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38 DESIGN007 MAGAZINE I MAY 2021 from oxidation and corrosion, as well as give the assembly a nice solderable surface for your parts. Solder Solder application is typically done through a hot air solder leveling process, frequently called HAL or HASL. Historically one of the more popular surface finishes because of its durability, manufacturability, and cost effec- tiveness, the process can leave uneven surface finishes and is not suitable for fine-pitch com- ponents. For many applications, solderability is a higher priority, making this process a bat- tle-tested choice for manufacturers. e process begins with cleaning the cop- per surface chemically and presenting a fresh copper surface for all the solderable pads and holes. A layer of flux is added to the panels to further clean the copper. e flux also alters the surface tension of the molten solder which helps during the soldering process. e freshly fluxed panel is then immersed in a pot of molten solder where the solder coats the copper surfaces, creating an inter- metallic bond. Aer dwelling in the pot of hot liquid solder for a few seconds, the panel is rapidly pulled out and passed between two high pressure air knives which blow off excess solder and level the deposit on the surface. e panels are cooled, washed, inspected, and moved on to the next step of the produc- tion process. ough not the perfect choice for every appli- cation, there is a reason this process has been used for many years. It produces a good quality product that makes assembly smoother, espe- cially for through-hole parts. Aer all, nothing solders quite like solder. Immersion Silver Immersion silver is a chemically applied sur- face finish. While more expensive than solder, it has become a more popular choice since it is RoHS and WEEE compliant. It's also an ideal choice for fine pitch components. e process also starts with a good cleaning of the copper surface. is time, the cleaned pan- els are immersed in the silver bath. rough the magic of chemistry, specifically a displacement reaction, copper atoms across all the surfaces are effectively displaced with a silver organometallic compound. It is a very efficient and effective reac- tion that produces a surface as planar as the cop- per deposit below it. Aer the reaction has com- pleted, the panels are rinsed, dried, inspected, and moved to the next step of the process. It's important to recognize during this stage of production that the immersion silver panel can be sensitive to contaminants and environ- mental conditions, and it should be packaged as soon as possible. ENIG ENIG (electroless nickel and immersion gold) is another chemically applied surface finish that is done by immersing the panels in several chemical baths. It offers a double layer metallic coating. Nickel serves as both a bar- rier to the copper and a surface to which com- ponents are soldered. In this process, the panel surface is again cleaned to create a pristine copper layer where we will apply the ENIG deposit. Unlike immer- sion silver, the first chemical step is an electro- less deposition reaction from the electroless nickel (EN). In this electroless deposition reac- tion, nickel ions in the solution are chemically reduced (autocatalytically) to nickel metal and deposited directly on the copper surface and subsequent nickel deposits. is deposit can be built up to a thickness capable of protecting against copper migration. Once the nickel deposit has been applied and rinsed, it quickly moves to the immersion gold (IG) part of the process. is reaction is similar to the immersion silver process above as it is a direct displacement reaction, displacing the nickel atoms in favor of the gold layer. Aer the reaction is complete the panels are rinsed, dried, inspected, and moved to the next step in the process.