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Connect the Dots: Designing for the Future of Manufacturing Reality—Surface Finish

When designing the complex boards that many electronic devices require to operate, designers should consider manufacturability at every step. This is my last article focused on designing for the always-evolving manufacturing reality.

The previous installment focused on the solder mask and legend process. The last phase of production involves applying surface finish to protect copper from oxidation and facilitate soldering components onto the board. Choosing the right surface finish has always been important. If you are creating intricate designs with a wide variety of components, like for an ultra-high density interconnect (UHDI) board, surface finish is a critical last step.

Surface finish for UHDI boards requires extreme flatness and high solderability for fine-pitch components. Applying surface finish to the solder pads can affect solderability during assembly to a limited extent, but today’s surface finishes are designed with solderability in mind and are necessary to sustain the productive life of the board.

Evaluating Surface Finishes
When selecting the right PCB surface finish for their project, designers find themselves weighing a variety of factors. Designers should consider the following when they establish their surface selection criteria:

• Cost
• Restriction of hazardous substances (RoHS)
• Flatness of the board surface
• Solder joint integrity
• Shelf life
• Wire bondable (if applicable)

With more boards having to accommodate larger components and more leads over multiple edges or surfaces, flatness is a more critical manufacturability consideration and surface finish plays a large role.

There are five commonly used surface finishes, plus several more that are used for specialty applications.

Hot Air Solder Leveling (HASL)

HASL has a long and successful history, and is still relied upon across many industries, most notably defense. It has become associated with the adage, “Nothing sells quite like solder.” HASL is cost-effective, durable in harsh environments, and very reliable. It also has a long shelf life (the time the board can sit in inventory before assembly) and provides excellent solderability.

But it has drawbacks. It often leaves a lack of flatness on the board that can be problematic during assembly, especially for IC components. The standard HASL surface finish includes lead, which is not compatible with RoHS requirements. Note that a lead-free HASL option is available. Designers should also consider the number of surface-mount components their board must accommodate before choosing HASL. If there are more than eight pads per side or fine pitch on an integrated circuit with many leads, flatness becomes an issue.

Immersion Silver

Immersion silver is a thin deposit that closely follows the contours of the copper and offers designers and manufacturers a very flat surface finish. It will be relatively level and have a good grain structure that the silver follows, creating exceptional flatness and planarity.

When we compare HASL to immersion silver, we are essentially contrasting old-school with new school. Bothcreate the best possible solder joints. [MS1] [KS2] [MS3] [KS4] The true advantage of immersion silver is its flatness and ability to handle fine-pitch components. It is also cost-effective compared to other options and in full compliance with RoHS standards.

Like your grandmother’s silver flatware, boards treated with immersion silver surface finish can be susceptible to heat, humidity, or even UV light. They will tarnish if left out in the elements. Boards with immersion silver surface finish that aren’t going directly to assembly should be stored and packaged as you would a moisture-sensitive component.

Electroless Nickel Immersion Gold (ENIG)

ENIG is a two-layer coating. The first layer is nickel electrolessly deposited atop the copper, with a layer of gold immersion-plated on top of the nickel. This surface finish includes two layers because nickel is a metal that, in its pure form (like it is here), passivates (becomes unreactive) in oxygen. The layer of gold protects the nickel from oxidation.

The solder joint forms between the tin in the solder and the nickel on the surface finish. This intermetallic is different from the copper-tin finish, meaning it has pluses and minuses, but ENIG produces solid solder joints for most applications. Like immersion silver, ENIG creates a flat surface—it’s a bit thicker than immersion silver. As you can imagine, cost is a big issue with ENIG as gold is among the most expensive raw materials on the planet, and the electroless immersion process is more costly than other surface finish application processes.

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

ENEPIG is the same surface finish as ENIG, but with a layer of palladium between the nickel and the gold. ENEPIG allows gold leads to be wire-bonded to the pads. Only specialized applications require wire bonding. ENEPIG is typically called out in the design specs, and entails the same cost considerations as ENIG, plus those associated with adding a palladium layer. That additional layer can also create solderability issues that may impact board durability and operability.

We recommend using ENEPIG surface finish only on the parts of the board where it is necessary, and ENIG on all other solderable surfaces. This can help contain costs, improve manufacturability, and avoid issues with board function.

Immersion Tin

As the name suggests, this method is similar to immersion silver. The result is a very flat and thin surface finish that is cost-effective, with a relatively long shelf life. This surface finish has low insertion friction, making it a viable surface finish where you have press-fit connectors. Though “nothing solders like solder,” the introduction of tin makes this a very solderable surface finish with a copper-tin intermetallic bond.

Immersion tin is more vulnerable to environmental conditions than immersion silver surface finish. Boards treated with immersion tin should be used promptly or stored with environmental concerns in mind.

These five methods represent the most common surface finishes, but there are others used for more specialized applications or to keep production costs down. Some of the more noteworthy finishes include:

• Electrolytic nickel with hard or soft gold 
• Electroless palladium immersion gold (EPIG)
• Organic solderability preservative (OSP)

Surface Finish Selection Tips
Every electronic device project has unique priorities that will impact decisions made by PCB designers. You often have to balance budgetary constraints with design constraints and functional requirements. Consider:

If containing production costs is your focus, HASL is probably your best bet. It is less expensive than other surface finishes, has good solderability, can sit on the shelf for up to a year, and is durable.

More complex boards—those with high-density interconnects—will likely have fine pitch components. We often use ENIG in these cases because it offers great solderability, resists oxidation, produces superior flatness, and ensures reliable joints. It’s a sound choice for boards operating in harsh environmental conditions.

Immersion silver is a solid choice for high-frequency applications because it delivers excellent signal integrity and limited signal loss.

For devices operating in harsh environments, ENEPIG offers solid protection against corrosion and oxidation.

For a deeper dive into the wide range of surface finish options, listen to the Designing for Reality: Surface Finish episode of On the Line With… .

This column originally appeared in the April 2026 issue of I-Connect007 Magazine.