In the design phase of a printed circuit board, one of the decisions that needs to be made is which surface treatment to choose. Unfortunately, there is no "best" PCB surface polish, and all surface treatments have advantages and disadvantages. The choice of surface finish is a matter of making a decision based on a compromise that best suits the intended use.

HASL

The most widely used surface treatment is HASL (Hot Air Solder Leveling). The production process is to immerse the PCB in molten solder, and then use a hot air knife to remove the excess part, leaving the solder layer as thin as possible. This is usually the cheapest finish and a good choice for general purpose boards. One disadvantage of HASL is that even after the hot air knife has removed as much excess as possible, the meniscus of the solder still causes the edge of the pad to become slightly rounded. This can lead to inaccurate placement of parts and can also cause visual problems with fiducial marks.

OSP

The biggest advantage of OSP (organic solderability preservative) finished boards is the price. OSP boards are cheaper than boards made with other surfaces such as HASL, Immersion Silver or ENIG. Another advantage of OSP, especially compared with HASL finished panels, lies in the flatness of the panel. The problem with OSP in the past was that the coating did not adapt well to multiple thermal cycles. In the past 10 years or so, this aspect has improved, but careful storage of PCBs should still be considered.

ENIG

ENIG (Electroless Nickel Immersion Gold) circuit board surface treatment costs are often higher, but its solderability, corrosion resistance and flatness have achieved the best results. It consists of a bottom layer of nickel and a thin gold top layer. The gold layer is very thin, not to provide the main structure of the track, it is only used as a protective coating of nickel to prevent it from discoloring before welding. Gold has strong corrosion resistance, so ENIG has the following advantages: it can be touched with fingers without discoloration, the shelf life is very long, the pad/track is very flat and square-this is good for fine surface mount parts.

Immersion Silver

Dipped silver printed circuit boards are flat and usually cost less than ENIG, but have more handling, packaging and storage requirements, resulting in a shorter shelf life. The problem is that it darkens quickly and is adversely affected by touch-usually silver jewelry changes over time and may be considered to have a rainbow effect. As long as the manufacturer’s guidelines are followed, silver PCB is a cost-effective solution, but additional quality issues should be considered.

All available surface treatments have their advantages, but generally speaking, the preferred surface treatment in the industry is ENIG because it has a longer shelf life, discoloration resistance, RoHS compliant and easy soldering compared to other surface treatments . It is more expensive than silver, but overall the result is worth it.