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Connect the Dots: A Closer Look at Surface Finish

The final surface finish of a PCB is an important consideration. This coating between your components and the bare board is applied to ensure solderability and protect any exposed copper circuitry. Selecting the right type of surface finish can be daunting, and for good reasons.

Designers need to consider whether the selected surface finish will provide an adequately reliable solder connection for the application. Is it the best fit for the type of components used in the design? Is it cost effective? Does it meet compliance requirements like the Restriction of Hazardous Substances Directive (RoHS) and the Waste Electrical and Electronic Equipment (WEEE)?

There are many choices out there and some are better than others at meeting each of these needs. Today, we will explore three of the most common surface finishes—examining how they are applied in manufacturing and looking at the pros and cons of each.

Once your board has been processed through solder mask and silkscreen, it is ready for surface finish application. At this stage of production, the PCB is fully functional, at least in theory. The solderable surfaces are copper. In an open environment, an oxidation layer will quickly form, keeping the solder from doing its job. That’s why we apply a surface finish like solder, immersion silver, or electroless nickel immersion gold (ENIG) to protect the copper 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 effectiveness, the process can leave uneven surface finishes and is not suitable for fine-pitch components. For many applications, solderability is a higher priority, making this process a battle-tested choice for manufacturers.

The process begins with cleaning the copper 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. The flux also alters the surface tension of the molten solder which helps during the soldering process.

The freshly fluxed panel is then immersed in a pot of molten solder where the solder coats the copper surfaces, creating an intermetallic bond. After 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. The panels are cooled, washed, inspected, and moved on to the next step of the production process. 

Though not the perfect choice for every application, there is a reason this process has been used for many years. It produces a good quality product that makes assembly smoother, especially for through-hole parts. After all, nothing solders quite like solder. 

Immersion Silver
Immersion silver is a chemically applied surface 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.

The process also starts with a good cleaning of the copper surface. This time, the cleaned panels are immersed in the silver bath. Through 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 reaction that produces a surface as planar 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. 

It’s important to recognize during this stage of production that the immersion silver panel can be sensitive to contaminants and environmental 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 barrier to the copper and a surface to which components 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 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. 

ENIG offers a surface finish that is ideal for complex surface components that cannot tolerate uneven surfaces. It is also lead-free and durable. The process can be more expensive and comes with some risk. Phosphorus has been known to build up between the gold and nickel layers, sometimes resulting in fractured surfaces and faulty connections, often called black pad. Many of the newer chemistry formulations have all but eliminated this possible defect. 

Whether your priority is cost-effectiveness, manufacturability, compliance, or some combination of the three, any of these surface finishes will allow a good solder joint to be formed. Which method you choose will be a function of your application’s requirements.

This column originally appeared in the May 2021 issue of Design007 Magazine.