: The nickel-gold intermetallic compound in ENIG can break down at elevated temperatures. This makes ENIG unsuitable for applications that require high-temperature resistance.

Applications of Immersion Gold (ENIG) PCB Surface Finish

Following are some applications of immersion gold surface finish in different industries:

1. Electronics Industry: The electronics industry is one of the biggest users of immersion gold surface finish. This surface finish is widely used in electronic components such as printed circuit boards (PCBs), integrated circuits (ICs), and connectors. Immersion gold provides excellent corrosion resistance, flat surface, and uniform thickness, which are crucial for electronic components that require high reliability, especially in harsh environments.
2. Medical Industry: The medical industry also uses immersion gold surface finish in its products. Medical devices, such as pacemakers, heart monitors, and defibrillators, require high reliability and long-term performance. Immersion gold provides excellent corrosion resistance and flat surfaces, which are necessary for these devices.
3. Aerospace Industry: The aerospace industry also uses immersion gold surface finish in its products. The aerospace industry demands high reliability, durability, and safety in its products. Immersion gold provides excellent corrosion resistance and uniform thickness, which are essential for aerospace products that operate in harsh environments and require long-term performance.
4. Telecommunications Industry: The telecommunications industry also uses immersion gold surface finish in its products. Telecommunications products, such as routers, switches, and servers, require high reliability, durability, and performance. Immersion gold provides a uniform layer thickness, which is critical for fine-pitch surface mount technology, and excellent wire bonding properties, which are necessary for high-speed data transmission.
5. Automotive Industry: The automotive industry also uses immersion gold surface finish in its products. Automotive electronics, such as engine control units (ECUs), airbag sensors, and anti-lock brake systems (ABS), require high reliability, durability, and safety. Immersion gold provides excellent corrosion resistance, flat surfaces, and uniform thickness, which are necessary for automotive electronics that operate in harsh environments.

Conclusion

In conclusion, PCB surface finishes are crucial for protecting exposed copper on a PCB from oxidation, ensuring a reliable connection between components and the board. Electroless Nickel Immersion Gold (ENIG) is a common and popular type of PCB surface finish in the electronics industry, widely used in high-reliability PCB applications, such as aerospace, defense, medical, and telecommunications industries. ENIG provides good corrosion resistance, and uniform thickness, and is an excellent surface finish for wire bonding applications because it provides a highly conductive surface that is easy to bond to.

However, ENIG is more expensive than other surface finishes and requires careful handling during the manufacturing process. It is also not suitable for high-temperature applications, and the nickel layer is difficult to inspect visually, which can make it challenging to detect defects during the manufacturing process.

Contact us to discuss your requirements of Industrial Control PCB Wholesale. Our experienced sales team can help you identify the options that best suit your needs.