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What is beneath the surface?
- by Matt Stevenson

The final surface finish of a PCB is an important consideration.  Does it meet RoHS guidelines, does it give me the reliable solder joint that I need for my application, is it the best fit for the type of components used in the design, is it cost-effective? There are many choices out there and some are better than others at meeting the needs, but the topic of this Tech Drill Down is not which one is the right choice, but rather how does it get on my PCB?

We will explore a couple of the common finishes and how they are applied in manufacturing.  Once the manufacturing panel has been processed through soldermask and silkscreen it is ready for surface finish application.  At this point of the process we have basically a fully functional PCB, however, the solderable surfaces are still copper which would not remain solderable very long in an open environment due to oxidation of copper, so we apply a surface finish like solder, immersion silver or ENIG to protect the copper from oxidation and corrosion and give the assembly a nice solderable surface to solder the parts onto.

Solder application is typically done through a Hot Air Leveling process, frequently called HAL or HASL. This process begins by cleaning the copper surface chemically, presenting a very fresh copper surface for all of the solderable pads and holes.  Once clean a layer of flux is added to the panels to further clean the copper and also by altering the surface tension of the molten solder helps in the soldering process.  This freshly fluxed panel is immersed in a pot of molten solder, where the solder coats the copper surface creating an intermetallic bond.  Once immersed and has dwelled in the pot for a period of a couple of seconds, the panel is rapidly pulled out and passed between two high-pressure air knives that blow off excess solder and level the deposit on the surface.  The panels are cooled, washed, inspected, and moved on to the next step of the process, usually route.  

There are other variations on this process but this process has been used for many years and still produces a good quality product.  Nothing solders quite like solder.  Immersion silver is a chemically applied surface finish.  Again the surface of the copper is cleaned to present a fresh copper surface.  Once cleaned the panels are immersed in the silver bath. In this bath, a displacement reaction occurs.  The chemistry basically displaces copper atoms and replaces them with a silver organometallic compound across all of the copper surfaces.  It is a very efficient and effective reaction that produces a surface as planer as the copper deposit below it.  After the reaction has completed, the panels are rinsed, dried, inspected, and moved to the next step of the process.

ENIG is another chemically applied surface finish that is done by immersing the panels in several chemical baths.  The surface is again cleaned to present a pristine copper layer to apply the ENIG deposit to.  Unlike Immersion silver the first chemical step is an electroless deposition reaction from the electroless nickel (EN). In this electroless deposition reaction, nickel ions in the solution are chemically reduced (autocatalytically) to nickel metal and deposited directly on the copper surface and subsequent nickel deposits.  This 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.  This 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.  After the reaction is complete the panels are rinsed, dried, inspected, and moved to the next step in the process.  

All of the surface finishes above will allow a good solder joint to be formed but there are advantages to each. Hope you enjoyed getting a little deeper into the manufacturing steps of PCBs.  If you have any questions please feel free to let me know!