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Printed circuit board surface finishes
Surface finishes are the coatings placed upon bare copper PCB pads whose purpose is to prevent contamination and oxidation and promote reliable soldering. Bare copper metal is chemically very active and will almost immediately oxide on exposure to air. Oxidised copper will not solder effectively, and the oxide layer must either be prevented by a suitable surface finish, or removed prior to soldering with application of an aggressive flux.
There four main classes of surface finish for printed circuit boards.
- Hot air solder level (either lead or lead-free), (HASL)
- Organic solderable preservative, (OSP)
- Electroless nickel immersion gold (ENIG) or (Au/Ni)
- Immersion silver (IS)
All the above surface finishes are lead-free unless stated otherwise.
Hot air solder level (either lead or lead-free), (HASL)
HASL submits the PCB to a severe heat shock, and the surface finish is often not flat enough for solder paste screen printing. Consequentially, this finish has largely been replaced by the other 3 finishes. HASL is multi-step soldering tolerant.
Organic solderable preservative, (OSP)
This is a simple organic anti-tarnish coating, very cheap but not robust. Widely used for high volume production of consumer electronics in the far-east. It is rarely available in Europe. It is not suitable for multi-step soldering process.
Electroless nickel immersion gold (ENIG) or (Au/Ni)
This is a long-established finish of choice for many customers because they are familiar with it. It is an expensive and complicated process but yields a good flat and solderable pad. It is good for wire bonding and key-pads. It has some well understood weaknesses such as skipped pads, black pads and brittle solder joints, but these defects are rare. The finish is multi-step soldering tolerant. Overall, ENIG is a good choice for a reliable surface finish with a long shelf-life.
Immersion silver (IS)
Although immersion silver finish has been widely used for over 20 years, it is the newest of all the commonly used PCB surface finishes. It is now the most popular finish of all. It is low cost, and simple to process. It is also an environmentally friendly process. The finish is flat, and there is no heat-shock to the PCB. It is wire bondable and suitable for keypads. The general shelf life of IS finished PCBs is 6 months although they will keep fresh longer if kept sealed from air. Any oxidation will manifest itself with a greying of the silver layer. In practice, this does not prevent effective soldering because the flux in solder paste will easily penetrate this layer. This finish is multi-step soldering tolerant.
Gold edge connectors
Edge connectors are commonly electroplated with hard gold (commonly 5 microns). Hard gold is generally plated on the edges of a circuit board, but it is possible to plate hard gold pads within the body of the PCB. For light use, an ENIG surface finish may suffice, and this can be achieved with the standard ENIG process upon any region of a PCB.
Surface finish availability by service
| Finish | PCB Train (online) | PCB Train Express (online) | Newbury Electronics PCB fabrication (by RFQ) |
|---|---|---|---|
| Hot air solder level (either lead or lead-free), (HASL) | No | No | Yes |
| Electroless nickel immersion gold (ENIG) or (Au/Ni) | Yes | No | Yes |
| Immersion silver (IS) | Yes | Yes | Yes |
| Hard gold plating | No | No | Yes |
| Silver Ink | No | No | Yes |
| Carbon Ink | No | No | Yes |
| Solder resist | Yes | No | Yes |
| Legend | Yes | No | Yes |
Surface finish characteristics
| Surface finish table | Hot air solder level (either lead or lead-free), (HASL) | Electroless nickel immersion gold (ENIG) or (Au/Ni) | Immersion silver (IS) |
|---|---|---|---|
| Soldering | |||
| Flatness for SMD | Not the best | Good | Good |
| Soldering process compatibility | Good | Good | Good |
| Multi-step soldering tolerant | Yes | Yes | Yes |
| Tolerance for pre-assembly baking | Good | Good | Good |
| Physical | |||
| Thickness | Up to 20 microns | Nickel 3-6 microns and gold 0.05-0.10 microns | 0.2 microns |
| Lead-free (RoHS) | Choice | Yes | Yes |
| Complications | |||
| Electrolytic gold edge connector possible | Yes | Yes | Yes |
| Peelable mask compatible | Yes | Yes | No |
| Wire bonding possible | No | Yes | Yes |
| Key-pad possible | No | Yes | Yes |
| Other factors | |||
| Process cost | Low | High | Low |
| Envronmental impact | Medium | High | Low |
| Heat shock on PCB | High | None | None |
| Shelf-life | Good | Excellent | Good if stored correctly |
| Nature of packaging in storage | Keep covered | Keep covered | Sealed packaging if possible |
