A finishing coating is a thin protective top layer on the landing areas made to preserve the solderability properties of the printed circuit boards. Also the flat coating contributes to a reliable connection with the solder and the materials of the landing area when assembling the components.
The finishing coating is applied either to the landing areas or open areas of topology. Depending on the required protective characteristics, the nuances of use and the type of basic material, various finishing coatings are applied to the printed circuit boards. For example, to protect:
- Landing areas designed for soldering, the main criteria for choosing the finishing coating are the characteristics of solderability/compatibility with the coating of the components leads, and the type of solder as well.
- Components/landing areas designed for surface assembly – the characteristics of the possibility to create flatness are important.
- Topologies from mechanical action (edge connectors) - it is necessary to take into account the friction coefficient, mechanical resistance and general wearing properties.
To ensure an effective protection of the copper PCB surface, and also to create an opportunity to realize high-quality soldering of components to the finishing coatings, there are following requirements:
- Period of storage.
- Chemical and mechanical resistances.
- Absence of peeling during operation.
- Metal alloys.
- Organic (feature - decompose during soldering).
Main types of finishing coatings
- HASL (HAL or POS-63) - solder coatings, made using air blade leveling technology.
- Lead Free HASL (LF HASL) –lead free tinning method.
- Based on chemical nickel ( (Electroless Ni).
- Golding of knife plugs - soft (Gold Finger) and hard/thick (Hard Gold).
- ENIG (immersion gold, ImAu) - a combination of chemical nickel and immersion gold.
- Immersion silver (ImAg).
- Immersion tin (ImSn).
- Organic coating OSP.
- Carbon (graphite).
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