Surface Finish Selection for PCB Assembly
The surface finish on a bare board is a compromise between three requirements that pull in different directions. It has to protect the copper before assembly, it has to allow the solder to wet during it, and it has to do both at a cost and with a shelf life that the product can live with.
Because the finish sits between the copper and the joint, it also participates in the metallurgy of the joint. The choice therefore affects not only whether the board solders, but how the joint behaves afterwards in a product that will see thermal cycling.
What the Finish Has to Do
Before assembly the finish has to keep the copper from oxidising during storage and handling. A copper surface that has been left bare will not wet after a few days, and the finish is what prevents that.
During assembly the finish has to dissolve into the solder or allow the solder to displace it, so that the joint is formed between the solder and the copper beneath. A finish that does not dissolve leaves an interface that weakens the joint or produces a brittle layer.
Hot Air Solder Levelling
Hot air levelling coats the copper with solder and levels it with a blast of hot air. It is the cheapest finish, it is easy to inspect, and it has been used for decades. Its weakness is flatness: the coating thickness varies, and the surface is not level enough for a fine pitch device.
The process also exposes the board to a thermal shock, which is a consideration for a thin or a high layer count board. For a product with a coarse pitch and a short life, it remains a reasonable choice.
Organic Solderability Preservative
An OSP is a thin organic film that protects the copper and is displaced by the flux during soldering. It is flat, it is inexpensive, and it allows the pad to be almost pure copper, which is good for the joint.
Its limitations are handling and thermal exposure. The film is damaged by repeated reflow cycles, so an assembly with two passes on the same side is a poor match, and the shelf life is short compared with a metallic finish. Fingerprints also degrade it, which makes handling discipline part of the choice.
Immersion Silver and Immersion Tin
Immersion silver deposits a thin layer of silver over the copper by a chemical displacement reaction. It is flat, it solders well and it is compatible with fine pitch work. Its weaknesses are tarnishing in a sulphur bearing atmosphere and the potential for silver migration in a humid environment under bias.
Immersion tin behaves similarly in terms of flatness and solderability, and it has the advantage of being lead free by nature. The concern with tin is the formation of whiskers and the diffusion of copper into the tin layer, which reduces the shelf life and is sensitive to the storage temperature.
Electroless Nickel Immersion Gold
ENIG deposits a nickel barrier with a thin gold layer on top. The gold protects the nickel from oxidation and dissolves quickly into the solder, and the nickel provides a barrier against the copper diffusing into the joint. It is the finish of choice for fine pitch and for wire bonding.
The risks are process related. Excessive corrosion of the nickel during the gold deposition produces the condition known as black pad, in which the joint fails at the nickel interface. Controlling the chemistry and measuring the gold thickness are what prevent it, and the thickness specification should be explicit.
Thickness, Porosity and Joint Quality
Every finish has a thickness window. Too thin and the protection fails during storage; too thick and the gold or the silver does not dissolve completely, leaving inclusions in the joint that reduce its fatigue life.
Porosity matters for the same reason. A coating with pores allows the underlying metal to oxidise and lets the plating chemistry attack the base, and the defect appears as a joint that wets unevenly for no apparent reason. Thickness measurement on a coupon is the routine control.
Flatness and Fine Pitch Assembly
Flatness is the property that decides whether a fine pitch device can be assembled at all. A finish that forms a dome over the pad changes the deposit height and the stand off, and at a pitch of a few tenths of a millimetre that variation is the difference between a joint and a bridge.
This is why the levelled finishes are less common on a dense board even though they are cheaper. The cost of the finish is small compared with the cost of a yield problem caused by an uneven surface.
Shelf Life and Storage
Shelf life follows from the chemistry. An organic film degrades within months, an immersion silver tarnishes, and a gold surface remains solderable for much longer. The storage conditions change all of them, and a board stored in a humid environment ages faster than the datasheet suggests.
Because the shelf life is part of the product’s logistics, it should be considered when the finish is chosen rather than when the boards arrive. A product that is built in small batches over a long period is a poor match for a finish with a short life, whatever the cost per panel.
Cost and Availability
The cost order is roughly the same everywhere: levelled solder is cheapest, followed by the organic film, then immersion silver and tin, and finally ENIG. The difference per panel is modest and the difference in yield is not.
Availability matters too. A finish that only one supplier in the region can produce limits the options if the project has to be moved, and a second source may not be able to match the process exactly. Where the requirement is critical, it is worth confirming that more than one fabricator can meet it.
Choosing for a Product
The choice starts with the finest feature and the number of reflow cycles, and it is then qualified by the environment and the shelf life. A dense board with a fine pitch device and a long storage period points clearly towards a metallic finish, while a simple board built immediately points the other way.
The decision should be recorded with the reason, so that a later change is made deliberately. That record belongs with the rest of the fabrication requirements, in the same way that the manufacturing tolerances are, and it should be revisited if the product or its supply chain changes.
Process Control and Verification
On a design of this kind, solderability is the item that decides how the rest of the board is arranged. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
FAQ
Can the finish be changed without changing the design? Usually yes, and it is one of the easier changes to make. The stencil and the profile may need to be re-qualified because the wetting behaviour changes.
Is a thicker gold layer better? No. Above the specified range the gold does not dissolve completely and the joint becomes brittle, which is worse than a marginal thickness.
How long is a finish solderable? It depends on the finish and the storage. A protected board in a sealed bag keeps its solderability for the period the fabricator quotes, and beyond that it should be tested.
