Surface Finishes: ENIG, OSP, HASL and Silver
Why a Finish Is Needed
Copper oxidises, and an oxidised pad does not wet. The surface finish is the thin layer that protects the copper between fabrication and soldering and that allows the solder to form a joint. Every finish is a compromise between solderability, shelf life, cost, planarity, the number of reflow cycles it tolerates and the applications it suits. The choice is therefore made from the product’s requirements rather than from the price alone, because a finish that fails at assembly or in the field costs far more than the difference in the price of the board.
Hot Air Solder Levelling
HASL coats the pads with solder and levels it with a blast of hot air. It is cheap, it is robust, it gives a long shelf life and it has a well understood assembly behaviour. Its limitations are the planarity, since the coating thickness varies and the surface is not flat, and the thermal shock it applies to the board, which makes it less suitable for a fine pitch design and for a board that is already stressed. A lead free HASL is now standard, and it runs at a higher temperature than the tin lead version. It remains a good choice for a coarse design that will be assembled soon after fabrication.
ENIG
Electroless nickel immersion gold deposits a nickel barrier with a thin gold layer over it. It is flat, which suits fine pitch and area array packages, and it has good shelf life and good solderability. Its weaknesses are the cost, the nickel layer’s role in joint embrittlement under some conditions, and the phenomenon known as black pad, where an over active immersion gold step attacks the nickel and leaves a surface that wets poorly. Controlling the process is what avoids black pad, and the finish should be qualified with the assembly rather than assumed to be interchangeable between suppliers.
OSP
Organic solderability preservative is a thin organic film that protects the copper and that is removed by the flux during soldering. It is cheap, it is flat, and it is the simplest finish for a board that will be assembled quickly, because it has the shortest shelf life and it is sensitive to handling. Multiple reflow cycles degrade it, so it suits a single sided assembly better than one that must be reflowed twice. Its advantage beyond the cost is that it leaves copper under the solder, which produces a joint with a consistent composition where a lead free alloy is used.
Immersion Silver and Tin
Immersion silver is a thin silver layer over the copper, which gives good solderability, good planarity and a long shelf life at a moderate cost. Its weakness is tarnishing, which occurs when the board is stored in an environment with sulphur compounds, and the tarnish can reduce solderability if it is severe. It also requires careful handling, since the surface is soft and easily marked. Immersion tin is similar in concept and gives a flat surface, but it has a limited shelf life and a risk of tin whiskers, and it requires a controlled storage. Both are used where a flat finish is needed at a lower cost than ENIG.
Choosing the Finish
The decision should follow a few questions. Does the design have fine pitch or an area array package that needs planarity? How long will the board be stored before assembly? How many reflow cycles will it see? Is the assembly process sensitive to the surface? Does the application require a specific metal for a wire bond, a connector edge or a press fit? Where a wire bond is required, the finish must be compatible with the bonding process, and a gold over nickel is usually the choice. Where the design is coarse and the schedule is short, HASL or OSP may be the best value. The finish should be recorded with the product and not changed without re-qualification.
Effects on Assembly and Reliability
The finish affects the process. The joint’s intermetallic composition depends on the metals involved, so a nickel barrier changes the joint’s behaviour under thermal cycling compared with a copper surface. The wetting behaviour affects the paste’s flux requirement and the reflow profile window. The surface’s roughness affects the paste’s release and the print’s consistency. Where a finish is changed, the assembly parameters may need to change with it, which is why a finish is a process decision rather than a purchasing convenience. A board that assembles well with one finish may have a different defect profile with another.
Qualifying a Finish with the Assembly
A finish should be qualified on the actual assembly rather than on a test coupon alone, because the joint quality depends on the paste, the profile and the finish together. The qualification should include a print and reflow trial, an inspection of the joints, a thermal cycling result and, where a wire bond or a press fit is involved, the specific test for that process. Where a finish is changed by the fabricator without notice, the assembly parameters may need to change with it, which is why the finish belongs in the controlled drawing. A supplier change that seems trivial can move a defect that had been stable for years.
FAQ
Which finish is the most versatile? ENIG, because it is flat, has good shelf life and suits fine pitch, at a higher cost.
Which is the cheapest? OSP or HASL, but each has limits on shelf life or planarity.
What is black pad? A nickel surface attacked during the immersion gold step that wets poorly, associated with ENIG.
How many reflows does OSP tolerate? Fewer than the metal finishes, which makes it better suited to a single sided assembly.
Can the finish be changed freely? No. It affects the assembly process and the joint, so a change should be re-qualified.
Conclusion
A surface finish is a compromise, so choose it from the planarity, the shelf life, the reflow count and the application, and record it with the product. Changing it changes the process. Finish selection belongs to PCB manufacturing, its effect on the joints appears in SMT PCB assembly, and the options the factory offers are described under PCB capabilities. The finish for a new product is fixed during prototype PCB assembly in 2026.
