Which PCB surface finish best fits your project’s requirements?
An often-overlooked decision in printed circuit board design is choosing the right surface finish. At least nine different choices are commonly available, and the right one depends on factors specific to your design, from regulatory requirements to how tightly your components are spaced. Thinking through these factors will help you make the best decision.
What Are PCB Surface Finishes?
A PCB surface finish is a thin coating applied to the exposed copper on a bare circuit board before components are assembled onto it. It serves two purposes: protecting the copper from oxidation, which would otherwise make it unsolderable within weeks of fabrication, and providing a solderable surface that lets components form a reliable connection during assembly. Without a finish, exposed copper degrades quickly enough that a board could become unusable before it ever reaches the assembly line.
The most common finishes include hot air solder leveling (HASL) and lead-free HASL, organic solderability preservative (OSP), immersion silver, immersion tin, electroless nickel immersion gold (ENIG), electroless nickel electroless palladium immersion gold (ENEPIG), and two electrolytic gold options: wire bondable gold and hard gold. Each behaves differently under the specific pressures a project puts on it, which is why the right choice comes down to matching the finish to your constraints rather than defaulting to whichever is most common.
Lead-Free Solder
Depending on where the product will be used after PCB assembly, you may face government regulations on the type of solder used in the project. Choices include:
- Lead-free hot air solder leveling
- Organic solderability preservative
- Immersion silver (BESTProto’s preferred finish for lead-free requirements)
- Immersion tin
- Electroless nickel immersion gold (ENIG)
- Electroless nickel
- Electroless nickel electroless palladium immersion gold
- Electrolytic wire bondable gold
- Electrolytic hard gold
Tight Pitch
Pitch refers to the center-to-center spacing between conductors, such as pads and pins. If you have a project, such as ball grid arrays, where that space is extremely minimal, it is vital to choose the right finish. Making a poor decision often leads to weak connections.
If you need a tight pitch, consider:
- Immersion silver
- Electroless nickel immersion gold (ENIG)
- Electroless nickel electroless palladium immersion gold
Shelf Life
Many different factors determine the shelf life of PCBs. Keeping the boards in a low-humidity environment that is dust-free helps extend their lives. Choosing quality materials for the PCB assembly can help. High frequencies and high loads on PCBs can make them fail more quickly. Boards should be stored in an environment that is colder than 30 degrees Celsius and has less than 85% relative humidity.
When shelf life is one of the deciding factors, choose:
- Hot-air solder leveling (HASL)
- Lead-free hot-air solder leveling (Lead-free HASL)
- Immersion silver
- Immersion tin
- Electroless nickel immersion gold (ENIG)
- Electroless nickel electroless palladium immersion gold
- Electrolytic wire bondable gold
- Electrolytic hard gold
Extra Costs
Some raw materials can be more costly to obtain. The manufacturer must pass these costs on to the company using the PCB. The market can change rapidly, but some types of PCBs that typically come at a higher cost include:
- Electroless nickel electroless palladium immersion gold (ENEPIG)
- Electrolytic wire bondable gold
- Electrolytic hard gold
Considering these different factors can help you make a more informed choice based on your specific needs. Contact BESTProto, and we will be happy to discuss the specifics of your PCB needs with you.
Frequently Asked Questions
What is the most widely used PCB surface finish today?
Historically, HASL (hot air solder leveling) was the most common surface finish due to its low cost and long track record, but ENIG has become increasingly common as fine-pitch and BGA components have become more widespread, since ENIG provides the flat, uniform surface those components need.
Can a single PCB use more than one surface finish?
Yes. A multi-finish board applies one finish, such as ENIG, to fine-pitch or BGA areas that need a flat surface, and a lower-cost finish, such as HASL, to the rest of the board, which can reduce overall cost without compromising performance where it matters most.
Does choosing a lead-free surface finish guarantee RoHS compliance for the whole board?
Not entirely. The surface finish is one part of a board’s lead content, but the solder paste and other materials used during assembly also factor into RoHS compliance, so a lead-free finish alone does not guarantee the whole assembly meets the standard.