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:
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.
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.
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.
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.
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.
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.
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:
- Cleaning: The copper surface of the PCB is cleaned of oil, fingerprints and other contaminants that could affect the flatness of the applied finish.
- 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.
- Acid rinse: The PCB undergoes an acid rinse in a sulfuric acid solution.
- OSP application: At this point in the process, the OSP solution is applied to the PCB.
- 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.
- 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.
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.
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:
- Applying the activator.
- Applying the electroless nickel.
- 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.