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64 DESIGN007 MAGAZINE I JANUARY 2025 ere are five commonly used surface fin- ishes, plus several others used for specialty applications. e five are: Hot Air Solder Leveling (HASL) HASL has been around for a long time and many industries rely on it, most notably the defense industry. HASL's long and successful history is associated with the adage, "Noth- ing solders quite like solder." HASL has been around the longest and is still one of the most widely used surface finish methods. It is cost- effective, durable, and reliable. It also has a long shelf life (the duration that the board can sit in inventory waiting for assembly) and pro- vides great solderability. However, HASL does have drawbacks. It oen leaves a lack of flatness on the board that can be problematic during assembly. ere is also lead (Pb) in the HASL surface finish, which is not com- patible with RoHS requirements. How- ever, there is also a Pb-free HASL option. Designers should also consider the num- ber of surface mount components their board must accommodate before choos- ing HASL. If there are more than eight pads per side on an integrated circuit with a lot of leads, flatness becomes an issue. Immersion Silver Immersion silver provides a very flat surface finish. It is a thin deposit that closely follows the contours of copper. It has a good grain structure, creating exceptional flatness and planarity. Comparing HASL with immersion sil- ver is essentially contrasting old school with new school. We oen recommend immersion silver over HASL because it creates the best solder joint. It can with- stand vibrations, is hospitable to surface mount components, and is cost-effective. Like your grandmother's silver flatware, boards treated with immersion silver sur- face finish can be susceptible to heat, humidity, or UV light. ey will tarnish if le out in the elements. You should store and pack- age boards with an immersion silver surface fin- ish that won't be going directly to assembly as you would a moisture-sensitive component. Electroless Nickel Immersion Gold (ENIG) ENIG is a two-layer coating. e first layer is nickel, electrolessly deposited atop the copper, and then a layer of gold is immersion plated on top of the nickel. It includes two layers because nickel is a metal that, in its pure form (like it is here), passivates (becomes unreactive) in oxygen. e layer of gold on top protects the nickel from oxidation. Tin from the solder and nickel from the sur-