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64 I-CONNECT007 MAGAZINE I APRIL 2026 flatness on the board that can be problematic dur- ing assembly, especially for IC components. The standard HASL surface finish includes lead, which is not compatible with RoHS requirements. Note that a lead-free HASL option is available. Design- ers should also consider the number of surface- mount components their board must accommo- date before choosing HASL. If there are more than eight pads per side or fine pitch on an integrated circuit with many leads, flatness becomes an issue. Immersion Silver Immersion silver is a thin deposit that closely fol- lows the contours of the copper and offers design- ers and manufacturers a very flat surface finish. It will be relatively level and have a good grain struc- ture that the silver follows, creating exceptional flatness and planarity. When we compare HASL to immersion silver, we are essentially contrasting old-school with new school. Both create the best possible solder joints. The true advantage of immersion silver is its flat- ness and ability to handle fine-pitch components. It is also cost-effective compared to other options and in full compliance with RoHS standards. Like your grandmother's silver flatware, boards treated with immersion silver surface finish can be susceptible to heat, humidity, or even UV light. They will tarnish if left out in the elements. Boards with immersion silver surface finish that aren't going directly to assembly should be stored and packaged as you would a moisture-sensitive component. Electroless Nickel Immersion Gold (ENIG) ENIG is a two-layer coating. The first layer is nickel electrolessly deposited atop the copper, with a layer of gold immersion-plated on top of the nickel. This surface finish includes two layers because nickel is a metal that, in its pure form (like it is here), passivates (becomes unreactive) in oxygen. The layer of gold protects the nickel from oxidation. The solder joint forms between the tin in the solder and the nickel on the surface finish. This intermetallic is different from the copper-tin fin- ish, meaning it has pluses and minuses, but ENIG produces solid solder joints for most applications. Like immersion silver, ENIG creates a flat surface— it's a bit thicker than immersion silver. As you can imagine, cost is a big issue with ENIG as gold is among the most expensive raw materials on the planet, and the electroless immersion process is more costly than other surface finish application processes. Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) ENEPIG is the same surface finish as ENIG, but with a layer of palladium between the nickel and the gold. ENEPIG allows gold leads to be wire- bonded to the pads. Only specialized applications require wire bonding. ENEPIG is typically called out in the design specs, and entails the same cost C O N N ECT T H E D OTS