Bare copper oxidises within hours, and oxidised copper does not take solder. The surface finish is a thin coating on the pads that keeps the copper solderable until assembly. It also decides how flat the pads are, which matters for small parts, and whether the board can carry contacts or wire bonds.
The finishes
HASL (hot air solder leveling). The board is dipped in molten solder and the extra is blown off with hot air. It comes in tin-lead and lead-free versions. Cheap and robust, with good solderability and a long shelf life. The surface is not flat: pads end up domed, and small pads vary in thickness. It is not a good choice for fine-pitch parts or for BGAs. The board goes through a hot dip, which stresses it and can warp a thin board. Lead-free HASL is needed where lead is restricted.
ENIG (electroless nickel immersion gold). A layer of nickel, a few micrometres thick, is deposited chemically, then a very thin layer of gold, a small fraction of a micrometre, protects the nickel. The surface is very flat, so it suits fine pitch and BGAs, and it has a long shelf life. The solder bonds to the nickel; the gold dissolves into the solder. Known risk: poor nickel surface quality in the process can give weak joints (often called black pad). Good process control avoids it. It costs more than HASL or OSP.
OSP (organic solderability preservative). A thin organic film on the copper that protects it until the first reflow, then burns off. Flat, cheap and lead-free. The shelf life is shorter than the metallic finishes, and the film is damaged by repeated reflow, so it suits boards that are assembled soon after manufacture and soldered once or twice. Bare copper stays exposed at test points after assembly, and the film is invisible, so handling marks are a risk.
Immersion silver. A thin silver layer plated by displacement. Flat, good for fine pitch, and good at high frequencies. It tarnishes in air containing sulfur, so it needs careful packing and storage and has a shorter shelf life. It is moderate in cost.
Immersion tin. A thin tin layer. Flat, lead-free, and cheap. Tin over copper slowly forms a compound that hurts solderability, so shelf life is limited, and tin can grow whiskers (very thin metal hairs) in some conditions. Used for press-fit connections and for some fine-pitch work.
ENEPIG (electroless nickel, electroless palladium, immersion gold). Like ENIG with a palladium layer between nickel and gold. The palladium blocks the black pad failure and allows wire bonding as well as soldering. Flat and long lasting; costs more than ENIG.
Hard gold (electroplated nickel and gold). A much thicker, harder gold layer, put on by electroplating and alloyed with a little cobalt or similar. It resists wear. It is used for edge connectors (gold fingers) and contact pads that are plugged and unplugged. It is usually applied only to those areas, and a different finish covers the rest of the board. Expensive, and a poor choice for ordinary solder pads.
- HASL: molten solder blown level. The pad ends up domed, not flat.
- ENIG: a nickel layer with a very thin gold layer on top. Flat.
- ENEPIG: nickel, a palladium layer, then thin gold.
- OSP: an organic film on bare copper. Very thin and invisible.
- Immersion silver or tin: a thin, flat metal layer.
- Hard gold: nickel with a much thicker gold layer, for contacts that wear.
Comparison
| Finish | Flatness | Fine pitch and BGA | Shelf life | Cost class | Typical use |
|---|---|---|---|---|---|
| HASL (tin-lead) | Poor | No | Long | Low | Simple through-hole and larger parts, where lead is allowed |
| HASL (lead-free) | Poor | No | Long | Low | The same, where lead is restricted |
| ENIG | Very good | Yes | Long | Medium to high | General fine-pitch boards, BGAs, boards with contacts |
| OSP | Good | Yes, if soldered soon | Short | Low | Consumer boards assembled quickly |
| Immersion silver | Good | Yes | Medium, storage-sensitive | Medium | RF, fine pitch |
| Immersion tin | Good | Yes | Limited | Low to medium | Press-fit, some fine pitch |
| ENEPIG | Very good | Yes | Long | High | Wire bonding plus soldering, demanding boards |
| Hard gold | Good | Not a solder finish | Long | High | Edge connectors, contacts |
The shelf life figures here are relative. Actual figures vary by maker and by packing, and are usually stated in months; ask the factory for the figure that holds for them. If boards will be stored for a long time before assembly, ask the factory how to store them.
Flatness matters at fine pitch. A flat part or a stencil meets a domed pad only at its highest point.
- HASL pads are domed, and each dome has its own height.
- The flat plane of a part or a stencil rests on the highest dome only.
- ENIG is a thin nickel layer and a thinner gold layer: the pad stays flat.
- Mask between pads, the same in both pictures.
How to choose
- If you do not know, choose between lead-free HASL and ENIG. HASL for simple boards with larger parts, ENIG for fine pitch, BGAs and boards where flat pads matter.
- Fine-pitch or BGA means a flat finish. ENIG, OSP, immersion silver or tin, ENEPIG.
- Gold fingers mean hard gold on the fingers. Say it, and say which areas.
- A wire bond means ENEPIG or another finish rated for bonding.
- A board that will wait before assembly needs a long-lived finish. ENIG and ENEPIG are the safe choices.
- Lead-free where required. If your product falls under restrictions on hazardous substances, choose a lead-free finish. The product regulations define what is needed, and you are responsible for them.
- Mixing finishes costs more. Selective hard gold on a board with ENIG elsewhere is a standard combination, but each combination adds steps.
Common problems
- Solder joints weak on ENIG. Rare when the process is controlled; ask for a process report if you have a reliability requirement.
- Pads dull or non-wetting after storage. Typically oxidation or tarnish from long storage or poor packing.
- Uneven solder on HASL pads. An effect of the process and not a fault; choose a flat finish.
- Gold on the wrong area. Describe gold areas in the drawing, not only in a note.
How this shows up when you order here
You choose the finish as one of the options, and the price follows. The finishes available are listed on the capabilities page. The check can tell when your files need a finish you did not order: for example, it reports "an edge connector (gold fingers)" as a feature found when it is not among the ordered options. An engineer confirms every order. For the related price effects, see What drives the price of a PCBA.