What is PCB Surface Finish?
A surface finish is a metallic coating covering the exposed copper, which will help protect the copper from environmental damage. Copper can oxidize when exposed to the natural environment, and the appropriate surface plating can protect against chemical degradation. Surface finishes are applied as one of the final steps in the PCB fabrication process. The surface finish on a PCB is typically applied after the solder mask process, so it only coats exposed conductors. This finish is designed to prevent oxidation of the remaining exposed copper and to provide a surface where components can be reliably soldered.
What are the types of PCB surface finish?
Some of the surface finishes designers can choose from include:
- Electroplated tin-lead solder
- Electroless nickel immersion gold (ENIG)
- Electroless nickel electroless palladium immersion gold (ENEPIG)
- Electroplated gold over electroplated nickel (hard gold)
- Immersion silver
- Hot air solder leveling (HASL)
- Organic solderability preserve (OSP)
These different surface finishes will give the exposed conductors on the finished PCB their characteristic color and luster.
How to Choose PCB Surface Finish?
For many years, HASL was the most popular surface finish, but commercialization of other techniques and the limitations of HASL has limited its use recently. Electroplated tin-lead solder is the entry-level surface finish material used in most PCBs as it will be compatible with tin-lead solder, creating strong solder joints during assembly. If your device will need to comply with Restriction of Hazardous Substances (RoHS) regulations, then you will need to use an alternative surface finish. ENIG is arguably the most common surface finish that is used in RoHS-compliant devices and is the entry-level alternative to tin-lead finish.
More specialized designs may use an alternative plating material. For example, RF designs tend to use immersion silver as it provides low losses. If a board needs to support gold or aluminum wire bonding directly to the PCB, then ENEPIG is normally used. For the RAM cards shown above, hard gold is used to ensure the conductor contacts along the board edge do not rub off over time.
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