The most basic purpose of the surface treatment is to ensure good solderability or electrical properties.Because the copper in the air tend to exist in the form of oxides, it is unlikely to maintain the original copper for a long time,so copper require additional processing. Although in the subsequent assembly ,a strong flux can be used to remove oxide of most copper,but flux itself is difficult to remove,so the industry generally do not use strong flux.There are many PCB surface treatment.The common treatments have hot air leveling (HASL), organic coating (OSP), electroless nickel / immersion gold (ENIG), immersion silver and immersion tin.The following will be introduced one by one.
Hot-air solder leveling. The most common PCB surface treatment menthod. Lead and lead-free should be considered.
Organic solderability protective coating (OSP, Organic solderability preservative) is an organic coating to prevent oxidation of copper in the welding before, that is to protect PCB(printed circuit board) pad solderability from destruction.
Nickel Immersion Gold. The copper surface is plated of Ni/Au. The deposition thickness of the inner Ni layer is generally 3~6um, the deposition thickness of the outer Au thin, usually 0.05 ~ 0.1um. Ni as the barrier layer between Cu and soldering tin. Welding, outside Au in the solder which melts rapidly with soldering tin and then turn into Ni/Sn intermetallic compounds with inner Ni. The gold-plated to prevent Ni oxidation during storage or passive, it should be enough gold plating density, the thickness can not be too thin.
4. Immersion Silver
Immersion silver process through the thin (about 0.1~0.4um) and dense to provide a layer of silver deposited organic protective film, the surface of the silver with the greatly extended life. Immersion Silver surface is very flat and very good with weldability.
5. Immersion Tin
Only used for two reasons for immersion tin process: first, the surface is very flat, coplanarity good; the other is lead-free. But in the immersion tine process easy to be with Cu/Sn intermetallic compound, Cu / Sn intermetallic is with poor solderability.
With the rising demand of customers, the increasingly stringent requirements of environment,surface treatment process more and more, in the end choosing the kind of promising, more versatile surface treatment process, it seems like a bit confusion. Where will PCB surface treatment process go in the future,and now it is not able to accurately predict. In any case, meeting customer requirements and the protection of the environment must be done!