Choosing a PCB Surface Finish: Five Options Compared

The surface finish is a thin layer of metal or organic material that protects the copper from oxidation and provides a solderable surface. It is chosen in seconds on most projects and it decides whether the assembly process runs smoothly, how long the boards can be stored, and what the joints look like afterwards. This article compares the common options on the four properties that actually matter.

What the Finish Has to Do

Four requirements apply to every finish, and the options trade them against each other.

The first is solderability: the surface has to wet and form a joint within the thermal budget of the process, and it has to do so after whatever storage and handling the board experiences.

The second is flatness. A finish that adds significant and uneven thickness cannot be used with fine pitch components, because the paste deposit and the component lead have to sit on a defined plane.

The third is shelf life. A finish that oxidises in months forces a production schedule, while one that lasts a year or more gives the supply chain room to breathe.

The fourth is durability: the surface has to survive handling, test probing and, where applicable, multiple reflow cycles without losing its solderability.

A fifth property applies to some designs only. Aluminium wire bonding, gold wire bonding and press fit connectors all place requirements on the finish that have nothing to do with soldering, and where those processes exist on the same board the finish choice is often made for them.

Hot Air Solder Levelling

HASL coats the copper with solder and then blows it flat with hot air. It is the cheapest finish, it is the most forgiving, and it has the longest history of use.

Its disadvantage is the surface. The levelling leaves a meniscus on each pad and a variation in thickness that is manageable for a 1 mm pitch component and marginal at 0.5 mm. For a fine pitch assembly it is unsuitable, which is why lead free HASL has been largely replaced by other finishes on modern boards.

It remains a good choice where the pitch is coarse, the volume is high and the cost matters. It is also more tolerant of poor storage than the thin finishes, because the solder layer is relatively thick.

Organic Solderability Preservative

OSP is an organic coating a few tens of nanometres thick that protects the copper and is removed by the flux during soldering. It is flat, inexpensive and simple to apply, and it produces a surface that is chemically clean at the moment of soldering.

Its limitations are shelf life and handling. The coating degrades with heat and with time, so the storage life is measured in months rather than years, and each reflow cycle consumes some of it. A board that will be assembled twice may need the finish reapplied or handled with care. OSP is also difficult to probe: the test probe has to penetrate the coating, which it does reliably on a bare board but not always on a pad that has already been reflowed once.

Where the assembly is a single reflow with a short interval between fabrication and build, OSP is an excellent and economical choice. Where the boards will be stored for a year or assembled in stages, it is a risk.

Immersion Tin and Immersion Silver

Both are thin metallic finishes deposited by a chemical displacement reaction, and both are flat enough for fine pitch work.

Immersion tin gives a solderable surface with good shelf life and a lower cost than gold. Its weakness is the intermetallic layer that grows between the tin and the copper over time, which consumes the tin and can eventually affect solderability, and its sensitivity to handling, since it is soft and easily contaminated.

Immersion silver is flat, has good solderability and good high frequency behaviour, which makes it popular on radio boards. Its weakness is tarnish: silver reacts with sulphur in the atmosphere and darkens, and while the tarnish does not prevent soldering, it makes inspection harder and it requires sulphur free packaging. Our article on immersion silver covers the storage requirements.

Electroless Nickel Immersion Gold

ENIG deposits a nickel layer of a few micrometres with a very thin gold layer on top. The nickel provides the barrier and the mechanical strength; the gold protects the nickel from oxidation and provides a surface that stays solderable for a long time.

It is flat, durable, has a shelf life of a year or more, withstands multiple reflow cycles and probing, and can be used for aluminium wire bonding. Those properties explain why it dominates fine pitch, high reliability and long life products despite costing several times as much as HASL.

The failure mode to know about is black pad: if the nickel layer is over attacked during the gold deposition, the surface can become brittle and produce a weak joint that fails at the interface. It is a process control issue, so the specification should include the nickel thickness and the corrosion resistance test, and the supplier should be able to demonstrate both.

Hard Gold and ENEPIG

Two other finishes cover the specialist cases.

