PCB Surface Finish: Preparing for Assembly

One of my first neighborhood jobs growing up was washing cars. With my dad’s trusted shammy, I would go around the block cleaning exteriors and vacuuming interiors to save a few bucks for video games. Part of my toolkit was wax – I didn’t understand its true purpose, but I could see the difference between a waxed car and an unwaxed car a mile away. Later, I would learn that much more than just leaving a car looking good, wax offered a protective layer to the elements.

Much like an exterior coat on a washed car, PCB surface finishes protect against environmental contaminants that can detract from the long-term reliability of the solder joint. There’s an added benefit, however: surface finishes improve the baseline solderability, creating a superior joint from the get-go. As such, the process is vital to ensuring a successful assembly during manufacturing and a long service life.

A List of Surface Finishes By Material Type
Organic	Metallic
Organic Solderability Preservative (OSP) Carbon ink Combination of an organic and metallic surface finish, e.g., OSP and ENIG/hard gold	Hot air solder leveled (HASL) Electroless nickel immersion gold (ENIG) Electrolytic nickel/gold Immersion silver Immersion tin Reflow tin/lead Electroless nickel electroless palladium immersion gold (ENEPIG) Tin nickel* Electroless nickel immersion palladium*

*: Less common surface finishes

PCB Surface Finish Mechanisms

PCB surface finishes are the critical final step of manufacturing. As the outermost layer of the PCB, the surface finish protects from corrosion while improving the solderability between components and conductive surfaces. Surface finish options are numerous, and suitability will depend on multiple factors such as cost, temperature, thickness, storage potential, and more. Surface finishes fall into one of two general categories:

Deposition

An additive process that places directly onto the surface of the PCB. There is no further processing besides some clean-up or resurfacing to even the thickness.

Immersion

A replacement process that uses a displacement mechanism; a metal in solution attacks the surface substrate and takes its place in the metallic structure. Because the immersion process relies on a substrate to chemically react, it is self-limiting–no further reaction can occur after the complete replacement of the substrate by the soluble metal. Reagent cost aside, this step of the process is economically optimized.

Your Contract Manufacturer Has All Aspects of PCBA Covered

The choice of surface finish will ultimately depend on the board’s requirements and expected operating conditions. Although some applications will preclude specific surface finishes, it’s worthwhile to consider the strengths and weaknesses of each type to devise the most suitable plan for manufacturing.

Hot Air Solder Leveling (HASL)

As the saying goes, “Nothing solders like solder.” A relatively simple and readily applicable method, the HASL process dips a PCB into molten solder before removing any excess solder with hot air knives. The primary benefits of HASL are its cost and availability: since it uses the same material as the soldering process, it is inexpensive with well-characterized process controls. On the downside, the process adds considerable heat stress to the entire board and lacks the precision of other finishes for fine-pitch assemblies.

HASL Pros/Cons
Widely used, easily incorporable Same equipment/process RoHS compliant Relatively long shelf life	Surface planarity variance is ill-suited for HDI and some SMT Greatest material aging due to thermal excursion of dip

Electroless Nickel Immersion Gold (ENIG)

ENIG is more expensive and spans a more complex process than many surface finishes owing to the two distinct metal layers (nickel and gold) comprising the final finish. A chemical agent reduces nickel in the plating bath before deposition (additional materials may intentionally deposit alongside the nickel). Gold then plates in an immersion replacement process that substitutes the surface nickel and leaves behind a gold layer approximately a magnitude less thick than the nickel layer. The outer gold layer provides exceptional protection against corrosion, and the intermediate nickel layer prevents copper diffusion from the base conductor, allowing for many reflow cycles. ENIG is susceptible to black pad corrosion, a mechanism that attacks the nickel layer during gold replacement; theoretically, black pad corrosion should occur only at the pad’s surface, but grain boundaries and other structure defects allow for deeper penetration that can undermine solderability.