Hard gold is an electroplated gold alloy that is thick and wear resistant, and it is used on edge connectors, test pads that are probed thousands of times, and switch contacts. It is a selective finish: it is applied to the connector fingers and not to the rest of the board, usually over a nickel barrier, and the process requires a plating bar that is later removed. Its cost is high and its solderability is poor, which is why a board with hard gold fingers normally has a different finish on the solderable areas.

ENEPIG inserts a thin palladium layer between the nickel and the gold, which prevents the nickel from being attacked during the gold deposition and eliminates the black pad mechanism. It costs more than ENIG and is used where wire bonding and soldering have to coexist on the same surface, or where the reliability requirement justifies the extra step.

Comparing the Options

• HASL: cheapest, coarse pitch only, long shelf life in practice, tolerant of handling.
• OSP: cheapest flat finish, fine pitch suitable, short shelf life, single reflow preferred.
• Immersion tin: flat, low cost, medium shelf life, sensitive to handling.
• Immersion silver: flat, good high frequency behaviour, requires sulphur free storage.
• ENIG: flat, durable, long shelf life, multiple reflows and probing, highest cost of the common finishes.
• Hard gold: for wear surfaces and connectors, not for general soldering.
• ENEPIG: ENIG plus wire bonding capability, at a further cost.

The comparison that matters most is between cost and flatness, and it is decided by the finest pitch on the board. If the smallest component pitch is above about 0.65 mm, HASL is usually acceptable. Below that, a flat finish is required, and the choice among OSP, immersion tin, immersion silver and ENIG comes down to shelf life, handling and budget. The differences between immersion tin and ENIG are covered in more detail in our article on immersion tin versus ENIG.

Cost and Handling

Finish cost is the most visible part of the difference between two quotes, and it is worth understanding where it comes from. The precious metal finishes carry a material cost that follows the metal market, so a quote for ENIG can move with the gold price rather than with the manufacturing cost. The thin organic finishes carry almost no material cost but need a controlled process and, for silver, carefully chosen packaging.

Handling requirements differ as much as cost. OSP and immersion silver dislike contamination and require gloves and sulphur free packaging. Immersion tin is soft and marks easily. ENIG is the most forgiving of the common finishes and is often chosen for that reason alone, since a stored board that cannot be soldered costs more than the finish ever did. The fabricator will normally specify the packaging and the storage conditions if asked, and those should be written into the purchase specification rather than left to the shipping department.

FAQ

• Which finish is best for fine pitch? Any flat finish. ENIG is the most durable and the most expensive; OSP is the cheapest if the boards are used quickly.
• Can HASL be used with 0.5 mm pitch parts? It can be attempted, but the uneven surface makes paste deposition and placement unreliable. A flat finish is the correct answer.
• How long can a board be stored? It depends on the finish and the packaging. ENIG is good for a year or more, OSP for months, and immersion silver requires sulphur free packaging whatever the duration.
• Does the finish affect impedance? Only very slightly, through the conductivity of the surface layer. Impedance models account for it as a thin surface layer rather than as part of the trace geometry.
• Can one board have two finishes? Yes. Hard gold on connector fingers with ENIG elsewhere is a common combination, and it requires a selective plating process.
• What causes black pad? Over attack of the nickel layer during gold deposition in the ENIG process. It is controlled by the process rather than by the design, and it is verified by a corrosion resistance test.

Summary

Surface finish choice follows from four questions: what is the smallest pitch on the board, how long will the boards be stored, how many times will they be soldered or probed, and what does the budget allow. HASL answers the coarse pitch case cheaply, OSP answers the fine pitch case cheaply when time is short, immersion tin and immersion silver fill the middle, and ENIG answers the demanding case at a price that is generally justified where reliability or storage life matters.

Two practices make the choice safer. Specify the finish by its properties, the thickness and the required test, rather than by name alone, because two suppliers offering ENIG may be offering different thicknesses of nickel. And state the storage and handling requirement in the purchase document, since the assembly result depends on how the board was stored as much as on what was deposited, and the capability of the supplier to hold the process is what makes the difference over a long production life.