ENIG Pros/Cons
The thickness and hardness of the gold layer is variable Long shelf life (12 months) Excellent planarity	Undercut of copper by nickel layer can lead to shorts One of the more expensive surface finishes

Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)

ENEPIG is similar to ENIG but with an additional metal layer of palladium between the nickel and gold layers. Like the nickel it deposits onto, palladium occurs with an electroless step that adds to the surface instead of attacking the substrate for replacement. The goal is to provide a barrier between the gold and nickel to prevent black pad corrosion that would attack the nickel layer during gold substitution, ensuring the integrity of the surface finish. Advantageously, the gold layer does not need to plate as thick on the palladium as it would on the nickel in ENIG. ENEPIG requires additional cost due to the inclusion of a second precious metal, and the processing and chemistry become more complex than ENIG.

ENEPIG Pros/Cons
Excellent joint characteristics with lead-free solders Prevents black pad corrosion, which can plague ENIG Long shelf life (12 months) Wire bond capabilities (Au/Al)	The most expensive and complex surface finish method available Poor joint formation with leaded solders

Organic Solder Preservative (OSP)

OSP forms a thermally stable protective layer over the base conductor by a network of substituted phenylimidazoles (alternatively, benzimidazoles and derivative molecules) that coordinate covalent bonds to copper. It is a highly inexpensive water-based organic compound, making it a suitable candidate for low-reliability, high-volume boards. No physical chemistry process is involved besides the direct application of the OSP to the conductor surfaces, lending to easy rework. However, it exhibits less heat and handling durability, as oils in fingers can interact with the organic compound. Additionally, inspection becomes more difficult due to a transparent and nonconductive finish.

OSP Pros/Cons
Thin coating promotes high planarity Multiple manufacturing methods of integration (dip, conveyor) Extremely competitive cost	Short shelf life (<6 months) Poor handling practices can significantly impact solderability Less conducive to multi-step assemblies

Immersion Tin (ImmSn)

Tin applies to the surface copper conductor in a replacement process; the finish isn’t pure tin, but the intermetallic formed at the boundary between the tin and copper layers. While thickness is moderate (higher thickness plating slows according to the self-limiting nature of the copper substrate disappearance during tin replacement), the process offers cost-effective planarity and fine-pitch suitability. The major drawback of ImmSn is the formation of tin whiskers, which most believe occur in response to internal stresses. The relief mechanism displaces tin in a thin outward growth that can short to nearby conductive surfaces. Additional metals can help minimize internal stresses during formation.

ImmSn Pros/Cons
Sn/Cu intermetallic offers excellent material strength Fine-grain and non-porous characteristics prevent corrosion and material ingress	Short shelf life (<6 months) Thiourea, a processing chemical, is a known carcinogen Safety and performance handling issues Tin whisker formation

Immersion Silver (ImmAg)

Immersion silver uses a self-limiting replacement process, yet it plates very thin–despite using a precious metal alone as the surface finish, the thin finish renders it economical. ImmAg has good planarity and solderability, but it is vulnerable to corrosion from both sulfur and chloride compounds. Like tin whiskers, ImmAg can form dendrites at high temperatures and humidity in reactions with sulfur, leading to shorts. An organic top coat can protect the metal finish from corrosion.

ImmAg Pros/Cons
Flat and planar surface finish, ideal for HDI, surface mount, BGAs, etc. Moderate shelf life (6-12 months) Wire bondable (Al)	Sensitive to handling and tarnishing Requires additional treatment to prevent dendrite growth

Your PCB Contract Manufacturer From Start to Finish

A PCB surface finish is an integral manufacturing process that ensures a reliable assembly outcome. Surface finishes differ in cost, chemistry, storage, and many other supporting aspects, making the most suitable finish for any specific board highly dependent on the design goals and end-use environment. An experienced contract manufacturer can vastly simplify the decision process. At VSE, our engineers are committed to building electronics for our customers; we guide the manufacturing process with over 40 years of experience. Alongside our valued partners, we’ve been producing PCBAs for countless life-saving and life-changing applications.

If you are looking for a CM that prides itself on its care and attention to detail to ensure that each PCB assembly is built to the highest standards, look no further than VSE. Contact us today to learn more about partnering with us for your next project.